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2.1 — Introduction to functions

In the last chapter, we defined a function as a collection of statements that execute sequentially. While that is certainly true, that definition doesn’t provide much insight into why functions are useful. Let’s update our definition: A function is a reusable sequence of statements designed to do a particular job.

You already know that every program must have a function named main (which is where the program starts execution when it is run). However, as programs start to get longer and longer, putting all the code inside the main function becomes increasingly hard to manage. Functions provide a way for us to split our programs into small, modular chunks that are easier to organize, test, and use. Most programs use many functions. The C++ standard library comes with plenty of already-written functions for you to use -- however, it’s just as common to write your own. Functions that you write yourself are called user-defined functions.

Consider a case that might occur in real life: you’re reading a book, when you remember you need to make a phone call. You put a bookmark in your book, make the phone call, and when you are done with the phone call, you return to the place you bookmarked and continue your book precisely where you left off.

C++ programs can work the same way. A program will be executing statements sequentially inside one function when it encounters a function call. A function call is an expression that tells the CPU to interrupt the current function and execute another function. The CPU “puts a bookmark” at the current point of execution, and then calls (executes) the function named in the function call. When the called function ends, the CPU returns back to the point it bookmarked, and resumes execution.

The function initiating the function call is called the caller, and the function being called is the callee or called function.

An example of a user-defined function

First, let’s start with the most basic syntax to define a user defined function. For this lesson, all user-defined functions (except main) will take the following form:

void identifier() // identifier replaced with the name of your function
{
// Your code here
}

We’ll talk more about the different parts of this syntax in the next few lessons. For now, identifier will simply be replaced with the name of your user-defined function. The curly braces and statements in between are called the function body.

Here is a sample program that shows how a new function is defined and called:

This program produces the following output:

Starting main()
In doPrint()
Ending main()

This program begins execution at the top of function main, and the first line to be executed prints Starting main().

The second line in main is a function call to the function doPrint. We call function doPrint by appending a pair of parenthesis to the function name like such: doPrint(). Note that if you forget the parenthesis, your program may not compile (and if it does, the function will not be called).

Warning

Don’t forget to include parenthesis () after the function’s name when making a function call.

Because a function call was made, execution of statements in main is suspended, and execution jumps to the top of called function doPrint. The first (and only) line in doPrint prints In doPrint(). When doPrint terminates, execution returns back to the caller (function main) and resumes from the point where it left off. Consequently, the next statement executed in main prints Ending main().

Calling functions more than once

One useful thing about functions is that they can be called more than once. Here’s a program that demonstrates this:

This program produces the following output:

Starting main()
In doPrint()
In doPrint()
Ending main()

Since doPrint gets called twice by main, doPrint executes twice, and In doPrint() gets printed twice (once for each call).

Functions calling functions calling functions

You’ve already seen that function main can call another function (such as function doPrint in the example above). Any function can call any other function. In the following program, function main calls function doA, which calls function doB:

This program produces the following output:

Starting main()
Starting doA()
In doB()
Ending doA()
Ending main()

Nested functions are not supported

Unlike some other programming languages, in C++, functions cannot be defined inside other functions. The following program is not legal:

The proper way to write the above program is:

Quiz time

Question #1

In a function definition, what are the curly braces and statements in-between called?

Show Solution

Question #2

What does the following program print? Do not compile this program, just trace the code yourself.

Show Solution


2.2 -- Function return values
Index
1.x -- Chapter 1 summary and quiz

567 comments to 2.1 — Introduction to functions

  • please guide me through this and help me solve it. I am unable to understand the error displayed, according to me there's no errors in this code but there's an error popping up all the time. Please Help. I am using https://ideone.com  

    Code Written

    Error

  • Helliarc

    Maybe it has already been mentioned, but I tried and it didn't work for me...  Are the functions built in sequence? I tried to call my second function with my first function (sequentially as declared), and function 1 failed to call function 2, but function 2 can call function 1...  I imagine the program isn't going to load ALL of the functions, but regardless of it's position of definition, shouldn't calling a function cause the program to "search" for that function that it is trying to call?

  • Oscar R.

    Ah! Now this version of learncpp.com is what I should of had 1 month ago! Great improvement guys

  • GG

    Alex, Relocating the DRY exercise from the chapter 1 Quiz (which was too tricky and so a bit demoralizing for newcomers) to an exercise in 2.2 section is a smart move...
    Nice to see uniform initialization being moved into chapter one, instead of creeping in after people got bad habits in the old 2.4 if i remember correctly....
    And so many other much needed improvements.....
    I was actually going to write you about all these things, glad to see everything being updated....

    in 1.10 compiler line error explanation for << being misused for cin  do not match the actually example lines  ( due to the omission of #include <iostream>), this might confuse some.

    Per our previous discussion: implementing top page navigation could be done with a simple small : < previous     next >   on the top right.

    I would also advise to change your header banner/logo on desktop as it is dated, it gives the wrong impression and might scare some away.
    Moreover, the title : learncpp.com and the your header: tutorials to help you master... are currently embedded as an image banner, these should be typed over your banner for SEO optimization, so search spiderscan read it and give you extra relevance.

    (while I'm at it: the ad banners background on each side being grey is feeling old or dull and does not make the ads or the center section  pop (=less ad clicks), a deeper grayish black would freshen it up but that's more of a taste thing)

    Thank you for your time.

    • Alex

      Thanks for all the feedback! The typos have been fixed. Still looking into the nav request. I'll look at the styling elements too. The site is overdue for a theme refresh but I'm not a designer so that's not my area of strength.

      • Tamir

        Hey, just want to say thanks for keeping up with all of this... this is great stuff and is really helping me out in my c++ endeavors. When I first saw some of the lessons I got worried bc some of the dates were old, but as I continued I saw updates being made so close to the current date. As a student, I just want to say I appreciate all of this and looking forward to completing the whole 'course' haha. Awesome stuff

        Tamir

  • Hi Alex!

    Quiz 1h says "What actually gets output depends on the compiler.".
    I don't see how this is the case. To my understanding, it's well defined.
    Function pointers can only be cast to bool, @std::basic_ostream::operator<< has an overload for bools, so that's used.

  • Kushal

    "In the third function call of return5(), the function returns the value of 5 back to the caller. However, main() does nothing with the return value so nothing further happens (the return value is ignored)."

    What will happen with 5 if it is ignored. My question is in terms of memory. When a function returns a value then where is the existence of the value even though it is gonna be ignored/deleted ? Any temporary storage area in memory ?

    Regards
    Kushal

    • Hi Kushal!

      This is implementation defined. Most likely, the called function (In this case @return5) stores the value in memory, but the memory is overridden later without ever being read from.

      • Kushal

        Ok , thanks for the explanation :)

        One more question. I just executed last example 1h in my case it always prints 1 when I run the program.

        I replaced the rreturn5 with return10 to check but it gives error. So why does it works only for return5 ?

        Not sure why.

        • > I just executed last example 1h in my case it always prints 1
          It's supposed to do that.

          > I replaced the rreturn5 with return10 to check but it gives error
          There is nothing named "return10", so you get an error. If you add a function or variable with that name, you'll get the same output.

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