What should go into an .h file?

When dividing your code up into multiple files just what exactly should go into an .h file and what should go into a .cpp file?

Header files (.h) are designed to provide the information that will be needed in multiple files. Things like class declarations, function prototypes, and enumerations typically go in header files. In a word, "definitions".

Code files (.cpp) are designed to provide the implementation information that only needs to be known in one file. In general, function bodies, and internal variables that should/will never be accessed by other modules, are what belong in .cpp files. In a word, "implementations".

The simplest question to ask yourself to determine what belongs where is "if I change this, will I have to change code in other files to make things compile again?" If the answer is "yes" it probably belongs in the header file; if the answer is "no" it probably belongs in the code file.

Fact is, in C++, this is somewhat more complicated that the C header/source organization.

##What does the compiler see?

The compiler sees one big source (.cpp) file with its headers properly included. The source file is the compilation unit that will be compiled into an object file.

##So, why are headers necessary?

Because one compilation unit could need information about an implementation in another compilation unit. So one can write for example the implementation of a function in one source, and write the declaration of this function in another source needing to use it.

In this case, there are two copies of the same information. Which is evil...

The solution is to share some details. While the implementation should remain in the Source, the declaration of shared symbols, like functions, or definition of structures, classes, enums, etc., could need to be shared.

Headers are used to put those shared details.

Move to the header the declarations of what need to be shared between multiple sources

##Nothing more?

In C++, there are some other things that could be put in the header because, they need, too, be shared:

• inline code
• templates
• constants (usually those you want to use inside switches...)

Move to the header EVERYTHING what need to be shared, including shared implementations

##Does it then mean that there could be sources inside the headers?

Yes. In fact, there are a lot of different things that could be inside a "header" (i.e. shared between sources).

• Forward declarations
• declarations/definition of functions/structs/classes/templates
• implementation of inline and templated code

It becomes complicated, and in some cases (circular dependencies between symbols), impossible to keep it in one header.

##Headers can be broken down into three parts

This means that, in an extreme case, you could have:

• a forward declaration header
• a declaration/definition header
• an implementation header
• an implementation source

Let's imagine we have a templated MyObject. We could have:

// - - - - MyObject_forward.hpp - - - - 
// This header is included by the code which need to know MyObject
// does exist, but nothing more.
template<typename T>
class MyObject ;

.

// - - - - MyObject_declaration.hpp - - - - 
// This header is included by the code which need to know how
// MyObject is defined, but nothing more.
#include <MyObject_forward.hpp>

template<typename T>
class MyObject
{
   public :
      MyObject() ;
   // Etc.
} ;

void doSomething() ;

.

// - - - - MyObject_implementation.hpp - - - - 
// This header is included by the code which need to see
// the implementation of the methods/functions of MyObject,
// but nothing more.
#include <MyObject_declaration.hpp>

template<typename T>
MyObject<T>::MyObject()
{
   doSomething() ;
}

// etc.

.

// - - - - MyObject_source.cpp - - - - 
// This source will have implementation that does not need to
// be shared, which, for templated code, usually means nothing...
#include <MyObject_implementation.hpp>

void doSomething()
{
   // etc.
} ;

// etc.

##Wow!

In the "real life", it is usually less complicated. Most code will have only a simple header/source organisation, with some inlined code in the source.

But in other cases (templated objects knowing each others), I had to have for each object separate declaration and implementation headers, with an empty source including those headers just to help me see some compilation errors.

Another reason to break down headers into separate headers could be to speed up the compilation, limiting the quantity of symbols parsed to the strict necessary, and avoiding unecessary recompilation of a source who cares only for the forward declaration when an inline method implementation changed.

##Conclusion

You should make your code organization both as simple as possible, and as modular as possible. Put as much as possible in the source file. Only expose in headers what needs to be shared.

But the day you'll have circular dependancies between templated objects, don't be surprised if your code organization becomes somewhat more "interesting" that the plain header/source organization...

^_^

in addition to all other answers, i will tell you what you DON'T place in a header file:
using declaration (the most common being using namespace std;) should not appear in a header file because they pollute the namespace of the source file in which it is included.

What compiles into nothing (zero binary footprint) goes into header file.

Variables do not compile into nothing, but type declarations do (coz they only describe how variables behave).

functions do not, but inline functions do (or macros), because they produce code only where called.

templates are not code, they are only a recipe for creating code. so they also go in h files.

In general, you put declarations in the header file and definitions in the implementation (.cpp) file. The exception to this is templates, where the definition must also go in the header.

This question and ones similar to it has been asked frequently on SO - see https://stackoverflow.com/questions/333889/in-c-why-have-header-files-and-cpp-files and https://stackoverflow.com/questions/280033/c-header-files-code-separation for example.

Header (.h)

• Macros and includes needed for the interfaces (as few as possible)
• The declaration of the functions and classes
• Documentation of the interface
• Declaration of inline functions/methods, if any
• extern to global variables (if any)

Body (.cpp)

• Rest of macros and includes
• Include the header of the module
• Definition of functions and methods
• Global variables (if any)

As a rule of thumb, you put the "shared" part of the module on the .h (the part that other modules needs to be able to see) and the "not shared" part on the .cpp

PD: Yes, I've included global variables. I've used them some times and it's important not to define them on the headers, or you'll get a lot of modules, each defining its own variable.

Mainly header file contain class skeleton or declaration (does not change frequently)

and cpp file contains class implementation (changes frequently).

Your class and function declarations plus the documentation, and the definitions for inline functions/methods (although some prefer to put them in separate .inl files).

the header file (.h) should be for declarations of classes, structs and its methods, prototypes, etc. The implementation of those objects are made in cpp.

in .h

    class Foo {
    int j;
    
    Foo();
    Foo(int)
    void DoSomething();
}

I'd expect to see:

• declarations
• comments
• definitions marked inline
• templates

the really answer though is what not to put in:

• definitons (can lead to things being multiply defined)
• using declarations/directives (forces them on anyone including your header, can cause nameclashes)

The header Defines something but doesn't tell anything about the implementation. ( Excluding Templates in this "metafore".

With that said, you need to divide "definitions" into sub-groups, there are, in this case, two types of definitions.

• You define the "layout" of your strucutre, telling only as much as is needed by the surrounding usage groups.
• The definitions of a variable, function and a class.

Now, I am of course talking about the first subgroup.

The header is there to define the layout of your structure in order to help the rest of the software use the implementation. You might want to see it as an "abstraction" of your implementation, which is vaughly said but, I think it suits quite well in this case.

As previous posters have said and shown you declare private and public usage areas and their headers, this also includes private and public variables. Now, I don't want to go into design of the code here but, you might want to consider what you put in your headers, since that is the Layer between the end user and the implementation.

• Header files - shouldn't change during development too often -> you should think, and write them at once (in ideal case)
• Source files - changes during implementation