2.7 — Programs with multiple code files

Adding files to your project

As programs get larger, it is common to split them into multiple files for organizational or reusability purposes. One advantage of working with an IDE is that they make working with multiple files much easier. You already know how to create and compile single-file projects. Adding new files to existing projects is very easy.

Best practice

When you add new code files to your project, give them a .cpp extension.

For Visual Studio users

In Visual Studio, right click on the Source Files folder in the Solution Explorer window, and choose Add > New Item….

Make sure you have C++ File (.cpp) selected. Give the new file a name, and it will be added to your project.

Note: If you create a new file from the File menu instead of from your project in the Solution Explorer, the new file won’t be added to your project automatically. You’ll have to add it to the project manually. To do so, right click on Source Files in the Solution Explorer, choose Add > Existing Item, and then select your file.

Now when you compile your program, you should see the compiler list the name of your file as it compiles it.

For Code::Blocks users

In Code::Blocks, go to the File menu and choose New > File….

In the New from template dialog, select C/C++ source and click Go.

You may or may not see a welcome to the C/C++ source file wizard dialog at this point. If you do, click Next.

On the next page of the wizard, select “C++” and click Next.

Now give the new file a name (don’t forget the .cpp extension), and click the All button to ensure all build targets are selected. Finally, select finish.

Now when you compile your program, you should see the compiler list the name of your file as it compiles it.

For GCC/G++ users

From the command line, you can create the additional file yourself, using your favorite editor, and give it a name. When you compile your program, you’ll need to include all of the relevant code files on the compile line. For example: g++ main.cpp add.cpp -o main, where main.cpp and add.cpp are the names of your code files, and main is the name of the output file.

A multi-file example

In lesson 2.6 -- Forward declarations and definitions, we took a look at a single-file program that wouldn’t compile:

When the compiler reaches the function call to add on line 5 of main, it doesn’t know what add is, because we haven’t defined add until line 9! Our solution to this was to either reorder the functions (placing add first) or use a forward declaration for add.

Now let’s take a look at a similar multi-file program:



Your compiler may decide to compile either add.cpp or main.cpp first. Either way, main.cpp will fail to compile, giving the same compiler error as the previous example:

main.cpp(5) : error C3861: 'add': identifier not found

The reason is exactly the same as well: when the compiler reaches line 5 of main.cpp, it doesn’t know what identifier add is.

Remember, the compiler compiles each file individually. It does not know about the contents of other code files, or remember anything it has seen from previously compiled code files. So even though the compiler may have seen the definition of function add previously (if it compiled add.cpp first), it doesn’t remember.

This limited visibility and short memory is intentional, so that files may have functions or variables that have the same names without conflicting with each other. We’ll explore an example of such a conflict in the next lesson.

Our options for a solution here are the same as before: place the definition of function add before function main, or satisfy the compiler with a forward declaration. In this case, because function add is in another file, the reordering option isn’t a good one.

The better solution here is to use a forward declaration:

main.cpp (with forward declaration):

add.cpp (stays the same):

Now, when the compiler is compiling main.cpp, it will know what identifier add is and be satisfied. The linker will connect the function call to add in main.cpp to the definition of function add in add.cpp.

Using this method, we can give files access to functions that live in another file.

Try compiling add.cpp and the main.cpp with the forward declaration for yourself. If you get a linker error, make sure you’ve added add.cpp to your project or compilation line properly.

Something went wrong!

There are plenty of things that can go wrong the first time you try to work with multiple files. If you tried the above example and ran into an error, check the following:

1. If you get a compiler error about add not being defined in main, you probably forgot the forward declaration for function add in main.cpp.

2. If you get a linker error about add not being defined, e.g.

unresolved external symbol "int __cdecl add(int,int)" (?add@@YAHHH@Z) referenced in function _main

2a. …the most likely reason is that add.cpp is not added to your project correctly. When you compile, you should see the compiler list both main.cpp and add.cpp. If you only see main.cpp, then add.cpp definitely isn’t getting compiled. If you’re using Visual Studio or Code::Blocks, you should see add.cpp listed in the Solution Explorer/project pane on the left side of the IDE. If you don’t, right click on your project, and add the file, then try compiling again. If you’re compiling on the command line, don’t forget to include both main.cpp and add.cpp in your compile command.

2b. …it’s possible that you added add.cpp to the wrong project.

2c. …it’s possible that the file is set to not compile or link. Check the file properties and ensure the file is configured to be compiled/linked. In Code::Blocks, compile and link are separate checkboxes that should be checked. In Visual Studio, there’s an “exclude from build” option that should be set to “no” or left blank.

3. Do not #include “add.cpp” from main.cpp. This will cause the compiler to insert the contents of add.cpp directly into main.cpp instead of treating them as separate files.


When the compiler compiles a multi-file program, it may compile the files in any order. Additionally, it compiles each file individually, with no knowledge of what is in other files.

We will begin working with multiple files a lot once we get into object-oriented programming, so now’s as good a time as any to make sure you understand how to add and compile multiple file projects.

Reminder: Whenever you create a new code (.cpp) file, you will need to add it to your project so that it gets compiled.

Quiz time

Question #1

Split the following program into two files (main.cpp, and input.cpp). Main.cpp should have the main function, and input.cpp should have the getInteger function.

Show Hint

Show Solution

2.8 -- Naming collisions and an introduction to namespaces
2.6 -- Forward declarations and definitions

443 comments to 2.7 — Programs with multiple code files

  • nascardriver

    You have more than 1 `main` function. If that's not the case, run a clean rebuild of your project.

  • 83338

    If I want to used iostream functions such as std::cout inside of add.cpp, I have to add #include <iostream> in add.cpp right? Why does that not create an error? I'm adding one module in two files.

    • nascardriver

      Right. Headers contain declarations. You can declare entities as often as you like.

      • 83338

        Thanks for your reply. I have another question, when would using more than one cpp file be useful as opposed to using a header? Why not just use multiple headers and include them all in your cpp file with the main() function?

  • Do not #include “add.cpp” from main.cpp

    "Do not #include “add.cpp” from main.cpp. This will cause the compiler to insert the contents of add.cpp directly into main.cpp instead of treating them as separate files. While it may compile and run for this simple example, it’s non-standard to do so and may cause problems down the road."

    If we do that, then our program won't compile as we violate the second ODR, which linker gives us "already defined" error. Right? But you said it can be run!

    • nascardriver

      If you try to compile both files, yes, that's a violation of the ODR and the program won't link. However, fairly often, someone forgets to compile one or more source files. If they compile only main.cpp, they'll get a working program.

      • Do not #include “add.cpp” from main.cpp

        "However, fairly often, someone forgets to compile one or more source files. If they compile only main.cpp, they'll get a working program."

        Thank you nascardriver, but when we add all files in the same project, all of them are compiled automatically. How does someone forget to compile one or more source files while compiler do that automatically?

  • Source Files folder

    1) "In Visual Studio, right click on the Source Files folder in the Solution Explorer window, and choose Add > New Item…."

    In the photo provided, the right click isn't on the "Source Files" folder; instead, it is on "Hello_World"!

  • Alek

    Hello,I don't get something from this sentence:This limited visibility and short memory is intentional, so that files may have functions or variables that have the same names without conflicting with each other.
    you say they have limited this intentionally so that if files have the same variable and functions, there
    won't be any conflict, but while we forward declare a function in main.cpp and then implement it below main function,we can't have the same function in another file becuase I tried to do it and it will complain about defining two functions with the exact same properties. so this will deny that sentence I getting something wrong there ?would be happy if you can clear my suspicion.thanks!

  • Harsh Gandhi

    how to print anything like hello world without using cout function

  • mmp52

    May I ask kind of problems does this method can cause in the future?

    • Alex

      I softened the language a bit. It may work fine, but you shouldn't do this, in part because it's non-standard, in part because it _can_ lead to issues (like violations of the one definition rule), and in part because it eliminates any benefits of separating your source into separate files (e.g. the need to recompile less if you make a change)

  • Ryan

    Quick typo! In "Our solution to this was to either reorder the functions (placing add first) or using a forward declaration for add", 'using' should be 'use', as in 'was to ... use a forward declaration'.

  • Hi your site is very helpful i learn a lot of better than in my school

    I think here

    you should initialize x and y with brace initialize as you teach us in lesson 1.4

  • Talon Reed

    Hey, Alex. I've been doing a decent job of keeping up with learncpp so far, but when it comes to adding multiple programs, I keep running into one particular issue that I can't quite seem to research away. I'm using VS 2017

    The error I need help with in particular is LNK2019 {unresolved external symbol main referenced in function "int __cdecl invoke_main(void)" (?invoke_main@@YAHXZ)} I believe it might be something to do with the linker not recognizing main() as main()? I don't know.

    I have 2 separate .cpp files, (getInteger.cpp, TestingThings2.cpp) and in both files, it gives me "no issues found" for both. I have no warning, yet I still end up with the error tied to getInteger.

    #include <iostream>

    int    getInteger()
        std::cout << "Enter an integer: ";
        int x;
        std::cin >> x;
        return x;

    and TestingThings2:

    #include <iostream>

    int getInteger();

    int main()
        int x = getInteger();
        int y = getInteger();

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

        return 0;

    I'm doing my best to solve this error, but no matter what I do, I can't seem to find a way out.
    Any help would be incredibly appreciated.

    • Alex

      My best guess is that TestingThings2.cpp isn't being compiled into your program. When you recompile your program, do you see the compiler actually compiling it? If not, you need to add it to your project.

  • MOJI

    Hi, i have a problem!! I'm using VS Code!!
    I created a workspace and added add.cpp & main.cpp with the codes above.
    but the files work independently and the multi-file doent work!!
    whta should i do!!?

    • nascardriver

      You need to compile both files at the same time. Assuming you're using clang++:

      clang++ ./add.cpp ./main.cpp

    • MOJI

      I got the result by using the default config of VS Code! ( creating task and run the program in powershell )
      but the Run button in the Upper-right corner of the codin window gives me the following error and wont compile!! even deletes the already created .exe file through Build Task!!

      So to sum it up:

      how can i work with Run button and multi-file together!?


    • Hey, I am in the same situation as you. Did you find a way eventually?

      • Madelain Yorick

        The default build tasks from VS code don't seem to handle multi-file.
        To make it work I had to create my own by looking up how it works. You have to edit the tasks.json file created by vs code and add the following one:

        It worked for me but all depends on what compiler you're using, I hope it will solve your problem.

  • Harshit Agrawal

    I am getting this error.

    1>------ Build started: Project: learn, Configuration: Debug Win32 ------
    1>MSVCRTD.lib(exe_winmain.obj) : error LNK2019: unresolved external symbol _WinMain@16 referenced in function "int __cdecl invoke_main(void)" (?invoke_main@@YAHXZ)
    1>C:\Users\Harshit Agrawal\source\repos\learn\Debug\learn.exe : fatal error LNK1120: 1 unresolved externals
    1>Done building project "learn.vcxproj" -- FAILED.
    ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

    My code:


    int doadd(int x, int y)
        return x + y;


    #include <iostream>

    int doadd(int x, int y);

    int main()
        std::cout << "The sum of 3 and 4 is " << doadd(3, 4);
        return 0;

    • Tommy

      You probably figured it out but I believe you forgot to add
      #include <iostream> At the top of add.cpp

      • nascardriver

        "add.cpp" should not include . "add.cpp" doesn't use anything from the standard library. The error is most likely caused by a misconfigured project.

  • David

    I tried putting everything I learned together by writing a program that asks the user for three integers, then calculates and prints the sum of the integers.

    The problem is I'm getting an error around sumOf(). I know I need to have the same number of arguments but I'm not sure how to fix it.

    The main.cpp file contains the following:

    and the add.cpp file contains the following:

    • nascardriver

      You need `x`, `y`, and `z` when you call `sumOf`. `main` doesn't have them, so you can't call `sumOf`.
      Write a function named "getInteger" or similar that reads an int from the user and returns it. Call that function 3 times in `main` and store its return values, which you then pass to `sumOf` (Should be renamed to "printSumOf" or similar). This makes `add` obsolete.

    • Omran

      hello i solved your problem you can write this code and then check it out

      this is main.cpp file

      this is add.cpp file

  • Crizonic

    I had to put a #include "input.cpp" at the top of main.cpp to get mine to compile. Im using Atom IDE, thought this might help someone.

    • nascardriver

      If you have the forward declaration in "main.cpp", the code compiles. If it doesn't for you, you didn't add "input.cpp" to the files that should be compiled. Never include "*.cpp" files, you're bound to break something.

  • Lord Voldemort

    In the section "A multi-file example", there is a line "Try compiling add.cpp and the main.cpp with the forward declaration for yourself. If you get a linker error, make sure you’ve added add.cpp to your project or compilation line properly." what is compilation line?

    • nascardriver

      If you're compiling on the command line, the compilation line is the command you use to compile your program.
      If you're using an IDE, this doesn't affect you.

  • NA

    Anyone know why syntax highlighting is only working for one source file in Visual Studio? If I open another file to do the multi-file solution, syntax highlighting only works for main.cpp

    Tried googling but couldn't find anyone else with the issue. Tools>Options>Environment>Fonts & Colors looks the same on both files.


  • Diana

    Why do you not need a header file such as add.h that contains the declaration of the add() function?

  • Chayim

    I pasted the solution from the quiz in a Code Playground” and received the error listed below. Is it because the Code Playground does not have a linker?

    Tried to post the error here but it’s too long and it’s not able to post it.

  • Chayim

    How is it able to understand this chapter correctly if it wasn’t explained how linkers work?

  • Chayim

    Why compile two files and not merging it in one file? Is it not more efficient and compiles and executes faster and better as one file and not to have it being linked? Is it only because of shortening the work of code not to have to merge it into one file and just linking existing files and done away with it only by linking? Or does it have any purpose?

    • nascardriver

      Writing everything in one file gets extremely messy and you'll run into problems with circular dependencies.
      The examples on learncpp are single-file for the most part because they're easier to reproduce that way. If you work on a project yourself, you should use multiple files.

  • fxr

    Im trying to make a program that prints and adds using functions on a separate page but I can't get the program to print the results what am I doing wrong.

    this is the function page
    [#include <iostream>

    int input()
        std::cout << "give num ";
        int R;
        std::cin >> R;
        return R;
    int add(int x, int y)
        return x + y;

    void print(int add1, int add2, int result)
         std::cout << add1 << "+" << add2 << "=" << result;

    This is the main page
    [#include <iostream>
    int add(int x , int y);
    int input();
    void print(int f, int g, int q);
    int main()
        int math{ input() };
        int math1{ input() };
        void print(int math, int math1, int add(int math, int math1));
            return 0;

  • Mike

    For me the issue was the HelloWorld.cpp file I was already using. By adding the files main.cpp and add.cpp, I then had three files, rather than just the two. Once I clicked Source Files, right clicked HelloWorld.cpp, and selected remove, I was good to go.

    Hopefully this will help someone else with a similar issue.

  • Vulryn

    I tried to make simple program that asks for 2 values and adds them. When i try to run 3 function in same page i get no problem but when i separate them into 3 file i get errors, they are all in source files of the same project. I tried to exclude forward decleration but it also didn't work. What am i doing wrong ?

    1>main.obj : error LNK2019: unresolved external symbol "int __cdecl calculate(int,int)" (?calculate@@YAHHH@Z) referenced in function _main
    1>  Hint on symbols that are defined and could potentially match:
    1>    "void __cdecl calculate(int,int)" (?calculate@@YAXHH@Z)

    My Codes :



  • David M.

    Hi. Love this site. In the first examples you have main.cpp and add.cpp and the function inside of add.ccp is called "int add(int x, int y)". Further down you have input.cpp with the function inside of it called int getInteger(). Question, is that ok to have the .ccp name different than the same name of the function contained within the .ccp file? Also, could the .ccp name be anything random like fdkfdkjdkfj.ccp and have multiple functions inside instead of just one? thanks in advance for any help.

  • fighter_fish

    I think you may have a typo in the section you're explaining how to add files in Code::Blocks

    I think you meant this:

  • GaryTruffles

    Does it matter the order in which you place cin and cin.ignore()?Thanks

  • waterBottle

    If you have your main.cpp, a source.cpp, and source.h files. You include source.h into your main.cpp. This basically copies and pastes the contents of source.h in the same location of #include "source.h" in your main.cpp. Then you link main.cpp and source.cpp to make an object file. And then run it? Is this right? Thanks. Also, if both main.cpp and source.cpp both have #include <iostream> for example, won't you get an error because it is defined twice?

    • Before linking, the files get compiled.

      > won't you get an error because it is defined twice?
      Headers should only contain declarations (Or definitions without the one-definition-rule). Unless the author of the header violates this convention, you're fine.

  • mits

    Question - why am I required, in app.cpp, to include iostream when it is already included in the main.cpp?

  • Do Vegans Kill Cockroaches?

    Have you guys considered making a night mode version of the site? Black background and white words? Y'know, just as an option for us night owls?

  • Prateek

    How to use multiple code files in cpp droid or cxx droid (android apps)?

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