Code files (with a .cpp extension) are not the only files commonly seen in programs. The other type of file is called a header file, sometimes known as an include file. Header files almost always have a .h extension. The purpose of a header file is to hold declarations for other files to use.
Using standard library header files
Consider the following program:
#include <iostream>
int main()
{
using namespace std;
cout << "Hello, world!" << endl;
return 0;
}
This program prints “Hello, world!” to the console using cout. However, our program never defines cout, so how does the compiler know what cout is? The answer is that cout has been declared in a header file called “iostream”. When we use the line #include <iostream>, we are telling the compiler to locate and then read all the declarations from a header file named “iostream”.
Keep in mind that header files typically only contain declarations. They do not define how something is implemented, and you already know that your program won’t link if it can’t find the implementation of something you use. So if cout is only defined in the “iostream” header file, where is it actually implemented? It is implemented in the runtime support library, which is automatically linked into your program during the link phase.
A library is a package of code that is meant to be reused in many programs. Typically, a library includes a header file that contains declarations for everything the library wishes to expose (make public) to users, and a precompiled object that contains all of the implementation code compiled into machine language. These libraries typically have a .lib or .dll extension on Windows, and a .a or .so extension on Unix. Why are libraries precompiled? First, since libraries rarely change, they do not need to be recompiled often, if ever. It would be a waste of time to compile them every time you wrote a program that used them. Second, because precompiled objects are in machine language, it prevents people from accessing or changing the source code, which is important to businesses or people who don’t want to make their source code available for intellectual property reasons.
Writing your own header files
Now let’s go back to the example we were discussing in the previous lesson. When we left off, we had two files, add.cpp and main.cpp, that looked like this:
add.cpp:
int add(int x, int y)
{
return x + y;
}
main.cpp:
#include <iostream>
int add(int x, int y); // forward declaration using function prototype
int main()
{
using namespace std;
cout << "The sum of 3 and 4 is " << add(3, 4) << endl;
return 0;
}
We’d used a forward declaration so that the compiler would know what add was when compiling main.cpp. As previously mentioned, writing forward declarations for every function you want to use that lives in another file can get tedious quickly.
Header files can relieve us of this burden. A header file only has to be written once, and it can be included in as many files as needed. This also helps with maintenance by minimizing the number of changes that need to be made if a function prototype ever changes (eg. by adding a new parameter).
Writing our own header files is surprisingly easy. Header files consist of two parts. The first part is called a header guard, which is discussed in the next lesson (on the preprocessor). The second part is the actual content of the .h file, which should be the declarations for all of the functions we want other files to be able to see. Our header files should all have a .h extension, so we’ll call our new header file add.h:
add.h:
#ifndef ADD_H #define ADD_H int add(int x, int y); // function prototype for add.h #endif
In order to use this header file in main.cpp, we have to include it. Here is the new main.cpp:
main.cpp that includes add.h:
#include <iostream>
#include "add.h" // this brings in the declaration for add()
int main()
{
using namespace std;
cout << "The sum of 3 and 4 is " << add(3, 4) << endl;
return 0;
}
When the compiler compiles the #include "add.h" line, it copies the contents of add.h into the current file. Because our add.h contains a function prototype for add(), this prototype is now being used as a forward declaration of add()!
Consequently, our program will compile and link correctly.
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. The double-quotes tell the compiler that this is a header file we are supplying, which causes it to look for that header file in the current directory containing our source code first.
Rule: Use angled brackets to include header files that come with the compiler. Use double quotes to include any other header files.
Another commonly asked question is “why doesn’t iostream have a .h extension?”. The answer is, because iostream.h is a different header file than iostream is! To explain requires a very short history lesson.
When C++ was first created, all of the files in the standard runtime library ended in .h. Life was consistent, and it was good. The original version of cout and cin lived in iostream.h. When the language was standardized by the ANSI committee, they decided to move all of the functions in the runtime library into the std namespace (which is generally a good idea). However, this presented a problem: if they moved all the functions into the std namespace, none of the old programs would work any more!
To try to get around this issue and provide backwards compatibility for older programs, 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 <iosteam.h> do not need to be rewritten, and newer programs can #include <iostream>.
Make sure when you include a header file from the standard library that you use the non .h version if it exists. Otherwise you will be using a deprecated version of the header that is no longer supported.
As a side note, many headers in the standard library do not have a non .h version, only a .h version. For these files, it is fine to include the .h version. Many of these libraries are backwards compatible with standard C programming, and C does not support namespaces. Consequently, the functionality of these libraries will not be accessed through the std namespace. Also, when you write your own header files, they will all have a .h extension, since you will not be putting your code in the std namespace.
Rule: use the non .h version of a library if it exists, and access the functionality through the std namespace. If the non .h version does not exist, or you are creating your own headers, use the .h version
Header file best practices
Here are a few best practices for creating your own header files.
- Always include header guards.
- Do not declare variables in header files unless they are constants. Header files should generally only be used for declarations.
- Do not define functions in header files unless they are trivial. Doing so makes your header files harder for humans to read.
- 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.
- Try to include as few other header files as possible in your header files.
 1.10 — A first look at the preprocessor
|
 Index
|
 1.8 — Programs with multiple files
|
Hi
I tried to use the add() function in a Class.
But I got this error :
error LNK2019: unresolved external symbol “public: int __thiscall Class1::add(int,int)” (?add@Class1@@QAEHHH@Z) referenced in function _main
error LNK1120: 1 unresolved externals
I am using Microsoft Visual Studio 2008. Created a win32 Console Application.
I have put the 3 files below. Main.cpp Class1.cpp and Class1.h
There seems to be a problem Linking but I can not figure out why.
Thanks a lot for your help, greatly appreciate your help.
FILE: MAIN.cpp_____________________________________________
#include
#include
#include “Class1.h”
int main()
{
Class1 c;
int res=c.add(3, 4);
return 0;
}
_____________________________________________
FILE: Class1.cpp_____________________________________________
#include
class Class1
{
public:
int add(int x, int y)
{
return x + y;
}
};
_____________________________________________
FILE:Class1.h_____________________________________________
#ifndef CLASS1_H
#define CLASS1_H
class Class1
{
public:
int add(int x, int y); // function prototype
};
#endif
What I did was change the content of “add.h” by adding:
{
return x + y;
}
This way, I didn’t even need the add.cpp and it compiled just fine.
I think this is a compatibility error on behalf of Alex, since this guide was made for C++03 originally.
[...] Now, when the compiler is compiling main.cpp, it will know what add is. Using this method, we can give files access to functions that live in another file. However, as programs grow larger and larger, it becomes tedious to have to forward declare every function you use that lives in a different file. To solve that problem, the concept of header files was introduced. We discuss header files in the lesson on header files. [...]
[...] Now, when the compiler is compiling main.cpp, it will know what add is. Using this method, we can give files access to functions that live in another file. However, as programs grow larger and larger, it becomes tedious to have to forward declare every function you use that lives in a different file. To solve that problem, the concept of header files was introduced. We discuss header files in the lesson on header files. [...]
amazing i find this website to be very helpful
The following source for
main.cpp, add.cpp and add.h
worked using Microsoft Visual C++ Express 2010
I still don’t understand the first two lines in the add.h file.
and
My first impression was “ifndef” was a typo of “define” with “MATH_H” being a derivative of “math.h”. Apparently I’m wrong since it does in fact work. I just don’t understand the where and how of those two lines substance.
makes sense
I don’t understand. It looks like “define” scrambled.
I don’t understand. “math.h” would seem to make more sense. I surmise “MATH_H” is a Microsoft addon to the standard library though I would think that would require an include statement, not a define.
I see the next page, Section 1.10 has this to say:
To prevent this from happening, we use header guards, which are conditional compilation directives that take the following form:
SO, the “MATH_H” is merely some unique name to ensure the particular header file is not included more than once. I still don’t understand the
I now understand. The next page we go to (1.10) explained the
term. Essentially the entire header file is an IF statment. Because the term MATH_H has to be unique, we ask
if not defined then perform all of the statements after until the next
end of if statment.
Guess I need to quit posting and just read on. All will be explained in good time… Sorry!
Ok I question first is the Header suppose to be made separate from the .cpp? or is the header suppose to be included inside the .cpp?
I’m using Visual studio 2010
ok this is what I’ve been trying to do. I first start making a header by going to File – New – File – Visual C++ – Header File C (.h)
I input:
#ifndef ADD_H
#define ADD_H
int add(int x, int y); // function prototype for add.h
#endif
After that I save it as ADD_H
After that I make a new .Cpp (File – New – Win32 Console Applications)
then I input:
#include
#include “stdafx.h”
#include “add.h” // this brings in the declaration for add()
int main()
{
using namespace std;
cout << "The sum of 3 and 4 is " << add(3, 4) <.<
I take it you had the same prob I had did you come up with a link error or did it run fine? you didn’t specify
Ok I understand wat I did wrong I didn’t the header file that I made into
to .cpp file that i’m using it for
BUT! everything said it was fine but… when I tried to complied with an fail… this is the error code
1>—— Build started: Project: Lesson1.9, Configuration: Debug Win32 ——
1> Lesson1.9.cpp
1>c:\users\culex\documents\visual studio 2010\projects\lesson1.9\lesson1.9\add.h(5): error C2447: ‘{‘ : missing function header (old-style formal list?)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
P.S. I wrote more on my original but.. it cutted it off…
Wooooo!!!! I understand what I did wrong and was able to compile it correctly ^^
Ok so im using Microsoft visual c++ express 2010 and I followed the instructions completely and I came up with a link error.? so any help? here is what i have for add.h
#ifndef ADD_H
#define ADD_H
int add(int x, int y);
#endif
// this is what i have for main.cpp
#include “stdafx.h”
#include
#include “add.h”
int main()
{
using namespace std;
cout << "The sum of 3 and 4 is:" << add(3,4) <—— Build started: Project: add, Configuration: Debug Win32 ——
1> add.cpp
1>add.obj : error LNK2019: unresolved external symbol “int __cdecl add(int,int)” (?add@@YAHHH@Z) referenced in function _main
1>C:\Users\\add\Debug\add.exe : fatal error LNK1120: 1 unresolved externals
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
i did finish the program it just didn’t post it all.
what did i do wrong or why did i get a link error? thanks in advance. also thanks for being patient.
Ok… this is how to make header files on Microsoft Visual 2010 and insert it into your other projects
Part1: First of all you’ll have to create a header file first go to
File –
New –
File –
Visual C++ –
Header File – (The file will be named by itself but I recommend you change it to the name you want by going to save as)[Also there is no need to add ".h" when naming it, it will be inserted by itself]{Also I named my “add” so i’ll be using my name for example}
Part 2
Now to input the code
Like this:
#ifndef ADD_H
#define ADD_H
int add(int x, int y )
{
return x + y;
}
#endif
After that save and close project
Part 3 Your Project
Insert the code:
// Lesson1.9.cpp : Defines the entry point for the console application.
//
#include “stdafx.h”
#include
#include “add.h” // this brings in the declaration for add()
int main()
{
using namespace std;
cout << "The sum of 3 and 4 is " << add(3, 4) << endl;
return 0;
}
Save it and close
Part 4
Now I suggest you start making a header folder for you header files
Now find your header file and then copy
After that find your project you just saved
{Good chance it will be where mine is}[Documents-Visual Studio 2010-Projects]
After that enter your project and go to “Name of your project”
For example my project was named “Lesson 1.9″
{Projects- Lesson 1.9 – {When you see other head files insert it in there}
Ok now open your “Project” and on the right of screen there should be “Solution Screen” and under Headers add your header
AND BAM your done try running it if it doesn’t work all hope is lost for you.. I guess. Well I got mine to work :)
That worked perfectly thanks!!!
[...] KNOWLEDGE FOR THIS PROGRAM Header Files – How To Use Them Class – Data Structure Enumerated List Typedef Do/While Loop Passing a Value By Reference Constant [...]
#include
#include
#include “Dll.h”
#include “guid.h”
can u say the meaning of these header files
[...] [...]
[...] KNOWLEDGE FOR THIS PROGRAM Header Files – How To Use Them Class – What Is It? Do/While Loop Passing a Value By Reference Roman Numerals – How Do You Convert [...]
[...] KNOWLEDGE FOR THIS PROGRAM Header Files – How To Use Them Class – What Is It? How To Read Data From A File String – Getline Array – Cin.Getline Strcpy – Copy [...]
why would a header file include another header file?
arn’t you supose to just declare stuff in a header file.
in that case i don’t see why header file will even include another header file
so add.cpp is still needed? since we don’t need to write the actual code in the header add.h ….
#ifndef ADD_H_INCLUDED // I don’t know why it has that “ADD_H_INCLUDED
#define ADD_H_INCLUDED // same for this one and the #endif …. can I change it to other names??? say ADD_H_WHATEVERIWANTHERE???
int add(int x, int y);
#endif // ADD_H_INCLUDED
I tried it in Code::Blocks and it worked! XD
[...] – Line 01, 02 and 22 are part of the so-called “include guard”. When C++ includes this header file the first time the name PERSON_H is assigned to it. If you try to include this header file a second time, which would result in an error, C++ checks if the name PERSON_H already exists. Since PERSON_H already exists from the first include, the header file is not included again which prevents an error. – More on the include guard can be found here: learncpp [...]
[...] Now, when the compiler is compiling main.cpp, it will know what add is. Using this method, we can give files access to functions that live in another file. However, as programs grow larger and larger, it becomes tedious to have to forward declare every function you use that lives in a different file. To solve that problem, the concept of header files was introduced. We discuss header files in the lesson on header files. [...]
[...] the lesson on header files, you learned that you can put functions inside header files in order to reuse them in multiple [...]
As an FYI to anyone else having issues with Visual Studio 2012 and having issues with with building:
If precompiled headers are turned on, you *must* have #include “stdafx.h” as the first include. Otherwise your includes don’t work.
See this link for more info:
http://en.wikipedia.org/wiki/Precompiled_header#stdafx.h
[...] this does yet, but just have faith and keep compiling’. It’s tricky to explain, but more info here if [...]
[...] have a simple program, which I copied exactly from the example in http://www.learncpp.com/cpp-tutorial/19-header-files/ because I’m learning how to make c++ programs with multiple [...]
add.cpp must contain the “add.h” header, otherwise the declaration wont be found, happened to me
the definition*