return Ends the Function

return does two things at once: it returns a value and ends the function right away. The rest of the function doesn't run:

#include <iostream>

int add(int a, int b) {
    return a + b;
    std::cout << "This line never runs." << std::endl;
}

int main() {
    std::cout << add(2, 3) << std::endl;
    return 0;
}

This program prints:

5

The line after return never runs, because the function already ended one line earlier.

This is especially handy together with conditions. The following function describes the weather by the temperature:

#include <iostream>
#include <string>

std::string describeWeather(int degrees) {
    if (degrees < 0) {
        return "freezing";
    }
    if (degrees < 20) {
        return "cold";
    }
    return "warm";
}

int main() {
    std::cout << describeWeather(-5) << std::endl;
    std::cout << describeWeather(12) << std::endl;
    std::cout << describeWeather(25) << std::endl;
    return 0;
}

This program prints:

freezing
cold
warm

Let's go through all three calls:

• describeWeather(-5): -5 < 0 is true, so the function returns "freezing" and ends. The second condition isn't checked at all.
• describeWeather(12): the first condition is false and the second is true, so the function returns "cold".
• describeWeather(25): neither condition is true, so the function gets all the way to the last line and returns "warm".

We didn't need any else: as soon as a return runs, the rest of the function is skipped anyway. (With else if and else, the function would work the same.) But the order of the conditions matters, because the first return that runs wins. If the function checked degrees < 20 first, it would return "cold" for -5, because -5 is less than 20 too.

Watch out for the last return. If you leave it out, the function has nothing to return for 25: no condition is true, and the function reaches its end without a return. The program then prints freezing and cold and stops with an error, below which you see the warning warning: non-void function does not return a value in all control paths. A function that isn't void must always return a value, whichever way the conditions turn out.

In a void function, you can write return; without a value. The function then simply ends:

#include <iostream>

void printDivision(int a, int b) {
    if (b == 0) {
        std::cout << "You can't divide by zero." << std::endl;
        return;
    }
    std::cout << a / b << std::endl;
}

int main() {
    printDivision(10, 2);
    printDivision(10, 0);
    return 0;
}

This program prints:

5
You can't divide by zero.