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1.9 — Header files

Headers, and their purpose

As programs grow larger and larger (and include more files), it becomes increasingly tedious to have to forward declare every function you want to use that lives in a different file. Wouldn’t it be nice if you could put all your declarations in one place?

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, sometimes known as an include file. Header files usually have a .h extension, but you will sometimes see them with a .hpp extension or no extension at all. The purpose of a header file is to hold declarations for other files to use.

Using standard library header files

Consider the following program:

This program prints “Hello, world!” to the console using cout. However, this 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’re requesting that all of the content from the header file named “iostream” be copied into the including file. That makes the content from the header file available for use in our code file.

Keep in mind that header files typically only contain declarations. They do not define how something is implemented. So if cout is only declared in the “iostream” header file, where is it actually defined? It is implemented in the C++ runtime support library, which is automatically linked into your program during the link phase.

Consider what would happen if the iostream header did not exist. Wherever you used std::cout, you would have to manually copy in all of the declarations related to std::cout into the top of each file that used it! This would require knowledge of what’s relevant and what’s not. It’s much easier to just #include iostream!

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).

We 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). Header guards prevent a given header file from being #included more than once in the same file.

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:

In order to use this header file in main.cpp, we have to include it.

main.cpp that includes add.h:

add.cpp stays the same:

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 function prototype for add(), this prototype is now being used as a forward declaration of add()!

Consequently, our program will compile and link correctly.

Note: When you #include a file, the entire content of the included file is inserted at the point of inclusion.

If you get a compiler error about add.h not being 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”.

If you get a linker error about add() not being defined, make sure you’ve included add.cpp in your project so it compiles properly!

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 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.

Header files may include other header files. However, you should never rely on this. Always #include all of the header files you need to use from each .cpp file.

Rule: Each .cpp file should explicitly #include all of the header files it needs to compile.

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, 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 <iostream.h> do not need to be rewritten, and newer programs can #include <iostream>.

When you include a header file from the standard library, make sure 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.

In addition, many of the libraries inherited from C that were 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.

However, when you write your own header files, you should give them all 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

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:

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.

In Visual Studio, you can 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 your include directories there.

In Code::Blocks, go to the Project menu and select “Build Options”, then the “Search directories” tab. Add your include directories there.

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.

Can I put function or variable definitions in a header file?

You shouldn’t, as it may cause linker errors. We’ll explain why in the upcoming lesson on header guards.

Declarations are okay.

Header file best practices

Here are a few best practices for creating your own header files.

  • Always include header guards.
  • Do not define variables in header files unless they are constants. Header files should generally only be used for declarations.
  • Do not define functions in header files.
  • 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.
  • Give your header files the same name as the source files they’re associated with (e.g. grades.h goes with grades.cpp).
  • Try to minimize the number of other header files you #include in your header files. Only #include what is necessary.
  • Do not #include .cpp files.
1.10 -- A first look at the preprocessor
Index
1.8a -- Naming conflicts and the std namespace

355 comments to 1.9 — Header files

  • I actually figured it out. It appears that within project explorer, you can right click the file and select properties. It’s important to make sure that under the Build tab, the Debug and Release boxes are checked. Apparently, when I created the file I missed the step to check these boxes. All compiles as expected now.

  • So, I have been messing with the CodeBlocks Editor on Gentoo and the prototype definition doesn’t appear to pull in the add.cpp file. For this reason, when building, the build fails stating that add(int, int) is not defined. Now I’ve tried removing the header all together and place the prototype declaration directly above main.cpp and the build still fails stating that add(int, int) is not defined.

    I’m able to build from the command line by specifying:

    This process does not fail and creates the add.o executable, which runs correctly. Am I correct to assume this could be a bug in Codeblocks? So far I’m also pretty impressed with the tutorial. Thanks for your guidance Alex.

  • retro-starr

    I figured it out! So you need main.cpp, add.cpp and add.h all in the same folder because the main is including add.h which calls add.cpp to define what “int add()” is! Now my question is, how can you use files in different folders?

  • Luke

    Hi Alex - I’m having some issues with this -

    my code is identical to the website - and I have no add.cpp in this project
    when my code is this

    I get the error “add(int, int)” not defined

    but when I change to this

    it works

    I am guessing that this is because the header file only is added when ADD_H is included somewhere in main due to the if statements - but what happens when there is more than one function prototype in the library?

    • Alex

      I’m not sure why this is working for you. ADD_H should be meaningless.

      The header guard ensures that the header is not included more than once (that would lead to redefinition errors).

      You are allowed to have more than one function prototype in a header file.

  • Array

    Hey Alex, thanks for the awesome tutorial, i had major problems and confusions on how to actually write the header files, but now i understand it more. 🙂

  • Bladtman242

    Why actually make header files? why not just define the functions you need after main?

    • Alex

      In trivial examples like the ones we’re doing right now, there’s really no need for header files.

      However, once you get into writing more complicated things, you’ll use header files everywhere. They allow you to import a set of declarations to multiple source (.cpp) files with minimal effort.

      An analogy might be useful here: imagine every time you wanted to use std::cout, you had to copy every thing related to std::cout into the top of your .cpp. For every .cpp file. That would get annoying, right? Isn’t it much easier to just #include ?

  • CJ

    Hey Alex, these tutorials are awesome! :] I’m learning a whole lot.
    Thank you for making them.

    This is probably something really obvious, but I am really confused.
    I’m using Microsoft Visual C++ 2008 Edition, and when I add the add.cpp file, it automatically gives me an add.h file as well. But when I open it, in the tab it says add.h [Design] and there’s just a box named add, and I can’t put code anywhere.

    I realized that there is a seperate folder specifically for Header files, but I couldn’t create an add.h file in it, because the name is already used. So I created one with a different name (with a .h extension), and it gave me a .cpp and a .h for that, too. And the .h is still a design box.

    I’m probably going to feel really dumb after this, but where am I suppose to put the header code? D:
    I have like 8 files and I only need to use three…so confusing >:
    Also, the .cpp files had #include “theirname.h” on the top,
    should I leave that there?

  • nryoung

    Great tutorial and thanks for taking the time to respond to people’s questions. I have one myself. I am using Code Blocks 8.02 with Ubuntu 9.04.

    main.cpp

    here is my add.cpp

    here is my add.h

    Now every time I try to build this program I get this error.

    any help with what I am doing wrong would be great. Thanks

    • I don’t see anything wrong with the code. Sounds like an OS-specific issue. Perhaps ask on a Ubuntu forum?

    • bansidhar

      For gcc compiler you may require to declare the function as extern “C” or the linker won’t link it properly. Try the following header file

  • rob s

    what does “int” stand for or represent?

  • Justin

    How do i create a header in Dev C++? your help would be appreciated

    • Tyler

      In order to create a header file in dev-cpp, in the upper-left corrner there is a ‘new’ button (button with a pic of a blank peice of paper on it), click on this. once the file is created, right click the filename in the left hand column, and select rename. Enter the new name for the file and put .h at the end of the name. This will state it as being a header file and thus cuase the linker to treat it as such.

      -Tyler

  • Michael

    WOW!!!!!!! Finally I get it. I had to have a add.cpp file and a main.cpp file and a add.h file! I’m sure I’ll be reading all these tutorials over again because I can never understand everything the first time around even at a snails pace. Just knowing that I finally figured it out though is the best part and seeing my mistake from the last section makes it all the better. Thanks for the tutorials 🙂 I’ve gotten this far after my first day. But I’m sure I’ll be reading these tutorials over and over to fully understand everything! Thanks again.

    • csvan

      It is always a good idea to go over good, rich materials twice. I believe that the things you learn later on (not specifically in this tutorial, but anything) will help you better understand the basics when you go over them again. Generally, as you build a programmer mindset and learn to think, reason and think accordingly, I believe you will also find it easier to put things into perspective, so you might finally understand basic things you never understood the first thing you saw them.

  • 19bk87

    excellent tutorials(thank you!) but I have 3 questions.

    I don’t get the header file part.
    In the example you use add.cpp as the declaration for add(). In add.h you put in a forward declaration for add(). But why can’t you define add() in the header file, isn’t that more efficient since you don’t need add.cpp anymore and thus less files = less work.

    Second question (assuming you answered my first question):
    You use add.cpp as a declaration file for add(), is it common to use a .cpp file with more then one declarations? Meaning multiple function declarations in one .cpp file (seems more convenient) or do you have to use multiple cpp files for multiple function declarations?

    My last question:

    Will these tutorials lead to the explaination of actually using your knowledge of c++ in developing programs? I think that’s really important because: okay I learned c++, now how do I use my knowledge? Like this tutorial gives you an assignment to make a program in the end.

    Many thanks, keep it up!

    • You _could_ define add() in the header file if you wanted, but this is generally not done (unless the function is trivial, like 1 or 2 statements). Header files are generally used for prototyping functions and declaring classes, and then those things are actually implemented in .cpp files.

      It is VERY common to have multiple functions in one .cpp file. Typically a .cpp file will contain a whole set of related function (eg. math.cpp will contain functions to do square roots, exponents, and other mathy things).

      To address your last question, no, not really. It’s really up to you to figure out how to apply what you learn. At some point I’d love to go back and add that, but I haven’t had time. 🙁

  • sra1

    Never mind Alex I got it.
    This tutorial is really really helpful.
    I wish we have a tutorial for java too, please tell me if you have one…
    Thanks again.

  • SRA1

    Hi Alex,

    This tutorial is awesome and I cant believe I’m finally learning C++.
    I have a question. I was trying to compile the code you gave, it sounds crazy but should I add a header file in the header files folder or source files folder. Anyways I tried creating in both but none worked. I created the header file with .h extension.

    and the header file was

    I had an error:
    1>---- Build started: Project: HelloWorld, Configuration: Debug Win32 ----
    1>Linking…
    1>MSVCRTD.lib(crtexe.obj) : error LNK2019: unresolved external symbol _main referenced in function ___tmainCRTStartup
    1>C:\Documents and Settings\sravan\My Documents\Visual Studio 2008\Projects\HelloWorld\Debug\HelloWorld.exe : fatal error LNK1120: 1 unresolved externals
    1>Build log was saved at “file://c:\Documents and Settings\sravan\My Documents\Visual Studio 2008\Projects\HelloWorld\HelloWorld\Debug\BuildLog.htm”
    1>HelloWorld - 2 error(s), 0 warning(s)
    ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

    Please help I am badly stuck here.
    Thanks in advance.

  • Juan

    Amazing tutorials! It actually all makes sense LOL -- now do one on the economy for the new U.S. administration LOL Thank you very, very much for making these tutorials!!! Alex, is there a cd I can buy of these tutorials from you? Or a book you wrote? I would donate, but who knows, next week the website could be down 🙁

    • Sorry, there is currently no offline version of the site. It’s something I’d like to do but I just don’t have the time to put it together right now.

      This website has been running since May 2007, so we’ve been up for over a year and a half. As long as the meager advertising revenue exceeds the cost of hosting the site, it’ll probably be here. 🙂

  • Julian

    Header files sound really cool, because I could make a header file that had a function in it that would calculate the angle of the sun depending on the time/day of year etc, and put it on the internet for people to download 😛 (along with some instructions as to what functions to call in the main.cpp)

    I’m not advanced enough yet to calculate the angle of the sun but that would still be cool 😀

    I understand this whole tutorial.. except I couldn’t work out how to create a “.h” file :S

    I’m using Code::Blocks and when I create a new file it makes me save it as a “.cpp”

    Edit:
    Woooaahh nevermind, I just figured it out. 😀

    Awesome tutorials btw 🙂

    • Generally it’s a better idea to define your functions in .cpp files. If you wanted, you could distribute both the .cpp (containing the code) and a .h (containing a prototype for people to #include).

  • Tyler

    sorry about that last comment i figured out what i was doing wrong just as i finished posting it, i forgot to include the “using namespace std” thing. but now i have another problem: everything else comiles fine but i keep getting a linker error. it says that i have an undefined referance to add. i don’t know what i’m doing wrong.i’m using the same code only i fixed the “name space” issue and filled in all the missing ;’s at the end of some of the lines.

    • Did you actually define the add() function? If you defined it in another .cpp file (eg. add.cpp), are you sure you are compiling that .cpp file into your program? You probably need to add it to your project or compile command line.

      • Tyler

        so I need my main program, a header file, and a sepreate .cpp file to define add? and how would I include the .cpp file for the add function? would I just place it affter my ADD.h header file in the
        “# include” list? ex.

        • tyler

          so I made a .cpp file defineing add and included it in my program ( the .cpp file defining add is the same as the one in this lesson ). but when I tryed to compile the program, it gave me an error. it was complaining about a linker error to win16 or somthing like that. to be honest, i havent got the slihtest clue what it means by win16 ( exept that I think it might have something to do the windows operating system files???).

        • If you’re using a MS compiler, on the left hand side you should see your project and a list of all the files include in it. If you right click on “Source Files” and choose “add” you can add new files or existing files to your project. That’s where you need to add the file.

          Using #include with .cpp is almost never something you’ll have need of.

  • Tyler

    for some reason my compiler ( i’m useing bloodshed versoin 4.somthing )keeps saying that cout and cin are undeclared. i have wrote several programms before including fixeing Fluxx’s rectangle area calculator and they all have compiled and ran smoothly up until now. i have included both iostream and my own header file ( ADD.h ).here is my code:

    my header file for the add function is identical to yours ( only i named the file ADD.h instead of add.h ) .i have tryed rearanging them, retyping them, even writeing a whole new program. i’ve been stuck on this for days and i can’t see what i’m doing wrong. please help me Alex.

  • Richard

    Aaah. Nice one. Yes that worked a charm. Thank you Alex.

    So the compiling syntax is:

    C++ mainfile.cc Inclue1.cc Include2.cc … IncludeN.cc -o out.exe

    where InlcudeN are the N additional header files to be read in mainfile.

    Is that a generic compiling syntax, or is this just specific to Cygwin???

    Thanks again!

  • Spock

    Hey,

    Thoroughly enjoying this tutorial too! Also the best I’ve seen so far online.

    I’m having some trouble compiling too!! I’m using a Cygwin bash shell

    I have 3 programs:

    The first is main.cc:

    The next is add.cc

    and lastly I have add.h

    All files are contained in the same folder that I have been using for all my other programs.
    I have been compiling previous programs using c++ filename.cc -o filename.exe

    The error message I get reads:

    “MYFOLDER/cc9iZFXR.0:main.cc(.text+0x13b): undefined reference to ‘add(int,int)’
    collect2: Id returned 1 exit status

    ****************

    If you can provide any advice here it would be greatly appreciated!

    Rich

    PS I’ve tried changing filename.cc to filename.cpp etc.

    • It looks to me like the linker can’t resolve the function call to add(), which means that you’re probably not compiling in add.cc properly. If I remember correctly, you should be doing something like this:

      c++ main.cc add.cc -o out.exe

  • funmi

    thanks for your tutorials. I mean not even c++ for dummies can touch this with the way u respond. 🙂 its helping me a great deal.

  • Alex

    Thanks for this tutorial, I have a question:
    I the second graphic, you showed the work of the compiler and linker with source files,
    Why there is not an arrow from add.cpp to add.h, since add.h is only declaration
    (which include function prototypes and variables) that add.cpp implemented?

    Thank you,

  • Niels

    In the actual add.cpp return (x + y) was stored.
    In add.h this sentence is not part of the code. How does the compiler/linker know that the sum of
    x + y is needed as the result?
    Niels

    • When main.cpp #includes add.h, the function prototype for add() is imported into main.cpp. The compiler uses this add() prototype to ensure anyone calling add() is doing so correctly.

      Once the compiler is satisfied no syntax or type-checking errors exist, the linker takes over. The linker’s job is to combine all of the individual .cpp files into a single unit (generally an executable or DLL), and ensure that all of the function calls resolve to an actual defined function.

      If you forgot to include add.cpp in your project, your project would still compile okay (because the compiler could use the prototype to do type checking), but it would fail in the linker stage because the linker would be unable to resolve the call to add() to a specific function.

      • Girish Shetty

        Thanks Alex! I had this doubt while reading the tutorial. Greatly explained.
        Foremost, thanks a ton for such a wonderful tutorial.

  • Kameshwar Rao K

    Hello Alex,
    First of all I would like to thank you very much for taking time and posting such a wonderful tutorial. You have mentioned namespaces, but coming from ‘C’ programming background, I never came across ‘namspaces’. Can you please explain the basics and what these namespaces are? Thank you once again.

    Kkamudu

  • daneil

    i used the codes that you posted in the tuturial which are

    and since im using visual C++ 2008 i added #include "stdafx.h" to the top of the code but it always comes up with these errors

    1>---- Build started: Project: hmmm, Configuration: Debug Win32 ----
    1>Compiling…
    1>hmmm.cpp
    1>c:my documentsvisual studio 2008projectshmmmhmmmhmmm.cpp(3) : fatal error C1083: Cannot open include file: ‘add.h’: No such file or directory
    1>Build log was saved at “file://c:My DocumentsVisual Studio 2008ProjectshmmmhmmmDebugBuildLog.htm”
    1>hmmm - 1 error(s), 0 warning(s)
    ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

    i have no clue what im doing wrong =/

    BTW so far the tuturials have been pretty good so thanks for taking your time to write it all

  • Daniel Passwater

    This is a great tutorial. Thanks for the time that you put into it.

    I’ve got a question: I’m working on a dialog in a solution (in VS 2005) that contains several projects. I’m trying to access one of the classes from another project, but I keep getting a linking error. I have done a #include “headerfile.h” and have added the path to my “Additional include directories” in the properties pages via the IDE. I’m unable to copy the .h file into my local directory, because it references a resource.h file located in it’s project. My dialog has a resource.h file of its own. Has anyone had a similar problem?

    • Generally you’ll get a linker error when you include all the appropriate header files but don’t include the actual definition of what’s in the header files. I’m guessing you need to include some of the .cpp files from the other project in the project you’re getting the linker error for.

  • Another interesting part of the tutorial. I learnt a lot about namespaces here. Thanks again for the great tutorial

  • Chase

    I use Bloodshed Dev-C++ and i keep geting these error messages while trying to compile:

    C:Dev-CppMakefile.win [Build Error] [Project1.exe] Error 1

    ld returned 1 exit status

    [Linker error] undefined reference to `add(int, int)’

    I copied the add header and saved it as a .h file in the same directory as the project. Has anyone else had this error? How can I fix it?

    • Chase

      Nevermind, I got it to work by changing the header from:

      add(int x, int y)

      to:

      add(int x, int y)
      { return x + y: }

      • TBM

        The problem Chase had here as do to the omission of the semi colon ” ; ” after his prototype for his header file and the reason it was fixed when he changed it was simply do to the fact that he defined it within the header itself instead of in a separate .cpp file.

        In other words his declaration should have looked like this in his header file:

        add(int x, int y); <-- notice the semi colon.

        When changed to:

        add(int x, int y)
        {
        return x + y;
        }

        It was no longer a prototype but a definition, meaning that the add.cpp file was no longer needed do to the fact that the definition is also now held in the header file. At least this is my understanding so far.

        Oh, and yes I know his change uses a colon instead of a semi colon, I’m just guessing that it was a typo.

    • I had this problem too and couldn’t understand what was being wrong (I was trying out the IDE Geany).

      Until I realised that Geany didn’t compile both of the two .cpp files, and link them together.

      When I manually compiled/linked them with g++ it worked just fine.

  • Ewinz87

    Hmm, first of all great job with the tutorials. I have finally decided to familiarize myself with C++ and I really like it so far, however I’m a bit confused with this and have a question.

    So my question is the following: Should be include the header file for every source file (in this case I’m thinking about add.cpp or can we only use it for one source file. Also, isn’t it going to scream at us for “redefining?” the functions?

    Edit: I just found out. The header guard prevent that from happenning. As for using it in all the source files makes it easier and saves times?

    Excuse my english, it is not my native tongue. 😛

    • As you’ve noted, the header guards prevent the header files from being included multiple times. It’s a sloppy way of doing things, in my opinion, but it works and it’s the standard way to do it.

      Ideally, you should include your header file in every .cpp file in which you use the stuff it declares. However, in practice, since header files can include other header files, this may not be strictly necessary. For example, you might have a class named Foo that uses strings, so foo.h will include string.h. foo.cpp (which also uses strings) should ideally include both foo.h and string.h, but since foo.h already includes string.h, if you only include foo.h you will still be able to use strings.

  • jestuart

    Ok I have an issue here with the headers. I deviated from the last lesson slighy but it is close to the same. Please tell me where I went wrong with Header Files.

    Here is Program1.cpp

    Here is my output from VS2008:

    1>---- Build started: Project: Program1, Configuration: Debug Win32 ----
    1>Compiling…
    1>Program1.cpp
    1>c:\documents and settings\jstuart\my documents\visual studio 2008\projects\program1\program1\program1.cpp(18) : error C3861: ‘Multiply’: identifier not found
    1>c:\documents and settings\jstuart\my documents\visual studio 2008\projects\program1\program1\program1.cpp(19) : error C3861: ‘Add’: identifier not found
    1>c:\documents and settings\jstuart\my documents\visual studio 2008\projects\program1\program1\program1.cpp(20) : error C3861: ‘Subtract’: identifier not found
    1>MATH.cpp
    1>c:\documents and settings\jstuart\my documents\visual studio 2008\projects\program1\program1\math.cpp(28) : fatal error C1010: unexpected end of file while looking for precompiled header. Did you forget to add ‘#include “stdafx.h”‘ to your source?
    1>Generating Code…
    1>Build log was saved at “file://c:\Documents and Settings\jstuart\My Documents\Visual Studio 2008\Projects\Program1\Program1\Debug\BuildLog.htm”
    1>Program1 - 4 error(s), 0 warning(s)
    ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

    • The problem might be that you named your math.h file math.cpp. Try renaming your math.cpp to math.h and see if it fixes your issue. If that doesn’t work, try reposting your problem in the forum. The forums don’t have any many problems with posted programs as wordpress seems to.

      • jestuart

        Spot on that worked. I changed MATH.cpp to Math.h and it worked fine. Thanks for the tutorial and the response Alex.

        • Abdul

          First of all thank you Alex for the excellent tutorials.

          a questions please:

          from jestuart’s code; I thought he will at least need three files, one for the header, one for the actual function and then one for main e.g
          Math.h - which contains the forward declaration
          Math.cpp - which will contain the actual function definition
          main.cpp - which will reference the .h files

          here is my code for a similar operation; using code::block

          main.cpp

          #include <iostream>
          #include "math.h"

          int main()
          {
          using namespace std;
          cout << "3 + 4 = " << add(3, 4) << endl;
          cout << "3 - 4 = " << sub(3, 4) << endl;
          cout << "3 * 4 = " << multiply(3, 4) << endl;
          return 0;
          }

          math.cpp

          int add( int x, int y)
          {
          return x + y;
          }

          int sub(int x, int y)
          {
          return x - y;
          }

          int multiply(int x, int y)
          {
          return x * y;
          }

          math.h

          #ifndef MATH_H
          #define MATH_H

          int add(int x, int y);
          int sub(int x, int y);
          int multiply(int x, int y);

          #endif

          • isla

            thank you abdul for the code. i tried to run the code above, i am using codeblocks in Windows 7, but kept getting an error that says as follows:

            “undefined reference to `add(int, int)’
            “undefined reference to `sub(int, int)’
            “undefined reference to `multiply(int, int)’

            however, i was able to resolve this issue by adding this line of code

            inside the header file or the math.h file

            the following is the final code for the header (math.h) file:

            thank you for the nice tutorials everyone! 🙂

            • Alex

              It is bad form to #include .cpp files.

              It sounds to me like in this case, math.cpp wasn’t being compiled in the project. So instead, you have main.cpp #include math.h, which #includes math.cpp. This means the code from math.h and math.cpp get inserted into main.cpp. That defeats much of the point of having separate files.

              • Akash Aggarwal

                The problem here seems to be that math.cpp wasn’t added to the project correctly or not added at all. I ran into the same error while using command line when I did this:

                instead of:

      • jojo

        can you please explain that why we need to create add.h file when we have forward declaration?
        what is its main purpose?

        // This is start of the header guard.  ADD_H can be any unique name.  By convention, we use the name of the header file.
        #ifndef ADD_H
        #define ADD_H

        // This is the content of the .h file, which is where the declarations go
        int add(int x, int y); // function prototype for add.h -- don’t forget the semicolon!

        // This is the end of the header guard
        #endif

        • Alex

          In the case where we have just one function we need to access from another file, we’d probably just use a forward declaration.

          But in code of sufficient complexity, we often have many related functions, all of which need forward declarations to be used, and we need to use those functions in more than one place. Header files vastly simplify that process.

          For example, it’s much easier to say, “#include ” than add individual forward declarations for functions named add, subtract, multiple, divide, square root, exponent, circumference, etc…

          • @ALEX..Lol then we can make another cpp file(A) where all the other functions’ forward declaration has been made, and forward declare (A) in the main program….Header files just make the compilation slower as each time the program compiles,all the data from the header file is simply copypasted…What is the main advantage of a header file?lol
            thanx

            • Alex

              This won’t work -- you can’t forward declare a file. That’s what #includes are for.

              The main advantage of a header file is to import a set of declarations into any file that needs it. Header files typically contain forward declarations for functions defined in other files, as well as custom define types that need to be used (enums, classes, etc…).

          • By bad i mean i got it thanx

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