6.3.1. Functions with No Return Value

A return with no value may be used only in a function that has a return type of void. Functions that return void are not required to contain a return. In a void function, an implicit return takes place after the function’s last statement.

Typically, void functions use a return to exit the function at an intermediate point. This use of return is analogous to the use of a break statement (§ 5.5.1, p. 190) to exit a loop. For example, we can write a swap function that does no work if the values are identical:

void swap(int &v1, int &v2)
{
    // if the values are already the same, no need to swap, just return
    if (v1 == v2)
        return;
    // if we're here, there's work to do
    int tmp = v2;
    v2 = v1;
    v1 = tmp;
    // no explicit return necessary
}

This function first checks if the values are equal and, if so, exits the function. If the values are unequal, the function swaps them. An implicit return occurs after the last assignment statement.

A function with a void return type may use the second form of the return statement only to return the result of calling another function that returns void. Returning any other expression from a void function is a compile-time error.