Print or Return?

Beginners often mix up return and std::cout, because both seem to give a value out. But they do very different things:

• std::cout only shows a value to you, in the output. The program can't work with what was printed.
• return gives the value to the program, which can store it in a variable, calculate with it or print it.

Look at what happens when a function prints its result instead of returning it, and the program then wants to calculate with it:

#include <iostream>

void printSum(int a, int b) {
    std::cout << a + b << std::endl;
}

int main() {
    int result = printSum(2, 3);
    std::cout << result * 2 << std::endl;
    return 0;
}

The program doesn't compile:

main.cpp:8:9: error: cannot initialize a variable of type 'int' with an rvalue of type 'void'
    8 |     int result = printSum(2, 3);
      |         ^        ~~~~~~~~~~~~~~
1 error generated.

The error message says that a variable of type int can't be filled with a value of type void. The function printSum is void, so it returns nothing that could be stored. It would only print the number 5. printSum(2, 3) * 2 ends the same way, only with the error invalid operands to binary expression ('void' and 'int'): you can't calculate with “nothing”.

The fix is to change void to int and return the value instead of printing it: return a + b;.

It can also happen the other way round: a function returns a value, but nobody uses it. If you write only add(2, 3); in main, the program prints nothing. The function returned 5, but the program didn't store it or print it, so the value was lost. If you want to see it, print it: std::cout << add(2, 3) << std::endl;.

So when should a function print, and when should it return? A function that calculates something should return the result. The code that calls it then decides what to do with it: print it, store it or keep calculating with it. Print inside a function only when printing is its whole job, like in sayHello() or printArea().

You can return text too. A function that prepares a text with a number in it can return it as a std::string, and the program then prints it wherever it wants. But you can't attach a number to a text with +. First turn it into text with the function std::to_string():

#include <iostream>
#include <string>

std::string formatPrice(int price) {
    return std::to_string(price) + " EUR";
}

int main() {
    std::string text = formatPrice(120);
    std::cout << "Price: " << text << std::endl;
    std::cout << "With a discount: " << formatPrice(120 - 20) << std::endl;
    return 0;
}

This program prints:

Price: 120 EUR
With a discount: 100 EUR

std::to_string(120) returns the text "120", and + attaches " EUR" to it. If you wrote just return price + " EUR";, the program would compile, but it wouldn't work: it would print nonsense or stop with an error, because C++ doesn't join a number and a text in quotes with +. And watch out for decimal numbers: std::to_string(2.5) returns "2.500000", with six decimal places.