How to Program in C++

Basics

C++ is a compiled language, an upward compatible superset of C and an (incompatible) predecessor to Java. C++ compiles C programs but adds object oriented (OO) features (classes, inheritance, polymorphism), templates (generic functions and classes), function and operator overloading, namespaces (packages), exception handling, a library of standard data structures (string, vector, map, etc.) and formatted text I/O (istream, ostream). Unlike Java, C++ lacks a standard graphical user interface (GUI), network interface, garbage collection, and threads, and allows non-OO programming and unchecked low-level machine operations with pointers. However, C++ executes faster than Java and requires no run-time support.

A C++ program is a collection of function, object, and type declarations. Every program must have a function int main() { ... } where the curly braces enclose a block, a sequence of declarations and statements ending in semicolons which are executed in order. A statement is an expression, block, or control statement that alters the order of execution, such as if, while, for, break, return. Some types (std::string), objects (std::cout), and functions are defined in header files, requiring the line #include <header> before use. Items defined in the standard headers are in the namespace std. The std:: prefix may be dropped after the statement using namespace std;. For instance,

  // Comment: prints "Hello world!" and an OS-independent newline
  #include <string>    // Defines type std::string
  #include <iostream>  // Defines global object std::cout
  using namespace std; // Allow std:: to be dropped
  int main() {         // Execution starts here
    string s="Hello world!\n"; // Declares object s of type string
    cout << s;         // An expression as a statement, << is the output operator
    return 0;          // Execution ends here
  }

The symbol // denotes a comment to the end of the line. You may also use /* ... */ for multiline comments. Spacing and indentation is used for readability. C++ is mostly free-form, except that the end of line is significant after # and //. C++ is case sensitive.

C++ source code files should be created with a text editor and have the extension .cpp. If the above is called hello.cpp, it may be compiled and run as follows in a UNIX shell window:

  g++ hello.cpp -o hello -Wall -O
  ./hello

In Windows, the GNU C++ compiler is called DJGPP. To compile and run from an MS-DOS box:

  gxx hello.cpp -o hello.exe
  hello

To use the network or GUI interface in UNIX, you must use the X and socket libraries, which don't work in Windows. In Windows, you must use the Windows API and a compiler that supports them, such as from Microsoft, Borland, or Symantec. GUI/network programming is nonportable and outside the scope of this document.

A program consists of a collection of functions (one of which must be int main() {...}) and type and object declarations. A function may contain declarations and statements. Statements have the following forms, where s is a statement, and t is a true/false expression.

  for (int i=0; i<10; i=i+1)

A test of several alternatives usually has the form if (t) s; else if (t) s; else if (t) s; ... else s;. A switch statement is an optimization for the special case where an int expression is tested against a small range of constant values. The following are equivalent:

  switch (i) {                if (i==1)
    case 1: j=1; break;         j=1;
    case 2: // fall thru      else if (i==2 || i==3) // || means "or else"
    case 3: j=23; break;        j=23;
    default: j=0;             else
  }                             j=0;

throw x jumps to the first catch statement of the most recently executed try block where the parameter declaration matches the type of x, or a type that x can be converted to, or is .... At most one catch block is executed. If no matching catch block is found, the program aborts (Unexpected exception). throw; with no expression in a catch block throws the exception just caught. Exceptions are generally used when it is inconvenient to detect and handle an error in the same place.

  void f() {
    throw 3;
  }

  int main() {
    try {
      f();
    }
    catch(int i) {  // Execute this block with i = 3
      throw;        // throw 3 (not caught, so program aborts)
    }
    catch(...) {    // Catch any other type
    }
  }

Expressions

There are 18 levels of operator precedence, listed highest to lowest. Operators at the same level are evaluated left to right unless indicted, Thus, a=b+c means a=(b+c) because + is higher than =, and a-b-c means (a-b)-c. Order of evaluation is undefined, e.g. for sin(x)+cos(x) we cannot say whether sin() or cos() is called first.

The meaning of an expression depends on the types of the operands. (x,y) denotes a comma separated list of 0 or more objects, e.g. (), (x), or (1,2,3,4).

  const char* s="hello";
  int(*s);                 // static_cast
  (char*)s;                // const_cast
  (const int*)s;           // reinterpret_cast
  (int*)s;                 // reinterpret_cast and const_cast

Declarations

A declaration creates a type, object, or function and gives it a name. The syntax is a type name followed by a list of objects with possible modifiers and initializers applying to each object. A name consists of upper or lowercase letters, digits, and underscores (_) with a leading letter. (Leading underscores are allowed but may be reserved). An initializer appends the form =x where x is an expression, or (x,y) for a list of one or more expressions. For instance,

  string s1, s2="xxx", s3("xxx"), s4(3,'x'), *p, a[5], next_Word();

Built-in Types

  int a, b=0;       // a's value is undefined
  static double x;  // 0.0

Integer Types

An int value may be written in decimal (e.g. 255), hexadecimal with a leading X (e.g. xff or XFF) or octal (base 8) with a leading 0 (e.g. 0377). A trailing L denotes long (e.g. 255L or 255l), and U denotes unsigned. These may be combined (e.g. 255lu or 255UL is unsigned long). Most integer operations translate to a single machine instruction and are very fast.

  + - * / % -i      Add, subtract, multiply, divide, mod, unary negation
  =                 Assignment
  == != < <= > >=   Comparison, returns true or false
  ++i i++ --i i--   Pre/post increment and decrement
  & | ^ ~i << >>    Bitwise and, or, xor, not, left shift, right shift
  += -= *= /= %= &= |= ^= <<= >>=    Operate and assign, e.g. x+=y means x=x+y
  && || !i          Logical and then, or else, not

Assignment returns the value assigned, e.g. x=y=0 assigns 0 to y and the new y to x. The result is an lvalue, e.g. (x=y)=0 is also legal (but useless). It assigns y to x, then 0 to x.

++i and i++ both add 1 to i. However, ++i returns the new value, and i++ returns the old value. Likewise for decrement, --i and i--, which subtracts 1. The pre forms, ++i, --i, are lvalues.

Bitwise operators treat an int as a 2's compliment B-bit binary number (B=32) with weights -2^B-1, 2^B-2, 2^B-3, ... 8, 4, 2, 1. The leftmost bit is negative, and serves as the sign bit. Thus, 0 is all zero bits and -1 is all 1 bits. Bitwise operators x&y x|y x^y and ~x perform B simultaneous logical operations on the bits of x and y. For instance, if y is a power of 2, then x&(y-1) has the effect x%y, but is usually faster, and the result is always positive in the range 0 to y-1.

x<<y returns x shifted left by y places, shifting in zeros. The result is x*2^y. x>>y returns x shifted right by y places, shifting in copies of the sign bit (or zeros if unsigned). The result is x/2^y but rounding negative instead of toward 0. For instance, -100>>3 is -13. y must be in the range 0 to B-1 (0 to 31). Shifting is usually faster than * and /.

Any binary arithmetic or bitwise operator may be combined with assignment. The result is an lvalue. e.g. (x+=2)*=3; has the effect x=x+2; x=x*3;

Logical operators treat 0 as false and any other value as true. They return true (1) or false (0), as do comparisons. The && and || operators do not evaluate the right operand if the result is known from the left operand.

  if (i>=0 && i<n && a[i]==x)  // Do bounds check on i before indexing array a
  if (x=3)                     // Legal but probably wrong, assign 3 to x and test true

char

A char is a one byte value. Unlike other numeric types, it prints as a character, although it can be used in arithmetic expressions. Character constants are enclosed in single quotes, as 'a'. A backslash has special meaning. '\n' is a newline, '\\' is a single backslash, '\'' is a single quote, '\"' is a double quote. A backslash may be followed by 3 octal digits ('\377') or an X and 2 hex digits ('\xFF') (but not decimal). Most computers use ASCII conversion as follows:

  8-13:   \b\t\n\v\f\r  (bell, tab, newline, vertical tab, formfeed, return)
  32-47:   !\"#$%&\'()*+,-./                 (32=space, \' and \" are one char)
  48-63:  0123456789:;<=>\?                  (\? is one char)
  64-95:  @ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_  (\\ is one char)
  96-126: `abcdefghijklmnopqrstuvwxyz{|}~

Floating Point Types

A double is usually represented as a 64 bit number with a sign bit, an 11 bit exponent, and 52 bit mantissa. Therefore it can only represent numbers of the form M*2^E exactly, where -2⁵² < M < 2⁵² and 2^-10 < E < 2¹⁰. This is about + or - 1.797e308 with about 15 decimal digits of precision. Therefore, numbers like 1e14 and 0.5 have exact representations, but 1e20 and 0.1 do not.

  0.1 * 10 == 1    // false, they differ by about 10-15

  + - * / -x        Add, subtract, multiply, divide, unary negation (no %)
  = += -= *= /=     Assignment, may be combined with operators
  == != < <= > >=   Comparison, however only < and > are meaningful

In a declaration, modifiers before the type name apply to all objects in the list. Otherwise they apply to single objects.

  int* p, q;           // p is a pointer, q is an int
  const int a=0, b=0;  // a and b are both const

const

const objects cannot be modified once created. They must be initialized in the declaration. By convention, const objects are UPPERCASE when used globally or as parameters.

  const double PI=3.14159265359;  // Assignment to PI not allowed

References

A reference creates an alias for an object that already exists. It must be initialized. A reference to a const object must also be const.

  int i=3;
  int& r=i;         // r is an alias for i
  r=4;              // i=4;
  double& pi=PI;    // Error, would allow PI to be modified
  const double& pi=PI;  // OK

Functions

A function has a list of parameter declarations, a return type, and a block of statements. Execution must end with a return statement returning an expression that can be converted to the return type, unless void, in which case there is an implied return; at the end. Arguments passed to a function must match the parameters or allow implicit conversion (such as int to double). Functions must be defined before use, or have a matching declaration that replaces the block with a semicolon and may optionally omit parameter names. Functions are always global (not defined in other functions).

  void f(double x, double); // Declaration
  double g() {              // Definition
    return 3;               // Implied conversion to double (3.0)
  }
  int main() {              // Execution starts with function main
    f(g(), 5);              // Calls g, then f with implicit 5.0
    return 0;               // Return UNIX $status or Windows ERRORLEVEL
  }
  void f(double x, double y) { // Definition must match declaration
    cout << x+y;
    return;                 // Optional
  }

Command line arguments may be passed to main(int argc, char** argv) where argv is an array of argc elements of type char* ('\0' terminated array of char), one element for each word (separated by white spaces). In UNIX, the command line is expanded before being passed (* becomes a directory listing, etc). The following program prints the command line.

  // echo.cpp
  #include <iostream>
  using namespace std;
  int main(int argc, char** argv) {
    for (int i=0; i<argc; ++i)
      cout << argv[i] << endl;
    return 0;
  }

  g++ echo.cpp
  ./a.out hello world
  ./a.out
  hello
  world

Function parameters have local scope. They are initialized by copying the argument, which may be an expression. Reference parameters are not copied; they become references to the arguments passed, which must be objects that the function may modify. If the reference is const, then the argument may be an expression. Const reference is the most common for passing large objects because it avoids the run time overhead of copying.

  void assign_if(bool cond, string& to,  const string& from) {
              // value      reference    const reference
    if (cond)
      to=from;
  }
  int main() {
    string s;
    assign_if(true, s, "a");   // OK, s="a"
    assign_if(false, "b", s);  // Error: to refers to a const

Functions returning a reference must return an object which can be assigned to, and that object must exist after returning (global or static, but not local). The function may be called on the left side of an assignment. Functions returning by value make a temporary copy which is const.

  int  a=1;                           // Global
  int  f() {return a;}                // OK, returns copy of a
  int& g() {return a;}                // OK, g() is alias for a
  int& h() {return a+1;}              // Error, reference to const
  int& i() {int b; return b;}         // Error, b destroyed after return
  int& j() {static int b; return b;}  // OK, static has global lifespan
  int main() {
    f()=2;    // Error, assignment to const
    g()=f();  // OK, a=1;
    return 0;
  }

Functions with the same name may be overloaded by matching the arguments to the parameters.

  int abs(int);
  double abs(double);
  int main() {
    abs(3);    // int
    abs(3.0);  // double
    abs("3");  // Error, no match
    abs('a');  // Error, ambiguous, could convert char to int or double
    return 0;
  }

  string operator - (const string& s, int i);  // Defines s-i
  string operator - (const string& s);         // Defines -s

  string& operator++(const string& s);      // defines ++s
  string  operator++(const string& s, int); // defines s++

Functions may have default arguments by initializing the parameters. Defaults should be specified only once. Defaulted parameters must appear after all non-default parameters.

  void f(int i, int j=0, int k=0);  // OK
  void g(int i=0, int j);           // Error
  int main() {
    f(1, 2);  // f(1, 2, 0);
    f(1);     // f(1, 0, 0);
    f();      // Error
    return 0;
  }
  void f(int i, int j, int k) {}    // Defaults not specified again

  template <class T>
  void swap(T& a, T& b) {
    T tmp=a;
    a=b;
    b=tmp;
  }
  void swap(string& a, string& b);  // Overrides the case T=string
  int main() {
    int i=1, j=2;
    string a, b;
    swap(i, j);        // OK, T is int
    swap(a, b);        // OK, calls non-templated swap
    swap(i, a);        // Error, cannot resolve T
    swap(cout, cerr);  // Error, ostream does not allow =

inline is a hint to the compiler to optimize for speed by expanding the code where it is called, saving a call and return instruction. Unlike a macro, semantics are preserved. Only short functions should be inlined.

  inline int min1(int a, int b) {return a<b?a:b;}
  #define min2(a,b) ((a)<(b)?(a):(b))
  int main() {
    min1(f(), 0);  // calls f() once
    min2(f(), 0);  // calls f() twice, expands to ((f())<(0)?(f()):(0))

Pointers

A pointer stores the address of another object, and unlike a reference, may be moved to point elsewhere. The expression &x means "address of x" and has type "pointer to x". If x has type T, then &x has type T*. If p has type T*, then *p is the object to which it points, which has type T. The * and & operators are inverses, e.g. *&x == x.

Two pointers are equal if they point to the same object. All pointer types are distinct, and can only be assigned pointers of the same type or 0 (NULL). There are no run time checks against reading or writing the contents of a pointer to invalid memory. This usually causes a segmentation fault or general protection fault.

  int i=3, *p=&i;    // p points to i, *p == 3
  *p=5;              // i=5
  p=new int(6);      // OK, p points to an int with value 6
  p=new char('a');   // Error, even though char converts to int
  p=6;               // Error, no conversion from int to pointer
  p=0;               // OK
  p=i-5;             // Error, compiler can't know this is 0
  *p=7;              // Segmentation fault: writing to address 0
  int *q; *q;        // Segmentation fault: q is not initialized, reading random memory

A pointer to a const object of type T must also be const, of type const T*, meaning that the pointer may be assigned to but its contents may not.

  const double PI=3.1415926535898;
  double* p=Π              // Error, would allow *p=4 to change PI
  const double* p=Π        // OK, can't assign to *p (but may assign to p)
  double* const p=Π        // Error, may assign to *p (but not to p)
  const double* const p=Π  // OK, both *p and p are const

A function name used without parenthesis is a pointer to a function. Function pointers can be assigned values and called.

  int f(double);     // functions f and g take double and return int
  int g(double);
  int *h(double);    // function h takes double and returns pointer to int
  int (*p)(double);  // p is a pointer to a function that takes double and returns int
  int main() {
    p=f; p(3.0);     // calls f(3.0)
    p=g; p(3.0);     // calls g(3.0)
    p=h;             // Error, type mismatch

Explicit pointer conversions are allowed but usually unsafe.

  int i, *p=&i;
  i=int(3.0);        // OK, rounds 3.0
  *(double*)p = 3.0; // Crash, writes beyond end of i
  *(double*)&PI = 4; // Overwrites a const

  i=static_cast<int>(3.0);            // Apply standard conversions
  *reinterpret_cast<double*>p = 3.0;  // Pretend p has type double*
  *const_cast<double*>&PI = 4;        // Same type except for const

Arrays

The size of an array must be specified by a constant, and may be left blank if the array is initialized from a list. Array bounds start at 0. There are no run time checks on array bounds. Multi-dimensional arrays use a separate bracket for each dimension. An array name used without brackets is a pointer to the first element.

  int a[]={0,1,2,3,4};    // Array with elements a[0] to a[4]
  int b[5]={6,7};         // Implied ={6,7,0,0,0};
  int c[5];               // Not initialized, c[0] to c[4] could have any values
  int d[2][3]={{1,2,3},{4,5,6}};  // Initialized 2-D array
  int i=d[1][2];          // 6
  d[-1][7]=0;             // Not checked, program may crash

  int a[5];               // a[0] through a[4]
  int* p=a+2;             // *p is a[2]
  p[1];                   // a[3]
  p-a;                    // 2
  p>a;                    // true because p-a > 0
  p-1 == a+1              // true, both are &a[1]
  *a;                     // a[0] or p[-2]
  a=p;                    // Error, a is const (but not *a)

A literal string enclosed in double quotes is an unnamed static array of const char with an implied '\0' as the last element. It may be used either to initialize an array of char, or in an expression as a pointer to the first char. Special chars in literals may be escaped with a backslash as before. Literal strings are concatenated without a + operator (convenient to span lines).

  char s[]="abc";                       // char s[4]={'a','b','c','\0'};
  const char* p="a" "b\n";              // Points to the 'a' in the 4 element array "ab\n\0"
  const char* answers[2]={"no","yes"};  // Array of pointers to char
  cout << answers[1];                   // prints yes (type const char*)
  cout << answers[1][0];                // prints y (type const char)
  "abc"[1]                              // 'b'

  int a[5], b[5]=a;       // Error: can't initialize b this way
  b=a;                    // Error: can't assign arrays
  b==a;                   // false, comparing pointers, not contents
  "abc"=="abc"            // false, comparing pointers to 2 different locations

  int n, *p;
  cin >> n;
  p=new int[n];  // Elements p[0] to p[n-1] with values initially undefined
  delete[] p;    // Use delete with new or new(), delete[] with new[]
  p[0] = 1;      // May crash

static

  +----------+
  |          |
  |   Heap   |  Allocated with new until deleted or program exits.
  |          |
  +^^^^^^^^^^+  
  |   Stack  |  Local objects, parameters, temporaries, function return addresses.
  +----------+     +---------+
  |   Data   | <-- |  Data   |  Initial values for static and global objects.
  +----------+     +---------+
  |   Code   | <-- |  Code   |  Executable machine instructions.
  +----------+     +---------+  (Cannot be read or written by program.)
  | Reserved |    a.out on disk
  | for OS   |
  | and other|  Cannot be read or written by program, will cause segmentation
  | programs |  fault or general protection fault.
  +----------+
     Memory

  int& f() {        // Return by reference, f() is an alias for s, not a temporary copy
    static int s=1; // Initialized only once
    ++s;
    return s;       // Safe to return by reference
  }
  int main() {
    cout << f();    // 2
    cout << f();    // 3
    f()=5;          // OK, s=5;
    s=6;            // Error, s is not in scope

register

  register int x;

volatile

  const volatile unsigned short& port=*(const short*)0xfffe; // 16 bit port at address xfffe

Standard Library Types

Standard library types (string, vector, map...) and objects (cin, cout...) require a #include <header> and must be extracted from namespace std, either with a using namespace std; statement or by using the fully qualified names preceded with std::, as in std::cout.

  #include <iostream>                    #include <iostream>
  int main() {                           using namespace std;
    std::cout << "Hello\n";              int main() {
    return 0;                              cout << "Hello\n";
  }                                        return 0;
                                        }

<iostream>

The header <iostream> defines global object cin of type istream, and global objects cout, cerr, clog of type ostream. cin represents standard input, normally the keyboard, unless redirected to a file or piped on the command line. cout represents standard output, which is normally the screen unless redirected or piped. Writing to cerr or clog both write to the screen even if output is redirected. The difference is that writing a newline ('\n') flushes any buffered output to cerr but not to cout or clog.

In the following, in is an istream (cin), out is an ostream (cout, cerr, clog), i is int, c is char, and cp is char*.

  in >> x;               // Read 1 word to numeric, string, or char* x, return in
  in.get();              // Read 1 char (0-255) or EOF (-1) as an int
  in.get(c);             // Read 1 char into c, return in
  in.unget();            // Put back last char read, return in
  in.getline(cp, i);     // Read up to i chars into char cp[i] or until '\n', return in
  in.getline(cp, i, c);  // Read to c instead of '\n', return in
  getline(in, s);        // Read up to '\n' into string s, return in
  in.good();             // true if no error or EOF
  bool(in);              // in.good();
  in.bad();              // true if unexpected char in formatted input
  in.clear();            // Allow more input after bad, or throw an ios::failure
  in.eof();              // true if end of file
  in.fail();             // true if system input error

  out << x;                // Formatted output, redirected with >
  out << endl;             // Print '\n' and flush

Input with >> reads a contiguous sequence of non-whitespace characters. If x is numeric and the next word contains invalid characters (such as "1.5" or "foo" for an int), then the first offending character remains unread, in.bad() is set, and no further input can occur until in.clear() is called. Input into a char* array is not bounds checked. Input returns the istream to allow chaining, and has a conversion to bool to test for success. Output also returns the ostream to allow chaining.

  // Read and print pairs of strings and ints until something goes wrong
  // Input:  hi 3 there 5 this is 1 test
  // Output: hi 3
             there 5

  string s; int i;
  while (cin >> s >> i)
    cout << s << " " << i << endl;
  cin.clear();

Defines manipulators for formatted output of numeric types. They have no effect on strings. setw() applies only to the next object printed, but the others remain in effect until changed.

  out << setw(i);          // Pad next output to i chars, then back to 0
  out << setfill(c);       // Pad with c (default ' ')
  out << setprecision(i);  // Use i significant digits for all float, double

  cout << setw(6) << setprecision(3) << setfill('0') << 3.1; // print "003.10"

<fstream>

  ifstream in(cp);          // Open file named cp for reading
  ifstream in(cp, ios::in | ios::binary);  // Open in binary mode
  bool(in);                 // true if open successful

cp is the file name. It must be a char*, not string (use s.c_str() to convert string s). Input is normally in text mode. In Windows, carriage returns ('\r') are discarded, and an ASCII 26 ('\032') signals end of file. In binary mode and in UNIX, no such translation occurs. The file is closed when the ifstream is destroyed.

  {
    ifstream f("input.dat", ios::in | ios::binary);
    if (!f)
      cerr << "File not found\n";
    else {
      int i=f.get();  // First byte or EOF if empty
    }
  } // f closed here

ofstream is derived from ostream, inheriting all its operations (such as <<). In addition,

  ofstream os(cp);          // Open file named cp for writing
  ofstream os(cp, ios::out | ios::binary);  // Open in binary mode

  string()           // Empty string
  string(cp)         // Convert char* cp to string
  string(n, c)       // string of n copies of char c
  s=s2               // Assign char* or string s2 to string s
  s1<s2              // Also ==, !=, >, <=, >=, either s1 or s2 may be char*
  s.size()           // Length of string s
  string::size_type  // Type of s.size(), usually unsigned int
  s.empty()          // True if s.size() == 0
  s[i]               // i'th char, 0 <= i < s.size() (unchecked), may be assigned to
  s.at(i)            // s[i] with bounds check, throws out_of_range
  s1+s2              // Concatenate strings, either s1 or s2 may be char or char*
  s+=s2              // Append string, char, or char* s2 to string s
  s.c_str()          // string s as a const char* with trailing '\0'
  s.substr(i, j)     // Substring of string s of length j starting at s[i]
  s.substr(i)        // Substring from s[i] to the end
  s.find(s2)         // Index of char, char*, or string s2 in s, or string::npos if not found
  s.rfind(s2)        // Index of last occurrence of s2 in s
  s.find_first_of(s2)     // Index of first char in s that occurs in s2
  s.find_last_of(s2)      // Index of last char in s that occurs in s2
  s.find_first_not_of(s2) // Index of first char in s not found in s2
  s.find_last_not_of(s2)  // Index of last char in s not found in s2
  s.replace(i, j, s2)     // Replace s.substr(i, j) with s2

  string s(3,'a');   // "aaa"
  s += "b"+s;        // "aaabaaa"
  for (int i=0; i!=int(s.size()); ++i) {  // print s one char at a time
    cout << s[i];
  s.size() > -1;     // false!  -1 is converted to unsigned

  s.begin()          // Iterator pointing to s[0]
  s.end()            // Iterator pointing 1 past last char
  string::iterator   // Iterator type, like char*
  string::const_iterator  // Type if s is const, like const char*
  string(b, e)       // string initialized from sequence [b,e)
  s.erase(b)         // Remove char in s pointed to by b
  s.erase(b, e)      // Remove substring [b,e) from s
  s.replace(b, e, s2)  // Replace substring [b,e) with string s2

  char* cp="ABCDE";
  string s(cp, cp+5); // "ABCDE"
  string s2(s.begin()+1, s.end()-1);  // "BCD"
  for (string::const_iterator p=s.begin(); p!=s.end(); ++p)  // Print s one char at a time
    cout << *p;       // or p[0] 

  string s(-1, 'x');              // Crash, negative size
  string s2(s.end(), s.begin());  // Crash, negative size
  s[-1]='x';                      // Crash, out of bounds
  *s.end()='x';                   // Crash, out of bounds
  string::iterator p; *p='x';     // Crash, dereferencing uninitialized iterator

<vector>

  vector<T>()            // Empty vector, elements of type T
  vector<T>(n)           // n elements, default initialized
  vector<T>(n, x)        // n elements each initialized to x
  vector<T> v2=v;        // Copy v to v2
  v2=v;                  // Assignment
  v2<v                   // Also >, ==, !=, <=, >= if defined for T
  vector<T>(b, e)        // Initialize to sequence [b, e)
  v.size()               // n
  vector<T>::size_type   // Type of v.size(), usually unsigned int
  v.empty()              // true if v.size() == 0
  v[i]                   // i'th element, 0 <= i < v.size() (unchecked), may be assigned to
  v.at(i)                // v[i] with bounds check, throws out_of_range
  v.begin(), v.end()     // Iterators [b, e)
  vector<T>::iterator    // Iterator type, also const_iterator
  v.back()               // v[v.size()-1] (unchecked if empty)
  v.push_back(x)         // Increase size by 1, copy x to last element
  v.pop_back()           // Decrease size by 1 (unchecked if empty)
  v.front()              // v[0] (unchecked)
  v.resize(n)            // Change size to n >= 0 (unchecked)
  v.insert(d, x)         // Insert x in front of iterator d, shift, increase size by 1
  v.insert(d, n, x)      // Insert n copies of x in front of d
  v.insert(d, b, e)      // Insert copy of [b, e) in front of d
  v.erase(d)             // Remove *d, shift, decrease size by 1
  v.erase(d, e)          // Remove subsequence [d, e)
  v.clear()              // v.erase(v.begin(), v.end())
  v.reserve(n)           // Anticipate that v will grow to size n >= v.size()
  v.capacity()           // Reserved size

To implement push_back() efficiently, a vector typically doubles the reserved space when it runs out in order to minimize memory reallocation and copying. reserve() allows this strategy to be optimized.

  // Read words from input into a stack, print in reverse order
  string s;
  vector<string> v;
  while (cin >> s)
    v.push_back(s);
  while (!v.empty()) {
    cout << v.back() << endl;
    v.pop_back();
  }

<deque>

  v.push_front(x)        // v.insert(v.begin(), x)
  v.pop_front()          // v.erase(v.begin())

<list>

  v.splice(d, v2, b);  // Move *b from list v2 to in front of d in v
  v.splice(d, v2);     // Move all elements of list v2 to in front of d in v
  v.splice(d, v2, b, e); // Move [b,e) in v2 to in front of d at v
  v.remove(x);         // Remove all elements equal to x
  v.remove_if(f);      // Remove elements x where f(x) is true
  v.sort();            // Sort list
  v.sort(f);           // Sort list using function bool f(x,y) instead of x < y
  v.merge(v2);         // Merge sorted list v2 into sorted list v
  v.merge(v2, f);      // Merge using f(x,y) instead of x < y to sort v
  v.unique();          // Remove duplicates from sorted list
  v.unique(f);         // Use f(x,y) instead of x == y
  v.reverse();         // Reverse order of elements

  char* cp="ABCDE";
  list<char> v(cp, cp+5);  // v.size() is 5
  for (list<char>::const_iterator p=v.begin(); p!=v.end(); ++p)  // Print ABCDE
    cout << *p;

<map>

  pair<K,V> x(k,v);    // Create a pair x containing copies of k and v
  x.first              // k
  x.second             // v
  x=make_pair(k,v)     // x.first=k; x.second=v;

  map<K,V> m;          // map sorted by < on K
  map<K,V,f>()         // map sorted by f(x,y) instead of x<y on K
  m[k]=v;              // Associate v (type V) with unique key k of type K
  m[k]                 // Retrieve v, or associate V() with k if new
  m.size()             // Number of unique keys
  m.empty()            // true if m.size() == 0
  map<K,V>::iterator   // bidirectional, points to a pair<const K, V>
  map<K,V>::const_iterator   // points to a pair<const K, const V>
  m.begin()            // Points to first pair (lowest k)
  m.end()              // Points 1 past last pair
  m.find(k)            // Points to pair containing k or m.end() if not found
  m.erase(k)           // Remove key K and its associated value
  m.erase(b)           // Remove pair pointed to by iterator b
  m.erase(b, e)        // Remove sequence [b, e)
  m.clear()            // Make empty: m.erase(m.begin(), m.end())
  m==m2                // Compare maps, also !=, <, <=, >, >=

  // Read words, print an alphabetical index of words with their counts
  string s;
  map<string, int> m;
  while (cin >> s)
    ++m[s];
  for (map<string, int>::const_iterator p=m.begin(); p!=m.end(); ++p)
    cout << p->first << " " << p->second << endl;

  multimap<K,V,f> m;   // f defaults to < on K
  m.insert(make_pair(k,v))  // Insert a pair
  pair<multimap<K,V,f>::iterator, multimap<K,V,f>::iterator> p
    = m.equal_range(k) // Sequence with key k is [p->first, p->second)

f (when used as a template argument) is a functoid (or function object), a class that looks like a function by overloading (). For example:

  template <class T> class GreaterThan {
  public:
    bool operator()(const T& a, const T& b) const {return b < a;}
  };

  map<string, int, GreaterThan<T> > m;  // keys sorted in reverse order

<set>

  set<K> m;            // Elements are sorted by < on K
  m.insert(k)          // Add an element
  m.erase(k)           // Remove an element
  m.find(k)!=m.end()   // Test if k is in m

<queue>

  queue<T> q;          // Queue of type T
  q.size()             // Number of items in q
  q.empty()            // true if q.size() == 0
  q.push(x)            // Put x in the back
  x=q.back()           // The item last pushed, may be assigned to
  x=q.front()          // The next item to pop, may be assigned to
  q.pop()              // Remove the front item

  priority_queue<T> q; // Element type is T
  priority_queue<T, vector<T>, f> q;  // Use functoid f(x,y) instead of x < y to sort
  q.size(), q.empty()  // As before
  q.push(x)            // Insert x
  x=q.top()            // Largest item in q, cannot be assigned to
  q.pop()              // Remove top item

<stack>

  stack<T> s;          // Stack with elements of type T
  s.size(), s.empty()  // As with queue
  s.push(x);           // Put x on top
  x=s.top();           // Last item pushed, may be assigned to
  s.pop();             // Remove the top item

<bitset>

  bitset<N> b;         // N-bit bitset, N must be a compile time constant
  bitset<N> b=x;       // Initialize b[0]..b[31] from bits of long x
  b[i]                 // i'th bit, 0 <= i < N or throw out_of_range()
  b.size()             // N, cannot be changed
  b.set(i)             // b[i] = 1
  b.reset(i)           // b[i] = 0
  b.flip(i)            // b[i] = 1 - b[i]
  b.test(i)            // true if b[i] == 1
  b.set()              // Set all bits, also b.reset(), b.flip()
  b & b2               // Bitwise AND, also | ^ ~ << >> &= |= ^= <<= >>= == !=
  b.count()            // Number of bits set to 1
  b.any()              // true if b.count() > 0
  b.none()             // true if b.count() == 0
  cin >> b             // Read bits as '0' and '1' e.g. "10101"
  cout << b            // Write bits as '0' and '1'
  bitset<N> b(s);      // Initialize from string s of '0' and '1' or throw invalid_argument()
  s=b.template to_string<char>()  // Convert to string
  x=b.to_ulong()       // Convert to unsigned long, throw overflow_error() if bits > 31 set

<valarray>

<complex>

  complex<T> x;        // (0,0), T is a numeric type
  complex<T> x=r;      // (r,0), convert real r to complex
  complex<T> x(r, i);  // (r,i)
  x=polar<T>(rho, theta); // Polar notation: radius, angle in radians
  x.real()             // r
  x.imag()             // i
  abs(x)               // rho = sqrt(r*r+i*i)
  arg(x)               // tan(theta) = i/r
  norm(x)              // abs(x)*abs(x)
  conj(x)              // (r,-i)
  x+y                  // Also - * / == != = += -= *= /= and unary + -
  sin(x)               // Also sinh, sqrt, tan, tanh, cos, cosh, exp, log, log10, pow(x,y)
  cout << x            // Prints in format "(r,i)"
  cin >> x             // Expects "r", "(r)", or "(r,i)"

<stdexcept>, <exception>

The standard library provides a hierarchy of exception types. Not all of them are used by the library, but any may be thrown.

  Type                   Header                   Thrown by
  exception              stdexcept, exception
    logic_error          stdexcept
      length_error       stdexcept
      domain_error       stdexcept
      out_of_range       stdexcept                .at(i) (vector/string/deque index out of bounds)
      invalid_argument   stdexcept, bitset        bitset("xxx") (not '0' or '1')
    runtime_error        stdexcept
      range_error        stdexcept    
      overflow_error     stdexcept
      underflow_error    stdexcept
    bad_alloc            new                      new, new[] (out of memory)
    bad_cast             typeinfo                 dynamic_cast<T&> (can't convert to derived)
    bad_typeid           typeinfo                 typeid(*p) when p==0
    bad_exception        exception
    ios_base::failure    ios, iostream, fstream   istream::clear(), ostream::clear()

  throw exception(msg)   // Throw exception with char* or string msg
  throw exception();     // Default msg
  catch(exception e) {e.what();}  // msg as a char*

C++ Standard Library Functions

Many C++ standard library functions operate on sequences denoted by iterators or pointers. Iterators are a family of types that include pointers. They are classified by the operators they support.

• input: ++p, p++, p=q, p==q, p!=q, *p (read-only)
• output: p=q, p==q, p!=q, *p++ = x (alternating write/increment)
• forward: input and output and p->m, *p (multiple read-write)
• bidirectional: forward and --p, p--, implemented by list, map, multimap, set, multiset.
• random: bidirectional and p<q, p>q, p<=q, p>=q, p+i, i+p, p-i, p-q, p[i], implemented by arrays (as pointers), string, vector, deque.

Some algorithms require certain iterator types, but will accept more powerful types. For example, copy(b, e, d) require b and e to be at least input iterators and d to be at least an output iterator. But it will accept forward, bidirectional, or random iterators because these all support input and output operations. sort() requires random iterators and will accept no other type.

In the following, b and e are input iterators, and d is an output iterator, unless otherwise specified. Parameters eq and lt are optional, and default to functions that take 2 arguments x and y and return x==y and x<y respectively, e.g. bool eq(x,y) {return x==y;}. x and y are objects of the type pointed to by the iterators. p is a pair of iterators. f is a function or function object as noted.

  // Operations on ordinary objects
  swap(x1, x2);              // Swap values of 2 objects of the same type
  min(x1, x2);               // Smaller of x1 or x2, must be same type
  max(x1, x2);               // Larger of x1 or x2, must be same type

  // Properties of sequences (input iterators)
  equal(b, e, b2, eq);       // true if [b,e)==[b2,...)
  lexicographical_compare(b, e, b2, e2, lt);  // true if [b,e)<[b2,e2)
  i=min_element(b, e);       // Points to smallest in [b,e)
  i=max_element(b, e);       // Points to largest
  n=count(b, e, x);          // Number of occurrences of x in [b,e)
  n=count_if(b, e, f);       // Number of f(x) true in [b,e)

  // Searching, i points to found item or end (e) if not found
  i=find(b, e, x);           // Find first x in [b,e)
  i=find_if(b, e, f);        // Find first x where f(x) is true
  i=search(b, e, b2, e2, eq);// Find first [b2,e2) in [b,e) (forward)
  i=find_end(b, e, b2, e2, eq); // Find last [b2,e2) in [b,e) (forward)
  i=search_n(b, e, n, x, eq);// Find n copies of x in [b,e) (forward)
  p=mismatch(b, e, b2, eq);  // Find first *p.first in [b,e) != *p.second in [b2,.) (forward)
  i=adjacent_find(b, e, eq); // Find first of 2 equal elements (forward)

  // Modifying elements
  i=copy(b, e, d);           // Copy [b,e) to [d,i)
  fill(d, i, x);             // Set all in [d,i) to x (forward)
  i=fill_n(d, n, x);         // Set n elements in [d,i) to x
  generate(d, i, f);         // Set [d,i) to f() (e.g. rand) (forward)
  i=generate_n(d, n, f);     // Set n elements in [d,i) to f()
  f=for_each(b, e, f);       // Call f(x) for each x in [b,e)
  i=transform(b, e, d, f);   // For x in [b,e), put f(x) in [d,i)
  i=transform(b, e, b2, d, f);  // For x in [b,e), y in [b2,.), put f(x,y) in [d,i)
  replace(b, e, x, y)        // Replace x with y in [b,e)
  replace_if(b, e, f, y);    // Replace with y in [b,e) where f(x) is true
  i=replace_copy(b, e, d, x, y);    // Copy [b,e) to [d,i) replacing x with y
  i=replace_copy_if(b, e, d, f, y); // Copy replacing with y where f(x) is true

  // Rearranging sequence elements
  sort(b, e, lt);            // Sort [b,e) by < (random)
  stable_sort(b, e, lt);     // Sort slower, maintaining order of equal elements (random)
  partial_sort(b, m, e, lt); // Sort faster but leave [m,e) unsorted (random)
  nth_element(b, m, e, lt);  // Sort fastest but only *m in proper place (random)
  iter_swap(b, e);           // swap(*b, *e) (forward)
  i=swap_ranges(b, e, b2);   // swap [b,e) with [b2,i) (forward)
  i=partition(b, e, f);      // Moves f(x) true to front, [i,e) is f(x) false (bidirectional)
  i=stable_partition(b, e, f);  // Maintains order within each partition
  i=remove(b, e, x);         // Move all x to end in [i,e) (forward)
  i=remove_if(b, e, f);      // Move f(x) true to front in [b,i) (forward)
  i=remove_copy(b, e, d, x); // Copy elements matching x to [d,i)
  i=remove_copy_if(b, e, d, f);  // Copy elements x if f(x) is false to [d,i)
  replace(b, e, x1, x2);     // Replace x1 with x2 in [b,e)
  i=replace_copy(b, e, d, x1, x2);  // Copy [b,e) to [d,i) replacing x1 with x2
  reverse(b, e);             // Reverse element order in [b,e) (bidirectional)
  i=reverse_copy(b, e, d);   // Copy [b,e) to [d,i) reversing the order (b,e bidirectional)
  rotate(b, m, e);           // Move [b,m) behind [m,e) (forward)
  i=rotate_copy(b, m, e, d); // Rotate into [d,i)
  random_shuffle(b, e, f);   // Random permutation, f() defaults to rand()
  next_permutation(b, e, lt);// Next greater sequence, true if successful (bidirectional)
  prev_permutation(b, e, lt);// Previous permutation, true if successful (bidirectional)

  // Operations on sorted sequences
  i=unique(b, e, eq);            // Move unique list to [b,i), extras at end
  i=unique_copy(b, e, d, eq);    // Copy one of each in [b,d) to [d,i)
  i=binary_search(b, e, x, lt);  // Find i in [b,e) (forward)
  i=lower_bound(b, e, x, lt);    // Find first x in [b,e) or where to insert it (forward)
  i=upper_bound(b, e, x, lt);    // Find 1 past last x in [b,e) or where to insert it (forward)
  p=equal_range(b, e, x, lt);    // p.first = lower bound, p.second = upper bound (forward)
  includes(b, e, b2, e2, lt);    // true if [b,e) is a subset of [b2,e2)
  i=merge(b, e, b2, e2, d, lt);  // Merge [b,e) and [b2,e2) to [d,i)
  inplace_merge(b, m, e, lt);    // Merge [b,m) and [m,e) to [b,e) (bidirectional)
  i=set_union(b, e, b2, e2, d, lt);  // [d,i) = unique elements in either [b,e) or [b2,e2)
  i=set_intersection(b, e, b2, e2, d, lt);  // [d,i) = unique elements in both
  i=set_difference(b, e, b2, e2, d, lt);    // [d,i) = unique elements in [b,e) but not [b2,e2)
  i=set_symmetric_difference(b, e, b2, e2, d, lt);  // [d,i) = elements in one but not both

  int a[5]={3,1,4,1,6};
  vector b(5);
  copy(a, a+5, v.begin());   // Copy a to v
  remove(a, a+5, 1);         // {3,4,6,1,1}, returns a+3
  sort(a, a+4);              // {1,3,4,6,1}

<numeric>

  x=accumulate(b, e, x, plus);                // x + sum over [b,e)
  x=inner_product(b, e, b2, x, plus, times);  // x + sum [b,e)*[b2,e2)
  adjacent_difference(b, e, minus);           // for i in (b,e) *i -= i[-1]
  partial_sum(b, e, plus);                    // for i in [b,e) *i += sum [b,i)

<iterator>

An inserter is an output iterator that expands the container it points to by calling push_back(), push_front(), or insert(). The container must support this operation. A stream iterator can be used to do formatted input or output using >> or <<

  back_inserter(c);             // An iterator that appends to container c
  front_inserter(c);            // Inserts at front of c
  inserter(c, p);               // Inserts in front of p
  ostream_iterator<T>(out, cp); // Writes to ostream separated by char* cp (default " ")
  istream_iterator<T>(in);      // An input iterator that reads T objects from istream

  vector<int> from(10), to;
  copy(from.begin(), from.end(), back_inserter(to));

Functions in <functional> create function objects, which are objects that behave like functions by overloading operator(). These can be passed to algorithms that take function arguments, e.g.

  vector<int> v(10);
  sort(v.begin(), v.end(), greater<int>());  // Sort v in reverse order
  int x=accumulate(v.begin(), v.end(), 1, multiplies<T>);  // Product of elements  

   // Predicates (return bool)
   equal_to<T>()                 // x==y
   not_equal_to<T>()             // x!=y
   greater<T>()                  // x>y
   less<T>()                     // x<y
   greater_equal<T>()            // x>=y
   less_equal<T>()               // x<=y
   logical_and<bool>()           // x&&y
   logical_or<bool>()            // x||y
   logical_not<bool>()           // !x (unary)

   // Arithmetic operations (return T)
   plus<T>()                     // x+y
   minus<T>()                    // x-y
   multiplies<T>()               // x*y
   divides<T>()                  // x/y
   modulus<T>()                  // x%y
   negate<T>()                   // -x (unary)

  bind1st(f, c)                  // An object computing f(c,y)
  bind2nd(f, c)                  // An object computing f(x,c)

  i=find_if(v.begin(), v.end(), bind2nd(equal_to<int>(), 0));  // Find first 0

  ptr_fun(f)                     // Convert ordinary function f to object
  mem_fun(&T::f)                 // Convert member function of class T
  mem_fun_ref(T::f)              // Same

  i=find_if(v.begin(), v.end(), mem_fun(&string::empty));  // Find ""
  transform(v.begin(), v.end(), v.begin(), bind2nd(ptr_fun(pow), 2.0));  // Square elements

  not1(f)                        // Object computing !f(x)
  not2(f)                        // Object computing !f(x,y)

  i=find_if(v.begin(), v.end(), not1(bind2nd(equal_to<int>(), 0)));  // Find nonzero

<new>

The default behavior of new is to throw an exception of type bad_alloc if out of memory. This can be changed by writing a function (taking no parameters and returning void) and passing it to set_new_handler().

  void handler() {throw bad_alloc();} // The default
  set_new_handler(handler);

  int* p = new(nothrow) int[1000000000];  // p may be 0

C Library Functions

<cstdlib>

Miscellaneous functions. s is type char*, n is int

  atoi(s); atol(s); atof(s);// Convert char* s to int, long, double e.g. atof("3.5")
  abs(x); labs(x);          // Absolute value of numeric x as int, long
  rand();                   // Pseudo-random int from 0 to RAND_MAX (at least 32767)
  srand(n);                 // Initialize rand(), e.g. srand(time(0));
  system(s);                // Execute OS command s, e.g. system("ls");
  getenv(s);                // Environment variable or 0 as char*, e.g. getenv("PATH"); 
  exit(n);                  // Kill program, return status n, e.g. exit(0);
  void* p = malloc(n);      // Allocate n bytes or 0 if out of memory.  Obsolete, use new.
  p = calloc(n, 1);         // Allocate and set to 0 or return NULL.  Obsolete.
  p = realloc(p, n);        // Enlarge to n bytes or return NULL.  Obsolete.
  free(p);                  // Free memory.  Obsolete: use delete

<cctype>

Character tests take a char c and return bool.

  isalnum(c);               // Is c a letter or digit?
  isalpha(c); isdigit(c);   // Is c a letter?  Digit?
  islower(c); isupper(c);   // Is c lower case?  Upper case?
  isgraph(c); isprint(c);   // Printing character except/including space?
  isspace(c); iscntrl(c);   // Is whitespace?  Is a control character?
  ispunct(c);               // Is printing except space, letter, or digit?
  isxdigit(c);              // Is hexadecimal digit?
  c=tolower(c); c=toupper(c);   // Convert c to lower/upper case

<cmath>

Functions take double and return double.

  sin(x); cos(x); tan(x);   // Trig functions, x in radians
  asin(x); acos(x); atan(x);// Inverses
  atan2(y, x);              // atan(y/x)
  sinh(x); cosh(x); tanh(x);// Hyperbolic
  exp(x); log(x); log10(x); // e to the x, log base e, log base 10
  pow(x, y); sqrt(x);       // x to the y, square root
  ceil(x); floor(x);        // Round up or down (as a double)
  fabs(x); fmod(x, y);      // Absolute value, x mod y

<ctime>

Functions for reading the system clock. time_t is an integer type (usually long). tm is a struct.

  clock()/CLOCKS_PER_SEC;   // Time in seconds since program started
  time_t t=time(0);         // Absolute time in seconds or -1 if unknown
  tm* p=gmtime(&t);         // 0 if UCT unavailable, else p->tm_X where X is:
    sec, min, hour, mday, mon (0-11), year (-1900), wday, yday, isdst
  asctime(p);               // "Day Mon dd hh:mm:ss yyyy\n"
  asctime(localtime(&t));   // Same format, local time

<cstring>

Functions for performing string-like operations on arrays of char marked with a terminating '\0' (such as "quoted literals" or as returned by string::c_str(). Mostly obsoleted by type string.

  strcpy(dst, src);         // Copy src to dst. Not bounds checked
  strcat(dst, src);         // Concatenate to dst. Not bounds checked
  strcmp(s1, s2);           // Compare, <0 if s1<s2, 0 if s1==s2, >0 if s1>s2
  strncpy(dst, src, n);     // Copy up to n chars, also strncat(), strncmp()
  strlen(s);                // Length of s not counting \0
  strchr(s,c); strrchr(s,c);// Address of first/last char c in s or 0
  strstr(s, sub);           // Address of first substring in s or 0
    // mem... functions are for any pointer types (void*), length n bytes
  memcpy(dst, src, n);      // Copy n bytes from src to dst
  memmove(dst, src, n);     // Same, but works correctly if dst overlaps src
  memcmp(s1, s2, n);        // Compare n bytes as in strcmp
  memchr(s, c, n);          // Find first byte c in s, return address or 0
  memset(s, c, n);          // Set n bytes of s to c

<cstdio>

The stdio library is made mostly obsolete by the newer iostream library, but many programs still use it. There are facilities for random access files and greater control over output format, error handling, and temporary files. Mixing both I/O libraries is not recommended. There are no facilities for string I/O.

Global objects stdin, stdout, stderr of type FILE* correspond to cin, cout, cerr. s is type char*, c is char, n is int, f is FILE*.

  FILE* f=fopen("filename", "r");  // Open for reading, NULL (0) if error
    // Mode may also be "w" (write) "a" append, "a+" random access read/append,
    // "rb", "wb", "ab", "a+b" are binary
  fclose(f);                // Close file f
  fprintf(f, "x=%d", 3);    // Print "x=3"  Other conversions:
    "%5d %u %-8ld"            // int width 5, unsigned int, long left justified
    "%o %x %X %lx"            // octal, hex, HEX, long hex
    "%f %5.1f"                // double: 123.000000, 123.0
    "%e %g"                   // 1.23e2, use either f or g
    "%c %s"                   // char, char*
    "%%"                      // %
  sprintf(s, "x=%d", 3);    // Print to array of char s
  printf("x=%d", 3);        // Print to stdout (screen unless redirected)
  fprintf(stderr, ...       // Print to standard error (not redirected)
  getc(f);                  // Read one char (as an int, 0-255) or EOF (-1) from f
  ungetc(c, f);             // Put back one c to f
  getchar();                // getc(stdin);
  putc(c, f)                // fprintf(f, "%c", c);
  putchar(c);               // putc(c, stdout);
  fgets(s, n, f);           // Read line including '\n' into char s[n] from f.  NULL if EOF
  gets(s)                   // fgets(s, INT_MAX, f); no '\n' or bounds check
  fread(s, n, 1, f);        // Read n bytes from f to s, return number read
  fwrite(s, n, 1, f);       // Write n bytes of s to f, return number written
  fflush(f);                // Force buffered writes to f
  fseek(f, n, SEEK_SET);    // Position binary file f at n
    // or SEEK_CUR from current position, or SEEK_END from end
  ftell(f);                 // Position in f, -1L if error
  rewind(f);                // fseek(f, 0L, SEEK_SET); clearerr(f);
  feof(f);                  // Is f at end of file?
  ferror(f);                // Error in f?
  perror(s);                // Print char* s and last I/O error message to stderr
  clearerr(f);              // Clear error code for f
  remove("filename");       // Delete file, return 0 if OK
  rename("old", "new");     // Rename file, return 0 if OK
  f = tmpfile();            // Create temporary file in mode "wb+"
  tmpnam(s);                // Put a unique file name in char s[L_tmpnam]

Example: input file name and print its size

  char filename[100];              // Cannot be a string
  printf("Enter filename\n");      // Prompt
  gets(filename, 100, stdin);      // Read line ending in "\n\0"
  filename[strlen(filename)-1]=0;  // Chop off '\n';
  FILE* f=fopen(filename, "rb");   // Open for reading in binary mode
  if (f) {                         // Open OK?
    fseek(f, 0, SEEK_END);         // Position at end
    long n=ftell(f);               // Get position
    printf("%s has %ld bytes\n", filename, n);
    fclose(f);                     // Or would close when program ends
  }
  else
    perror(filename);              // fprintf(stderr, "%s: not found\n", filename);
                                   // or permission denied, etc.

  printf("%d %f %s", 2, 2.0, "2");  // OK
  printf("%s", 5);  // Crash: expected a char* arg, read from address 5
  printf("%s");     // Crash
  printf("%s", string("hi"));  // Crash: use "hi" or string("hi").c_str()

<cassert>

Provides a debugging function for testing conditions where all instances can be turned on or off at once. assert(false); prints the asserted expression, source code file name, and line number, then aborts. Compiling with g++ -DNDEBUG effectively removes these statements.

  assert(e);                // If e is false, print message and abort
  #define NDEBUG            // (before #include <assert.h>), turn off assert

Classes

Classes provide data abstraction, the ability to create new types and hide their implementation in order to improve maintainability. A class is a data structure and an associated set of member functions (methods) and related type declarations which can be associated with the class or instances (objects) of the class. A class is divided into a public interface, visible wherever the class or its instances are visible, and a private implementation visible only to member functions of the class.

  class T {          // Create a new type T
  private:           // Members are visible only to member functions of T (default)
  public:            // Members are visible wherever T is visible
    // Type, object, and function declarations
  };
  T::m;              // Member m of type T
  T x;               // Create object x of type T
  x.m;               // Member m of object x
  T* p=&x; p->m;     // Member m of object pointed to by p

  class Complex {    // Represents imaginary numbers
  private:
    double re, im;   // Data members, represents re + im * sqrt(-1)
  public:
    void set(double r, double i) {re=r; im=i;}  // Inlined member function definition
    double real() const {return re;}            // const - does not modify data members
    double imag() const;                        // Declaration for non-inlined function
  };
  double Complex::imag() const {return im;}     // Definition for imag()
  int main() {
    Complex a, b=a;                // Objects of type Complex
    a.set(3, 4);                   // Call a member function
    b=a;                           // Assign b.re=a.re; b.im=a.im
    b==a;                          // Error, == is not defined
    cout << a.re;                  // Error, re is private
    cout << a.real();              // OK, 3
    cout << Complex().real();      // OK, prints an undefined value
    Complex().set(5, 6);           // Error, non-const member called on const object

A class has two special member functions, a constructor, which is called when the object is created, and a destructor, called when destroyed. The constructor is named class::class, has no return type or value, may be overloaded and have default arguments, and is never const. It is followed by an optional initialization list listing each data member and its initial value. Initialization takes place before the constructor code is executed. Initialization might not be in the order listed. Members not listed are default-initialized by calling their constructors with default arguments. If no constructor is written, the compiler provides one which default-initializes all members. The syntax is:

  class::class(parameter list): member(value), member(value) { code...}

The destructor is named class::~class, has no return type or value, no parameters, and is never const. It is usually not needed except to return shared resources by closing files or deleting memory. After the code executes, the data members are destroyed using their respective destructors in the reverse order in which they were constructed.

  class Complex {
  public:
    Complex(double r=0, double i=0): re(r), im(i) {}  // Constructor
    ~Complex() {}                                     // Destructor
    // Other members...
  };
  Complex a(1,2), b(3), c=4, d;  // (1,2) (3,0) (4,0) (0,0)

A constructor defines a conversion function for creating temporary objects. A constructor that allows 1 argument allows implicit conversion wherever needed, such as in expressions, parameter passing, assignment, and initialization.

  Complex(3, 4).real();  // 3
  a = 5;                 // Implicit a = Complex(5) or a = Complex(5, 0)

  void assign_if(bool, Complex&, const Complex&);
  assign_if(true, a, 6); // Implicit Complex(6) passed to third parameter
  assign_if(true, 6, a); // Error, non-const reference to Complex(6), which is const

Operators may be overloaded as members. The expression aXb for operator X can match either operator X(a, b) (global) or a.operator X(b) (member function), but not both. Unary operators omit b. Operators =, [], and -> can only be overloaded as member functions.

  class Complex {
  public:
    Complex operator + (const Complex& b) const { // const because a+b doesn't change a
      return Complex(re+b.re, im+b.im);
    }
    // ...
  };

  Complex operator - (const Complex& a, const Complex& b) {
    return Complex(a.real()-b.real(), a.imag()-b.imag());
  }

  Complex a(1, 2), b(3, 4);
  a+b;                    // OK, a.operator+(b) == Complex(4, 6)
  a-b;                    // OK, operator-(a, b) == Complex(-2, -2)
  a+10;                   // OK, Complex(1, 12), implicit a+Complex(10, 0)
  10+a;                   // Error, 10 has no member operator+(Complex)
  a-10;                   // OK, Complex(1, -8)
  10-a;                   // OK, Complex(7, -4)

The member function (+) has the advantage of private access (including to other objects of the same class), but can only do implicit conversions on the right side. The global function (-) is symmetrical, but lacks private access. A friend declaration (in either the private or public section) allows private access to a global function.

  class Complex {
    friend Complex operator-(const Complex&, const Complex&);
    friend class T;       // All member functions of class T are friends
    // ...
  };

A conversion operator allows implicit conversion to another type. It has the form of a member function named operator T() const with implied return type T. It is generally a good idea to allow implicit conversions in only one direction, preferably with constructors, so this member function is usually used to convert to pre-existing types.

  class Complex {
  public:
    operator double() const {return re;}
    // ...
  }

  Complex a(1, 2);
  a-10;                  // Error, double(a)-10 or a-Complex(10) ?
  a-Complex(10);         // Complex(-9, 2);
  double(a)-10;          // -9

An explicit constructor does not allow implicit conversions.

  class Complex {
    explicit Complex(double r=0, double i=0);
    // ...
  };

  Complex a=1;           // Error
  Complex a(1);          // OK
  a-10;                  // OK, double(a)-10 = -9
  a-Complex(10);         // OK, Complex(-9, 0)

A class or member function may be templated. The type parameter must be passed in the declaration for objects of the class.

  template <class T>
  class Complex {
    T re, im;
  public:
    T real() const {return re;}
    T imag() const {return im;}
    Complex(T r=0, T i=0);
    friend Complex<T> operator - (const Complex<T>&, const Complex<T>&);
  };

  template <class T>
  Complex<T>::Complex(T r, T i): re(r), im(i) {}

  Complex<int> a(1, 2);               // Complex of int
  Complex<double> b(1.0, 2.0);        // Complex of double
  a=a-Complex<int>(3, 4);             // Complex<int>(-2, -2)
  Complex<Complex<double> > c(b, b);  // Note space, not >>
  c.real().imag();                    // 2.0

  template <class T, class U=T, int n=0> class V {};
  V<double, string, 3> v1;
  V<char> v2;  // V<char, char, 0>

Classes define default behavior for copying and assignment, which is to copy/assign each data member. This behavior can be overridden by writing a copy constructor and operator= as members, both taking arguments of the same type, passed by const reference. They are usually required in classes that have destructors, such as the vector<T>-like class below. If we did not overload these, the default behavior would be to copy the data pointer, resulting in two Vectors pointing into the same array. The assignment operator normally returns itself (*this) by reference to allow expressions of the form a=b=c;, but is not required to do so. this means the address of the current object; thus any member m may also be called this->m within a member function.

  template <class T>
  class Vector {
  private:
    T* data;  // Array of n elements
    int n;    // size()
  public:
    typedef T* iterator;                      // Vector::iterator means T*
    typedef const T* const_iterator;          // Iterators for const Vector
    int size() const {return n;}              // Number of elements
    T& operator[](int i) {return data[i];}    // i'th element
    const T& operator[](int i) const {return data[i];}  // i'th element of const Vector
    iterator begin() {return data;}           // First, last+1 elements
    iterator end() {return data+size();}
    const_iterator begin() const {return data;}  // Const versions
    const_iterator end() const {return data+size();}
    Vector(int i=0): data(new T[i]), n(i) {}  // Create with size i
    ~Vector() {delete[] data;}                // Return memory
    Vector(const Vector<T>& v): data(new T[v.n]), n(v.n) {  // Copy constructor
      copy(v.begin(), v.end(), begin());
    }
    Vector& operator=(const Vector& v) {      // Assignment
      if (&v != this) {                       // Assignment to self?
        delete[] data;                        // If not, resize and copy
        data=new T[n=v.n];
        copy(v.begin(), v.end(), begin());
      }
      return *this;                           // Allow a=b=c;
    }
    template <class P> Vector(P b, P e): data(new T[e-b]), n(e-b) {  // Templated member
      copy(b, e, data);                       // Initialize from sequence [b, e)
    }
  };

  Vector<int>::iterator p;        // Type is int*
  Vector<int>::const_iterator cp; // Type is const int*

  Vector<int> v(10);              // Uses non-const [], begin(), end()
  const Vector<int> cv(10);       // Uses const [], begin(), end()
  cv=v;                           // Error, non-const operator= called on cv
  v[5]=cv[5];                     // OK. assigns to int&
  cv[5]=v[5];                     // Error, assigns to const int&
  p=cv.begin();                   // Error, would allow *p=x to write into cv
  cp=cv.begin();                  // OK because can't assign to *cp

Inheritance

  class String: public Vector<char> {
  public:
    String(const char* s=""): Vector<char>(strlen(s)) {
      copy(s, s+strlen(s), begin());  // Inherits Vector<char>::begin()
    }
  };
  String a="hello"; // Calls Vector<char>::Vector(5);
  a.size();         // 5, inherits Vector<char>::size()
  a[0]='j';         // "jello", inherits Vector<char>::operator[]
  String b=a;       // Default copy constructor uses Vector's copy constructor on base part
  b=a;              // Default = calls Vector's assignment operator on base part

Although String inherits Vector<char>::data, it is private and inaccessible. A protected member is accessible to derived classes but private elsewhere.

  class B {
  protected:
    int x;
  } b;                            // Declare class B and object b
  b.x=1;                          // Error, x is protected

  class D: public B {
    void f() {x=1;}               // OK
  };

  class Stack: Vector<int> {  // or class Stack: private Vector<int>
  public:
    bool empty() const {return size()==0;}  // OK
  } s;
  s.size();   // Error, private
  s.empty();  // OK, public

A class may have more than one base class (called multiple inheritance). If both bases are in turn derived from a third base, then we derive from this root class using virtual to avoid inheriting its members twice further on. Any indirectly derived class treats the virtual root as a direct base class in the constructor initialization list.

  class ios {...};                                      // good(), binary, ...
  class fstreambase: public virtual ios {...};          // open(), close(), ...
  class istream: public virtual ios {...};              // get(), operator>>(), ...
  class ifstream: public fstreambase, public istream {  // Only 1 copy of ios
    ifstream(): fstreambase(), istream(), ios() {...}   // Normally ios() would be omitted
  };

Polymorphism

  String s="Hello";
  Vector<char> v=s;    // Discards derived part of s to convert
  Vector<char>* p=&s;  // p points to base part of s
  try {throw s;} catch(Vector<char> x) {}  // Caught with x set to base part of s
  s=Vector<char>(5);   // Error, can't convert base to derived

  // Allow output of Vector<char> using normal notation
  ostream& operator << (ostream& out, const Vector<char>& v) {
    copy(v.begin(), v.end(), ostream_iterator<char>(out, ""));  // Print v to out
    return out;        // To allow (cout << a) << b;
  }
  cout << s;           // OK, v refers to base part of s
  ofstream f("file.txt");
  f << s;              // OK, ofstream is derived from ostream

A derived class may redefine inherited member functions, overriding any function with the same name, parameters, return type, and const-ness (and hiding other functions with the same name, thus the overriding function should not be overloaded). The function call is resolved at compile time. This is incorrect in case of a base pointer or reference to a derived object. To allow run time resolution, the base member function should be declared virtual. Since the default destructor is not virtual, a virtual destructor should be added to the base class. If empty, no copy constructor or assignment operator is required. Constructors and = are never virtual.

  class Shape {
  public:
    virtual void draw() const;
    virtual ~Shape() {}
  };
  class Circle: public Shape {
  public:
    void draw() const;      // Must use same parameters, return type, and const
  };

  Shape s; s.draw();        // Shape::draw()
  Circle c; c.draw();       // Circle::draw()
  Shape& r=c; r.draw();     // Circle::draw() if virtual
  Shape* p=&c; p->draw();   // Circle::draw() if virtual
  p=new Circle; p->draw();  // Circle::draw() if virtual
  delete p;                 // Circle::~Circle() if virtual

An abstract base class defines an interface for one or more derived classes, which are allowed to instantiate objects. Abstractness can be enforced by using protected (not private) constructors or using pure virtual member functions, which must be overridden in the derived class or else that class is abstract too. A pure virtual member function is declared =0; and has no code defined.

  class Shape {
  protected:
    Shape();                 // Optional, but default would be public
  public:
    virtual void draw() const = 0; // Pure virtual, no definition
    virtual ~Shape() {}
  };
  // Circle as before

  Shape s;                   // Error, protected constructor, no Shape::draw()
  Circle c;                  // OK
  Shape& r=c; r.draw();      // OK, Circle::draw()
  Shape* p=new Circle();     // OK

Run time type identification

  #include <typeinfo>    // For typeid()
  typeid(*p)==typeid(T)  // true if p points to a T
  dynamic_cast<T*>(p)    // Convert base pointer to derived T* or 0.
  dynamic_cast<T&>(r)    // Convert base reference to derived T& or throw bad_cast()

  class B {public: virtual void f(){}};
  class D: public B {public: int x;} d;  // Bad design, public member in D but not B
  B* p=&d; p->x;                         // Error, no B::x
  D* q=p; q->x;                          // Error, can't convert B* to D*
  q=(D*)p;  q->x;                        // OK, but reinterpret_cast, no run time check
  q=dynamic_cast<D*>(p); if (q) q->x;    // OK

Other Types

  typedef char* Str;  // Str is a synonym for char*
  Str a, b[5], *c;    // char* a; char* b[5]; char** c;
  char* d=a;          // OK, really the same type

enum defines a type and a set of symbolic values for it. There is an implicit conversion to int and explicit conversion from int to enum. You can specify the int equivalents of the symbolic names, or they default to successive values beginning with 0. Enums may be anonymous, specifying the set of symbols and possibly objects without giving the type a name.

  enum Weekday {MON,TUE=1,WED,THU,FRI};  // Type declaration
  enum Weekday today=WED;                // Object declaration, has value 2
  today==2                               // true, implicit int(today)
  today=Weekday(3);                      // THU, conversion must be explicit
  enum {N=10};                           // Anonymous enum, only defines N
  int a[N];                              // OK, N is known at compile time
  enum {SAT,SUN} weekend=SAT;            // Object of anonymous type

A struct is a class where the default protection is public instead of private. A struct can be initialized like an array.

  struct Complex {double re, im;};     // Declare type
  Complex a, b={1,2}, *p=&b;           // Declare objects
  a.re = p->im;                        // Access members

A union is a struct whose fields overlap in memory. Unions can also be anonymous. They may be used to implement variant records.

  union U {int i; double d;};  // sizeof(U) is larger of int or double
  U u; u.i=3;                  // overwrites u.d

  // A variant record
  class Token {
    enum {INT, DOUBLE} type;   // which field is in use?
    union {int i; double d;} value;  // An anonymous union
  public:
    void print() const {
      if (type==INT) cout << value.i;
      else cout << value.d;
    }
  };

An enum, struct, class, or union type and a list of objects may be declared together in a single statement.

  class Complex {public: double re, im;} a, b={1,2}, *p=&b;

Program Organization

For C++ programs that only use one source code file and the standard library, the only rule is to declare things before using them: type declarations before object declarations, and function declarations or definitions before calling them. However, implicitly inlined member functions may use members not yet declared, and templates may use names as long as they are declared before instantiation.

  class Complex {
    double real() const {return re;}  // OK
    double re, im;
  };

Global and member functions (unless inlined or templated) and global or class static objects are separately compilable units, and may appear in separate source code (.cpp) files. If they are defined and used in different files, then a declaration is needed. To insure that the declaration and definition are consistent, the declaration should be in a shared header file. A shared header conventionally has a .h extension, and is inserted with a #include "filename.h", using double quotes to indicate that the file is in the current directory. Global variables are declared with extern without initialization.

// prog.h         // prog1.cpp        // prog2.cpp
extern int x;     #include "prog.h"   #include "prog.h"
int f();          int x=0;            int f() {
                  int main() {          return x;
                    f();              }
                    return 0;
                  }

  g++ prog1.cpp prog2.cpp -o prog

The UNIX make command updates the executable as needed based on the timestamps of source and .o files. It requires a file named Makefile containing a set of dependencies of the form:

  file: files which should be older than file
  (tab) commands to update file

  # Makefile comment
  prog: prog1.o prog2.o
        g++ prog1.o prog2.o -o prog

  prog1.o: prog1.cpp prog.h
        g++ -c prog1.cpp

  prog2.o: prog2.cpp prog.h
        g++ -c prog2.cpp

  g++ file1.cpp              Compile, produce executable a.out in UNIX
  g++ file1.cpp file2.o      Compile .cpp and link .o to executable a.out
  g++ -Wall                  Turn on all warnings
  g++ -c file1.cpp           Compile to file1.o, do not link
  g++ -o file1               Rename a.out to file1
  g++ -O                     Optimize executable for speed
  g++ -v                     Verbose mode
  g++ -DX=Y                  Equivalent to #define X Y
  g++ --help                 Show all g++ options
  gxx file1.cpp              Compile in Windows MS-DOS box (DJGPP) to A.EXE

  extern "C" {          // Turn off name mangling
  #include "header.h"   // Written in C
  }

Also, to guard against possible multiple inclusions of the header file, #define some symbol and test for it with #ifndef ... #endif on the first and last lines. Don't have a using namespace std;, since the user may not want std visible.

  #ifndef MYLIB_H       // mylib.h, or use #if !defined(MYLIB_H)
  #define MYLIB_H
  #include <string>
  // No using statement
  namespace mylib {
    class B {
    public:
      std::string f();  // No code
    }
  }
  #endif

  // mylib.cpp, becomes mylib.o
  #include <string>
  #include "mylib.h"
  using namespace std;  // OK
  namespace mylib {
    string B::f() {return "hi";}
  }

  #ifdef unix  // Defined by most UNIX compilers
  // ...
  #else
  // ...
  #endif

  #include <header>          // Standard header
  #include "header.h"        // Include header file from current directory
  #define X Y                // Replace X with Y in source code
  #define f(a,b) a##b        // Replace f(1,2) with 12
  #define X \                // Continue a # statement on next line
  #ifdef X                   // True if X is #defined
  #ifndef X                  // False if X is #defined
  #if !defined(X)            // Same
  #else                      // Optional after #if...
  #endif                     // Required

History of C++

C++ evolved from C, which in turn evolved from B, written by Ken Thompson in 1970 as a variant of BCPL. C was developed in the 1970's by Brian Kernighan and Dennis Ritchie as a "portable assembly language" to develop UNIX. C became widely available when they published "The C Programming Language" in 1983. C lacked standard containers (string, vector, map), iostreams, bool, const, references, classes, exceptions, namespaces, new/delete, function and operator overloading, and object-oriented capabilities. I/O was done using <stdio.h>. Strings were implemented as fixed sized char[] arrays requiring functions to assign or compare them (strcpy(), strcmp()). Structs could not be assigned, and had to be copied using memcpy(). Function arguments were not type checked. Functions could only modify arguments by passing their addresses. Memory allocation was done using malloc(), which requires the number of bytes to allocate and returns an untyped pointer or NULL if it fails. The language allowed unsafe implicit conversions such as int to pointers. Variables had to be declared before the first statement. There was no inline, so macros were often used in place of small functions. Hardware was slow and optimizers were not very good, so it was common to declare register variables. There were no // style comments. For instance,

  /* Copy argv[1] to buf and print it */
  #include <stdio.h>         /* No cout, use printf()                             */
  #include <string.h>        /* No string type, use char*                         */
  #include <stdlib.h>        /* No new/delete, use malloc/free                    */
  main(argc, argv)           /* Return type defaults to int */
  int argc;                  /* Old style parameter declaration, no type checking */
  char** argv;
  {                          /* No namespace std                                  */
    char* buf;               /* All declarations before the first statement       */
    if (argc>1) {
      buf=(char*)malloc((strlen(argv[1])+1)*sizeof(char));  /* Cast optional      */
      strcpy(buf, argv[1]);  /* Can't assign, no range check                      */
      printf("%s\n", buf);   /* Arguments not type checked                        */
      free(buf);             /* No delete */
    }
  }                          /* Return value is undefined (unchecked)             */

The ANSI C standard was finished in 1988. It added const, new style function declaration with type checking, struct assignment, strict type checking of pointer assignments, and specified the standard C library, which until now was widely used but with minor, annoying variations. However, many compilers did not become ANSI compliant until the early 1990's.

In the 1980's Bjarne Stroustrup at AT&T developed "C with Classes", later C++. Early implementations were available for UNIX as cfront (cc), a C++ to C translator around 1990. It added object oriented programming with classes, inheritance, and polymorphism, also references, the iostream library, and minor enhancements such as // style comments and the ability to declare variables anywhere. Because there were no namespaces, the iostream header was named <iostream.h> and no using statement was required. Unlike C programs which always have a .c extension, C++ didn't say, so .cpp, .cc and .C were all common, and .hpp for headers.

GNU gcc and g++, which compiled C and C++ directly to machine code, were developed in the early 1990's. Templates were added in 1993. Exceptions were added in 1994. The standard container library (originally called the standard template library or STL) was developed by researchers at Hewlett-Packard and made available free as a separate download in the mid 1990's and ported to several compilers.

ANSI standard C++ compilers became available in 1998. This added STL to the standard library, added multiple inheritance, namespaces, type bool, and run time type checking (dynamic_cast, typeid). The .h extension on headers was dropped.

C++ most likely succeeded where other early object oriented languages failed (Simula67, Actor, Eiffel, SmallTalk) because it was backwards compatible with C, allowing old code to be used, and because C programmers could use it immediately without learning the new features. However, there are a few incompatibilities.

• Old style function declarations are not allowed.
• Conversion from void* (returned by malloc()) requires a cast.
• There are many new reserved words.

There are also some incompatibilities between old (before 1998) and new versions of C++.

• new was changed to throw type bad_alloc if out of memory, instead of returning 0.
• The scope of a variable declared in a for loop was changed to be local to the loop and not beyond it (not yet implemented by Microsoft Visual C++)

g++ does not yet implement all ANSI C++ features. For instance,

• Type ostringstream allowing formatted writing to strings.
• Run time bounds checking of vector indexes using v.at(i)

The largest integer type is 32 bits in most implementations, but as 64 bit machines become common it is possible that type long could become a 64 bit type (as in Java) in the future. g++ supports the nonstandard 64-bit integer type long long, e.g.

  unsigned long long bigzero=0LLU;

Brian W. Kernighan, The C Programming Language, 2nd Ed., Prentice Hall, 1988.

Bjarne Stroustrup, The C++ Programming Language, 3rd Ed,, Addison Wesley, 1997.

Andrew Koenig, Barbara E. Moo, Accelerated C++, Addison Wesley, 2000.