2.2 — Function return values (value-returning functions)

Consider the following program:

#include <iostream>

int main()
{
	// get a value from the user
	std::cout << "Enter an integer: ";
	int num{};
	std::cin >> num;

	// print the value doubled
	std::cout << num << " doubled is: " << num * 2 << '\n';

	return 0;
}

This program is composed of two conceptual parts: First, we get a value from the user. Then we tell the user what double that value is.

Although this program is trivial enough that we don’t need to break it into multiple functions, what if we wanted to? Getting an integer value from the user is a well-defined job that we want our program to do, so it would make a good candidate for a function.

So let’s write a program to do this:

// This program doesn't work
#include <iostream>

void getValueFromUser()
{
 	std::cout << "Enter an integer: ";
	int input{};
	std::cin >> input;  
}

int main()
{
	getValueFromUser(); // Ask user for input

	int num{}; // How do we get the value from getValueFromUser() and use it to initialize this variable?

	std::cout << num << " doubled is: " << num * 2 << '\n';

	return 0;
}

While this program is a good attempt at a solution, it doesn’t quite work.

When function getValueFromUser is called, the user is asked to enter an integer as expected. But the value they enter is lost when getValueFromUser terminates and control returns to main. Variable num never gets initialized with the value the user entered, and so the program always prints the answer 0.

What we’re missing is some way for getValueFromUser to return the value the user entered back to main so that main can make use of that data.

Return values

When you write a user-defined function, you get to determine whether your function will return a value back to the caller or not. To return a value back to the caller, two things are needed.

First, your function has to indicate what type of value will be returned. This is done by setting the function’s return type, which is the type that is defined before the function’s name. In the example above, function getValueFromUser has a return type of void (meaning no value will be returned to the caller), and function main has a return type of int (meaning a value of type int will be returned to the caller). Note that this doesn’t determine what specific value is returned -- it only determines what type of value will be returned.

Second, inside the function that will return a value, we use a return statement to indicate the specific value being returned to the caller. The return statement consists of the return keyword, followed by an expression (sometimes called the return expression), ending with a semicolon.

When the return statement is executed:

• The return expression is evaluated to produce a value.
• The value produced by the return expression is copied back to the caller. This copy is called the return value of the function.
• The function exits, and control returns to the caller.

The process of returning a copied value back to the caller is named return by value.

A value-returning function will return a value each time it is called.

Let’s take a look at a simple function that returns an integer value, and a sample program that calls it:

#include <iostream>

// int is the return type
// A return type of int means the function will return some integer value to the caller (the specific value is not specified here)
int returnFive()
{
    // the return statement provides the value that will be returned
    return 5; // return the value 5 back to the caller
}

int main()
{
    std::cout << returnFive() << '\n'; // prints 5
    std::cout << returnFive() + 2 << '\n'; // prints 7

    returnFive(); // okay: the value 5 is returned, but is ignored since main() doesn't do anything with it

    return 0;
}

When run, this program prints:

5
7

Execution starts at the top of main. In the first statement, the function call to returnFive() is evaluated, which results in function returnFive() being called. The return expression 5 is evaluated to produce the value 5, which is returned back to the caller and printed to the console via std::cout.

In the second function call, the function call to returnFive is evaluated, which results in function returnFive being called again. Function returnFive returns the value of 5 back to the caller. The expression 5 + 2 is evaluated to produce the result 7, which is then printed to the console via std::cout.

In the third statement, function returnFive is called again, resulting in the value 5 being returned back to the caller. However, function main does nothing with the return value, so nothing further happens (the return value is ignored).

Note: Return values will not be printed unless the caller sends them to the console via std::cout. In the last case above, the return value is not sent to std::cout, so nothing is printed.

Fixing our challenge program

With this in mind, we can fix the program we presented at the top of the lesson:

#include <iostream>

int getValueFromUser() // this function now returns an integer value
{
 	std::cout << "Enter an integer: ";
	int input{};
	std::cin >> input;  

	return input; // return the value the user entered back to the caller
}

int main()
{
	int num { getValueFromUser() }; // initialize num with the return value of getValueFromUser()

	std::cout << num << " doubled is: " << num * 2 << '\n';

	return 0;
}

When this program executes, the first statement in main will create an int variable named num. When the program goes to initialize num, it will see that there is a function call to getValueFromUser(), so it will go execute that function. Function getValueFromUser, asks the user to enter a value, and then it returns that value back to the caller (main()). This return value is used as the initialization value for variable num. num can then be used as many times as needed within main().

Compile this program yourself and run it a few times to prove to yourself that it works.

Revisiting main()

You now have the conceptual tools to understand how the main() function actually works. When the program is executed, the operating system makes a function call to main(). Execution then jumps to the top of main(). The statements in main() are executed sequentially. Finally, main() returns an integer value (usually 0), and your program terminates.

In C++, there are two special requirements for main():

• main() is required to return an int.
• Explicit function calls to main() are disallowed.

void foo()
{
    main(); // Compile error: main not allowed to be called explicitly
}

void main() // Compile error: main not allowed to have non-int return type
{
    foo();
}

For now, you should also define your main() function at the bottom of your code file, below other functions, and avoid calling it explicitly.

Status codes

You may be wondering why we return 0 from main(), and when we might return something else.

The return value from main() is sometimes called a status code (or less commonly, an exit code, or rarely a return code). The status code is used to signal whether your program was successful or not.

By convention, a status code of 0 means the program ran normally (meaning the program executed and behaved as expected).

A non-zero status code is often used to indicate some kind of failure (and while this works fine on most operating systems, strictly speaking, it’s not guaranteed to be portable).

#include <cstdlib> // for EXIT_SUCCESS and EXIT_FAILURE

int main()
{
    return EXIT_SUCCESS;
}

A value-returning function that does not return a value will produce undefined behavior

A function that returns a value is called a value-returning function. A function is value-returning if the return type is anything other than void.

A value-returning function must return a value of that type (using a return statement), otherwise undefined behavior will result.

Here’s an example of a function that produces undefined behavior:

#include <iostream>

int getValueFromUserUB() // this function returns an integer value
{
 	std::cout << "Enter an integer: ";
	int input{};
	std::cin >> input;

	// note: no return statement
}

int main()
{
	int num { getValueFromUserUB() }; // initialize num with the return value of getValueFromUserUB()

	std::cout << num << " doubled is: " << num * 2 << '\n';

	return 0;
}

A modern compiler should generate a warning because getValueFromUserUB is defined as returning an int but no return statement is provided. Running such a program would produce undefined behavior, because getValueFromUserUB() is a value-returning function that does not return a value.

In most cases, compilers will detect if you’ve forgotten to return a value. However, in some complicated cases, the compiler may not be able to properly determine whether your function returns a value or not in all cases, so you should not rely on this.

Function main will implicitly return 0 if no return statement is provided

The only exception to the rule that a value-returning function must return a value via a return statement is for function main(). The function main() will implicitly return the value 0 if no return statement is provided. That said, it is best practice to explicitly return a value from main, both to show your intent, and for consistency with other functions (which will exhibit undefined behavior if a return value is not specified).

Functions can only return a single value

A value-returning function can only return a single value back to the caller each time it is called.

Note that the value provided in a return statement doesn’t need to be literal -- it can be the result of any valid expression, including a variable or even a call to another function that returns a value. In the getValueFromUser() example above, we returned a variable input, which held the number the user input.

There are various ways to work around the limitation of functions only being able to return a single value, which we’ll cover in future lessons.

The function author can decide what the return value means

The meaning of the value returned by a function is determined by the function’s author. Some functions use return values as status codes, to indicate whether they succeeded or failed. Other functions return a calculated or selected value. Other functions return nothing (we’ll see examples of these in the next lesson).

Because of the wide variety of possibilities here, it’s a good idea to document your function with a comment indicating what the return values mean. For example:

// Function asks user to enter a value
// Return value is the integer entered by the user from the keyboard
int getValueFromUser()
{
 	std::cout << "Enter an integer: ";
	int input{};
	std::cin >> input;  

	return input; // return the value the user entered back to the caller
}

Reusing functions

Now we can illustrate a good case for function reuse. Consider the following program:

#include <iostream>

int main()
{
	int x{};
	std::cout << "Enter an integer: ";
	std::cin >> x; 

	int y{};
	std::cout << "Enter an integer: ";
	std::cin >> y; 

	std::cout << x << " + " << y << " = " << x + y << '\n';

	return 0;
}

While this program works, it’s a little redundant. In fact, this program violates one of the central tenets of good programming: Don’t Repeat Yourself (often abbreviated DRY).

Why is repeated code bad? If we wanted to change the text “Enter an integer:” to something else, we’d have to update it in two locations. And what if we wanted to initialize 10 variables instead of 2? That would be a lot of redundant code (making our programs longer and harder to understand), and a lot of room for typos to creep in.

Let’s update this program to use our getValueFromUser function that we developed above:

#include <iostream>

int getValueFromUser()
{
 	std::cout << "Enter an integer: ";
	int input{};
	std::cin >> input;  

	return input;
}

int main()
{
    int x{ getValueFromUser() }; // first call to getValueFromUser
    int y{ getValueFromUser() }; // second call to getValueFromUser

    std::cout << x << " + " << y << " = " << x + y << '\n';

    return 0;
}

This program produces the following output:

Enter an integer: 5
Enter an integer: 7
5 + 7 = 12

In this program, we call getValueFromUser twice, once to initialize variable x, and once to initialize variable y. That saves us from duplicating the code to get user input, and reduces the odds of making a mistake. Once we know getValueFromUser works, we can call it as many times as we desire.

This is the essence of modular programming: the ability to write a function, test it, ensure that it works, and then know that we can reuse it as many times as we want and it will continue to work (so long as we don’t modify the function -- at which point we’ll have to retest it).

Conclusion

Return values provide a way for functions to return a single value back to the function’s caller.

Functions provide a way to minimize redundancy in our programs.

Quiz time

Question #1

Inspect (do not compile) each of the following programs. Determine what the program will output, or whether the program will generate a compiler error.

Assume you have “treat warnings as errors” turned off.

1a)

#include <iostream>

int return7()
{
    return 7;
}

int return9()
{
    return 9;
}

int main()
{
    std::cout << return7() + return9() << '\n';

    return 0;
}

Show Solution

1b)

#include <iostream>

int return7()
{
    return 7;

    int return9()
    {
        return 9;
    }
}

int main()
{
    std::cout << return7() + return9() << '\n';

    return 0;
}

Show Solution

1c)

#include <iostream>

int return7()
{
    return 7;
}

int return9()
{
    return 9;
}

int main()
{
    return7();
    return9();

    return 0;
}

Show Solution

1d)

#include <iostream>

int getNumbers()
{
    return 5;
    return 7;
}

int main()
{
    std::cout << getNumbers() << '\n';
    std::cout << getNumbers() << '\n';

    return 0;
}

Show Solution

1e)

#include <iostream>

int return 5()
{
    return 5;
}

int main()
{
    std::cout << return 5() << '\n';

    return 0;
}

Show Solution

Question #2

What does “DRY” stand for, and why is it a useful practice to follow?

Show Solution