Stats

Contributors: 11 Sunday, May 21, 2017
Licensed under: CC-BY-SA
Not affiliated with Stack Overflow
Rip Tutorial: riptutorial@gmail.com
Roadmap: roadmap

Function

Example

A function is a unit of code that represents a sequence of statements.

Functions can accept arguments or values and return a single value (or not). To use a function, a function call is used on argument values and the use of the function call itself is replaced with its return value.

Every function has a type signature -- the types of its arguments and the type of its return type.

Functions are inspired by the concepts of the procedure and the mathematical function.

  • Note: C++ functions are essentially procedures and do not follow the exact definition or rules of mathematical functions.

Functions are often meant to perform a specific task. and can be called from other parts of a program. A function must be declared and defined before it is called elsewhere in a program.

  • Note: popular function definitions may be hidden in other included files (often for convenience and reuse across many files). This is a common use of header files.

Function Declaration

A function declaration is declares the existence of a function with its name and type signature to the compiler. The syntax is as the following:

int add2(int i); // The function is of the type (int) -> (int)

In the example above, the int add2(int i) function declares the following to the compiler:

  • The return type is int.
  • The name of the function is add2.
  • The number of arguments to the function is 1:
    • The first argument is of the type int.
    • The first argument will be referred to in the function's contents by the name i.

The argument name is optional; the declaration for the function could also be the following:

int add2(int); // Omitting the function arguments' name is also permitted.

Per the one-definition rule, a function with a certain type signature can only be declared or defined once in an entire C++ code base visible to the C++ compiler. In other words, functions with a specific type signature cannot be re-defined -- they must only be defined once. Thus, the following is not valid C++:

int add2(int i);  // The compiler will note that add2 is a function (int) -> int
int add2(int j);  // As add2 already has a definition of (int) -> int, the compiler
                  // will regard this as an error.

If a function returns nothing, its return type is written as void. If it takes no parameters, the parameter list should be empty.

void do_something(); // The function takes no parameters, and does not return anything.
                     // Note that it can still affect variables it has access to.

Function Call

A function can be called after it has been declared. For example, the following program calls add2 with the value of 2 within the function of main:

#include <iostream>

int add2(int i);    // Declaration of add2

// Note: add2 is still missing a DEFINITION.
// Even though it doesn't appear directly in code,
// add2's definition may be LINKED in from another object file.

int main()
{
    std::cout << add2(2) << "\n";  // add2(2) will be evaluated at this point,
                                   // and the result is printed.
    return 0;  
}

Here, add2(2) is the syntax for a function call.

Function Definition

A function definition* is similar to a declaration, except it also contains the code that is executed when the function is called within its body.

An example of a function definition for add2 might be:

int add2(int i)       // Data that is passed into (int i) will be referred to by the name i
{                     // while in the function's curly brackets or "scope."
                    
    int j = i + 2;    // Definition of a variable j as the value of i+2.
    return j;         // Returning or, in essence, substitution of j for a function call to
                      // add2.
}

Function Overloading

You can create multiple functions with the same name but different parameters.

int add2(int i)           // Code contained in this definition will be evaluated
{                         // when add2() is called with one parameter.
    int j = i + 2;
    return j;
}

int add2(int i, int j)    // However, when add2() is called with two parameters, the
{                         // code from the initial declaration will be overloaded,
    int k = i + j + 2 ;   // and the code in this declaration will be evaluated
    return k;             // instead.
}

Both functions are called by the same name add2, but the actual function that is called depends directly on the amount and type of the parameters in the call. In most cases, the C++ compiler can compute which function to call. In some cases, the type must be explicitly stated.

Default Parameters

Default values for function parameters can only be specified in function declarations.

int multiply(int a, int b = 7); // b has default value of 7.
int multiply(int a, int b)
{
    return a * b;               // If multiply() is called with one parameter, the
}                               // value will be multiplied by the default, 7.

In this example, multiply() can be called with one or two parameters. If only one parameter is given, b will have default value of 7. Default arguments must be placed in the latter arguments of the function. For example:

int multiply(int a = 10, int b = 20); // This is legal 
int multiply(int a = 10, int b);      // This is illegal since int a is in the former

Special Function Calls - Operators

There exist special function calls in C++ which have different syntax than name_of_function(value1, value2, value3). The most common example is that of operators.

Certain special character sequences that will be reduced to function calls by the compiler, such as !, +, -, *, %, and << and many more. These special characters are normally associated with non-programming usage or are used for aesthetics (e.g. the + character is commonly recognized as the addition symbol both within C++ programming as well as in elementary math).

C++ handles these character sequences with a special syntax; but, in essence, each occurrence of an operator is reduced to a function call. For example, the following C++ expression:

3+3

is equivalent to the following function call:

operator+(3, 3)

All operator function names start with operator.

While in C++'s immediate predecessor, C, operator function names cannot be assigned different meanings by providing additional definitions with different type signatures, in C++, this is valid. "Hiding" additional function definitions under one unique function name is referred to as operator overloading in C++, and is a relatively common, but not universal, convention in C++.