Function declaration

A function declaration declares a function but doesn't provide an implementation:

// declares a function add2 that takes a a single int argument named i
// and returns a result of type int 
int add2(int i);

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); // in a declaration, name of the argument is optional

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);  // This is an error because add2 function (int) -> int has already been declared

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 doesn't return anything.

Function definition

A function definition defines the body of the function:

int add2(int i)
{
		// define variable j whose value is i + 2
    int j = i + 2;
		// return variable j as a result of function add2
    return j;
}