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.
Related content
We explore functions that return void
further in the next lesson (2.3 -- Void functions (non-value returning functions)).
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 specific value returned from a function is called the return value. When the return statement is executed, the function exits immediately, and the return value is copied from the function back to the caller. This process is called 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 indicates the specific value that will be returned
return 5; // return the specific 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. Function returnFive
returns the specific value of 5
back to the caller, which is then 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.
Tip
When a called function returns a value, the caller may decide to use that value in an expression or statement (e.g. by using it to initialize a variable, or sending it to std::cout
) or ignore it (by doing nothing else). If the caller ignores the return value, it is discarded (nothing is done with it).
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()
.
Tip
If you need to use the return value of a function call more than once, initialize a variable with the return value, and then use that variable as many times as needed.
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 anint
.- 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();
}
Key insight
C does allow main()
to be called explicitly, so some C++ compilers will allow this for compatibility reasons.
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).
Best practice
Your main
function should return the value 0
if the program ran normally.
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).
For advanced readers
The C++ standard only defines the meaning of 3 status codes: 0
, EXIT_SUCCESS
, and EXIT_FAILURE
. 0
and EXIT_SUCCESS
both mean the program executed successfully. EXIT_FAILURE
means the program did not execute successfully.
EXIT_SUCCESS
and EXIT_FAILURE
are preprocessor macros defined in the <cstdlib> header:
#include <cstdlib> // for EXIT_SUCCESS and EXIT_FAILURE
int main()
{
return EXIT_SUCCESS;
}
If you want to maximize portability, you should only use 0
or EXIT_SUCCESS
to indicate a successful termination, or EXIT_FAILURE
to indicate an unsuccessful termination.
We cover the preprocessor and preprocessor macros in lesson 2.10 -- Introduction to the preprocessor.
As an aside…
The status code is passed back to the operating system. The OS will typically make the status code available to whichever program launched the program returning the status code. This provides a crude mechanism for any program launching another program to determine whether the launched program ran successfully or not.
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.
Related content
We discuss undefined behavior in lesson 1.6 -- Uninitialized variables and undefined behavior.
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.
Best practice
Make sure your functions with non-void return types return a value in all cases.
Failure to return a value from a value-returning function will cause undefined behavior.
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).
Best practice
Follow DRY: “Don’t repeat yourself”. If you need to do something more than once, consider how to modify your code to remove as much redundancy as possible. Variables can be used to store the results of calculations that need to be used more than once (so we don’t have to repeat the calculation). Functions can be used to define a sequence of statements we want to execute more than once. And loops (which we’ll cover in a later chapter) can be used to execute a statement more than once.
Like all best practices, DRY is meant to be a guideline, not an absolute. Reader Yariv has noted that DRY can harm overall comprehension when code is broken into pieces that are too small.
As an aside…
The (snarky) opposite of DRY is WET (“Write everything twice”).
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;
}
1b)
#include <iostream>
int return7()
{
return 7;
int return9()
{
return 9;
}
}
int main()
{
std::cout << return7() + return9() << '\n';
return 0;
}
1c)
#include <iostream>
int return7()
{
return 7;
}
int return9()
{
return 9;
}
int main()
{
return7();
return9();
return 0;
}
1d)
#include <iostream>
int getNumbers()
{
return 5;
return 7;
}
int main()
{
std::cout << getNumbers() << '\n';
std::cout << getNumbers() << '\n';
return 0;
}
1e)
#include <iostream>
int return 5()
{
return 5;
}
int main()
{
std::cout << return 5() << '\n';
return 0;
}
Question #2
What does “DRY” stand for, and why is it a useful practice to follow?