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2.11 — Header files

Headers, and their purpose

As programs grow larger (and make use of more files), it becomes increasingly tedious to have to forward declare every function you want to use that is defined in a different file. Wouldn’t it be nice if you could put all your forward declarations in one place and then import them when you need them?

C++ code files (with a .cpp extension) are not the only files commonly seen in C++ programs. The other type of file is called a header file. Header files usually have a .h extension, but you will occasionally see them with a .hpp extension or no extension at all. The primary purpose of a header file is to propagate declarations to code files.

Using standard library header files

Consider the following program:

This program prints “Hello, world!” to the console using std::cout. However, this program never provide a definition or declaration for std::cout, so how does the compiler know what std::cout is?

The answer is that std::cout has been forward declared in the “iostream” header file. When we #include <iostream>, we’re requesting that the preprocessor copy all of the content (including forward declarations for std::cout) from the file named “iostream” into the file doing the #include.

Key insight

When you #include a file, the content of the included file is inserted at the point of inclusion. This provides a useful way to pull in declarations from another file.

Consider what would happen if the iostream header did not exist. Wherever you used std::cout, you would have to manually type or copy in all of the declarations related to std::cout into the top of each file that used std::cout! This would require a lot of knowledge about how std::cout was implemented, and would be a ton of work. Even worse, if a function prototype changed, we’d have to go manually update all of the forward declarations. It’s much easier to just #include iostream!

When it comes to functions and objects, its worth keeping in mind that header files typically only contain function and object declarations, not function and object definitions (otherwise a violation of the one definition rule could result). std::cout is forward declared in the iostream header, but defined as part of the C++ standard library, which is automatically linked into your program during the linker phase.

Best practice

Header files should generally not contain function and variable definitions, so as not to violate the one definition rule. An exception is made for global constant variables (which we’ll cover in a future lesson).

Writing your own header files

Now let’s go back to the example we were discussing in a previous lesson. When we left off, we had two files, add.cpp and main.cpp, that looked like this:

add.cpp:

main.cpp:

(If you’re recreating this example from scratch, don’t forget to add add.cpp to your project so it gets compiled in).

In this example, we used a forward declaration so that the compiler will know what identifier add is when compiling main.cpp. As previously mentioned, manually adding forward declarations for every function you want to use that lives in another file can get tedious quickly.

Let’s write a header file to relieve us of this burden. Writing a header file is surprisingly easy, as header files only consist of two parts:

  1. A header guard, which we’ll discuss in more detail in the next lesson (2.12 -- Header guards).
  2. The actual content of the header file, which should be the forward declarations for all of the identifiers we want other files to be able to see.

Adding a header file to a project works analogously to adding a source file (covered in lesson 2.8 -- Programs with multiple code files). If using an IDE, go through the same steps and choose “Header” instead of “Source” when asked. If using the command line, just create a new file in your favorite editor.

Best practice

Use a .h suffix when naming your header files.

Header files are often paired with code files, with the header file providing forward declarations for the corresponding code file. Since our header file will contain a forward declaration for functions defined in add.cpp, we’ll call our new header file add.h.

Best practice

If a header file is paired with a code file (e.g. add.h with add.cpp), they should both have the same base name (add).

Here’s our completed header file:

add.h:

In order to use this header file in main.cpp, we have to #include it (using quotes, not angle brackets).

main.cpp:

add.cpp:

When the compiler compiles the #include "add.h" line, it copies the contents of add.h into the current file at that point. Because our add.h contains a forward declaration for function add, that forward declaration will be copied into main.cpp. The end result is a program that is functionally the same as the one where we manually added the forward declaration at the top of main.cpp.

Consequently, our program will compile and link correctly.

Best practice

A code file should #include its paired header file (if it exists). If you’re curious as to why, see nascardriver’s comment here.

Troubleshooting

If you get a compiler error indicating that add.h isn’t found, make sure the file is really named add.h. Depending on how you created and named it, it’s possible the file could have been named something like add (no extension) or add.h.txt or add.hpp. Also make sure it’s sitting in the same directory as the rest of your code files.

If you get a linker error about function add not being defined, make sure you’ve included add.cpp in your project so the definition for function add can be linked into the program.

Angled brackets vs quotes

You’re probably curious why we use angled brackets for iostream, and double quotes for add.h. The answer is that angled brackets are used to tell the compiler that we are including a header file that was included with the compiler, so it should look for that header file in the system directories. The double-quotes tell the compiler that this is a user-defined header file we are supplying, so it should look for that header file in the current directory containing our source code first. If it doesn’t find the header file there, it will check any other include paths that you’ve specified as part of your compiler/IDE settings. That failing, it will fall back to checking the system directories.

Rule

Use angled brackets to include header files that come with the compiler. Use double quotes to include any other header files.

Why doesn’t iostream have a .h extension?

Another commonly asked question is “why doesn’t iostream (or any of the other standard library header files) have a .h extension?”. The answer is that iostream.h is a different header file than iostream! To explain requires a short history lesson.

When C++ was first created, all of the files in the standard runtime library ended in a .h stuffix. Life was consistent, and it was good. The original version of cout and cin were declared in iostream.h. When the language was standardized by the ANSI committee, they decided to move all of the functionality in the standard library into the std namespace to help avoid naming conflicts with user-defined identifiers. However, this presented a problem: if they moved all the functionality into the std namespace, none of the old programs (that included iostream.h) would work any more!

To work around this issue, a new set of header files was introduced that use the same names but lack the .h extension. These new header files have all their functionality inside the std namespace. This way, older programs that include #include <iostream.h> do not need to be rewritten, and newer programs can #include <iostream>.

In addition, many of the libraries inherited from C that are still useful in C++ were given a c prefix (e.g. stdlib.h became cstdlib). The functionality from these libraries was also moved into the std namespace to help avoid naming collisions.

Best practice

When including a header file from the standard library, use the no extension version (without the .h) if it exists. User-defined headers should still use a .h extension.

Including header files from other directories

Another common question involves how to include header files from other directories.

One (bad) way to do this is to include a relative path to the header file you want to include as part of the #include line. For example:

While this will compile (assuming the files exist in those relative directories), the downside of this approach is that it requires you to reflect your directory structure in your code. If you ever update your directory structure, your code won’t work any more.

A better method is to tell your compiler or IDE that you have a bunch of header files in some other location, so that it will look there when it can’t find them in the current directory. This can generally be done by setting an include path or search directory in your IDE project settings.

For Visual Studio users

Right click on your project in the Solution Explorer, and choose Properties, then the VC++ Directories tab. From here, you will see a line called Include Directories. Add the directories you’d like the compiler to search for additional headers there.

For Code::Blocks users

In Code::Blocks, go to the Project menu and select Build Options, then the Search directories tab. Add the directories you’d like the compiler to search for additional headers there.

For GCC/G++ users

Using g++, you can use the -I option to specify an alternate include directory.

The nice thing about this approach is that if you ever change your directory structure, you only have to change a single compiler or IDE setting instead of every code file.

Headers may include other headers

It’s common that a header file will need a declaration or definition that lives in a different header file. Because of this, header files will often #include other header files.

When your code file #includes the first header file, you’ll also get any other header files that the first header file includes (and any header files those include, and so on). The definitions of all these subsequent header files are also implicitly available for use in your code file.

In your code files, you should never rely on the contents of header files that are included by other header files. The implementation of header files may change over time, or be different across different systems. If that happens, your code may only compile on certain systems, or may compile now but not in the future.

Best practice

Each .cpp file should explicitly #include all of the header files it needs to compile. Do not rely on headers included from other headers.

Unfortunately, there is no easy way to detect when your code file is accidentally relying content of a header file that has been included by another header file.

Header file best practices

Here are a few more recommendations for creating and using header files.

  • Always include header guards (we’ll cover these next lesson).
  • Do not define variables and functions in header files (global constants are an exception -- we’ll cover these later)
  • Give your header files the same name as the source files they’re associated with (e.g. grades.h is paired with grades.cpp).
  • Each header file should have a specific job, and be as independent as possible. For example, you might put all your declarations related to functionality A in A.h and all your declarations related to functionality B in B.h. That way if you only care about A later, you can just include A.h and not get any of the stuff related to B.
  • Be mindful of which headers you need to explicitly include for the functionality that you are using in your code files
  • Every header you write should compile on its own (it should #include every dependency it needs)
  • Only #include what you need (don’t include everything just because you can).
  • Do not #include .cpp files.
  • Order your #includes as follow: your own user-defined headers first, then 3rd party library headers, then standard library headers. This is a minor one, but may help highlight a user-defined header file that doesn’t directly #include everything it needs.

2.12 -- Header guards
Index
2.10 -- Introduction to the preprocessor

490 comments to 2.11 — Header files

  • Juan

    I keep learning new things in this tutorial! As the difference between the headers with h and without it. Thanks

  • 5n1p3r_4rt3m1s

    "A code files should #include its paired header file (even if it’s not strictly needed), as this will in some cases allow the compiler to detect forward declarations with mismatched parameter. types)." ??? I cant understand this..

    • Alex

      I'm having a hard time finding a good example of this. I'm going to back the recommendation off a bit until I can find a better use case that illustrates why.

      • Although a rare encounter when writing programs, this is relevant when writing libraries.
        Not all functions are used, so the linker won't try to find function definitions.

        cat.hpp

        cat.cpp

        If we were writing a program, we might have a main.cpp

        The linker will abort, alerting us that no definition of @mew exists.
        Though, with libraries, we don't call all of the functions we write, we just offer them for other programmers to use.
        If we didn't call @mew from anywhere, the code would compile and link fine.
        By including @"mew.hpp" in @"mew.cpp"

        the compiler will error out, because there are 2 function declaration that only differ in their return type, which is illegal.

        Granted, compilation and linkage will succeed if the function parameters differ.
        ld has an option to disallow undefined symbols, that helps too.
        A 1-liner to avoid mistakes like this is worth a mention.

  • 5n1p3r_4rt3m1s

    "%Failed lesson reference, id 7938%" in Writing your own header files section. Seems like some error in this webpage's code.

  • Hadi

    Hi ALEX
    You said that iostream.h is a different header than iostream
    then why in Code::Blocks if i tried to #include<iostream.h>
    i get the error message ..No such file or directory..
    thank you so much for this great website..

    • Alex

      Because iostream.h is deprecated, and was deprecated so long ago that many compilers don't even provide a version of it any longer.

  • jih332

    Hi there,  
      
    I was practicing on problem 6.x.2), I wrote a main.cpp in current directory, and wanted to reuse some code in another directory ../lib (safe_input.h, safe_input.cpp, sort.h, sort.cpp to be specific). so I included both "safe_input.h" and "sort.h" in main.cpp, and compiled it with

    but it said

    then I changed it to

    it started to compile, but still complained that

    so how should I write compiler command in this situation? (p.s. I checked the solution for this problem, and found that it also gave the same [-Werror=effc++] error as mentioned above).
    Thanks!

  • Kol

    Please help! I'm using Atom and my code wont work unless I use #include "add.cpp" (Is this a setting I have to change?)
    Here is the code for all 3 files:

    //add.cpp  
    #include "add.h"
    int add(int x, int y)
    {
      return x + y ;
    }

    // add.h
    int add(int x, int y);

    //main.cpp
    #include <iostream>
    #include "add.h"
    #include "add.cpp"
    int main()
    {
      std::cout << "The sum of 3 and 4 is " << add(3,4) << "\n";
      return 0;
    }

  • James III

    Can anyone help me? I'm using Microsoft Visual Studio community. Is there a way to create another version of a project that is already created, along with the rest of the project so the solution explorer of the new version is the same as the original version?

    I have an application that is 1263 lines long that I wrote and everything in it works fine. Now I'd like to recreate this whole project so I can make major changes to modify the behavior of the original app without effecting the original app so as to evolve the app to be more advanced than the original.

    The original app is a Windows Console Application and has an #include "stdafx.h". Every time I create a new Windows Console Application project, it always #includes "pch.h" and if I add #include "stdafx.h", I get an error that it cannot be opened.

    If anyone can help me with this, it would be greatly appreciated. If you need more info, please let me know. Thank you very much.

    • stdafx.h has been replaced with pc.h
      I don't know about your other question

      • JamesIII

        Thank you nascardriver for getting back to me. If pch.h replaces stdafx.h, then I have to try and redo my code with the new #include.

        My program using #include "stdafx.h", the reading from a text file works fine. This program does all it's supposed to do. I have a menu with 11 selections to it that all do different things that count on the data in my text file. Everything works fine.

        Now I continue on creating a new project that has #include "pch.h" automatically added in, copying the main() function only from the old program and pasting it to the new program, the program compiles, but the reading from the text file no longer functions. Reading out the file output (using std::cout << ...) produces only blanks and when I get to the part where I use stoi to convert a string to an int, the program aborts.

        I'm sorry, but I cannot for the life of me figure out how to remedy this. Is there another header file I should be including to get the file reading working again in the new program? I'm already including <fstream>, <iostream>, <string>, "stdio.h" and <cmath> along with "pch.h". If I take the pch.h out, the program won't compile. If I include "stdafx.h", I get an error   "cannot open source file stdafx.h", but this file also isn't in the solution explorer. If I add it to the solution explorer, duplicating what I have with the old program, and try and compile, I get the following:

        1>------ Build started: Project: NewMain, Configuration: Debug Win32 ------
        1>NewMain.obj : error LNK2011: precompiled object not linked in; image may not run
        1>C:\Users\JamesIII\source\repos\NewMain\Debug\NewMain.exe : fatal error LNK1120: 1 unresolved externals
        1>Done building project "NewMain.vcxproj" -- FAILED.
        ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

        I'd really appreciate it if someone could help me out so I can see what I'm doing wrong. I can't see why file reading stops working with "pch.h". If you need any more info, please let me know. Thank you for considering my dilemma.

        • Disable precompiled headers in your project settings and remove the includes to @"stdafx.h" and @"pc.h". Those file are just some VS junk that you don't need. It causes more problems than it's helpful.
          You have 1 unresolved external (ie. a function in a library that cannot be found).
          I can only guess that this is caused by the precompiled headers and should be resolved by disabling them. If it doesn't, leave another comment with a minimal compilable code example that causes your problem.

          • JamesIII

            Hello Nascardriver, I solved that problem I've been having. It's not with pch.h or stdafx.h, but has to do with the placement of the data file I'm using with the program. Placing a copy of the data file in the same directory as the new program (with #include "pch.h") and giving it a new name like changing "afile.dat" to "tfile.dat", the program works just like the original program. I'd like to apologize for drawing your attention to a problem that was more of an oversight on my part, but also thank you very much for getting back to me.

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