Should every C or C++ file have an associated header file?

问题

Should every .C or .cpp file should have a header (.h) file for it?

Suppose there are following C files :

1. Main.C

2. Func1.C

3. Func2.C

4. Func3.C

where main() is in Main.C file. Should there be four header files

1. Main.h

2. Func1.h

3. Func2.h

4. Func3.h

Or there should be only one header file for all .C files?

What is a better approach?

回答1:

For a start, it would be unusual to have a main.h since there's usually nothing that needs to be exposed to the other compilation units at compile time. The main() function itself needs to be exposed for the linker or start-up code but they don't use header files.

You can have either one header file per C file or, more likely in my opinion, a header file for a related group of C files.

One example of that is if you have a BTree implementation and you've put add, delete, search and so on in their own C files to minimise recompilation when the code changes.

It doesn't really make sense in that case to have separate header files for each C file, as the header is the API. In other words, it's the view of the library as seen by the user. People who use your code generally care very little about how you've structured your source code, they just want to be able to write as little code as possible to use it.

Forcing them to include multiple distinct header files just so they can create, insert into, delete from, and search, a tree, is likely to have them questioning your sanity :-)

You would be better off with one btree.h file and a single btree.lib file containing all of the BTree object files that were built from the individual C files.

---

Another example can be found in the standard C headers.

We don't know for certain whether there are multiple C files for all the stdio.h functions (that's how I'd do it but it's not the only way) but, even if there were, they're treated as a unit in terms of the API.

You don't have to include stdio_printf.h, stdio_fgets.h and so on - there's a single stdio.h for the standard I/O part of the C runtime library.

回答2:

1. Header files are not mandatory.

2. #include simply copy/paste whatever file included (including .c source files)

3. Commonly used in real life projects are global header files like config.h and constants.h that contains commonly used information such as compile-time flags and project wide constants.

4. A good design of a library API would be to expose an official interface with one set of header files and use an internal set of header files for implementation with all the details. This adds a nice extra layer of abstraction to a C library without adding unnecessary bloat.

5. Use common sense. C/C++ is not really for the ones without it.

回答3:

I used to follow the "it depends" trend until I realized that consistency, uniformity and simplicity are more important than saving the effort to create a file, and that "standards are good even when they are bad".

What I mean is the following: a .cpp/.h pair of files is pretty much what all "modules" end up anyway. Making the existing of both a requirement saves a lot of confusion and bad engineering.

For instance, when I see some interface of something in a header file, I know exactly where to search for / place its implementation. Conversely, if I need to expose the interface of something that was previously hidden in .cpp file (e.g. static function becoming global), I know exactly where to put it.

I've seen too many bad consequences of not following this simple rule. Unnecessary inline functions, breaking any kind of rules about encapsulation, (non)separation of the interface and implementation, misplaced code, to name a few -- all due to the fact that the appropriate sibling header or cpp file was never added.

So: always define both .h and .c files. Make it a standard, follow it, and safely rely on it. Life is much simpler this way, and simplicity is the most important thing in software.

回答4:

Generally it's best to have a header file for each .c file, containing the declarations for functions etc in the .c file that you want to expose. That way, another .c file can include the .h file for the functions it needs, and won't need to be recompiled if a header file it didn't include got changed.

回答5:

Generally there will be one .h file for each .c/.cpp file.

回答6:

It depends. Usually your reason for having separate .c files will dictate whether you need separate .h files.

回答7:

Bjarne Stroustrup Explains it beautifully in his book "The C++ Programming Language"....

The single header style of physical partitioning is most useful when the program is small and its parts are not intended for separate use. When namespaces are used, the logical structure of the program can still be explained in a single header file.

For larger Programs, the single header file approach is unworkable in a conventional file-based development environment. A change to the common header forces recompilation of the whole program, and updates of that single header by several programmers are error prone. Unless strong emphasis is placed on programming styles relying heavily on namespaces and classes, the logical structure deteriorates as program grows.

An alternative physical organization lets each logical module have its own header defining the facilities it provides. Each .c file then has a corresponding h. file specifying what it provides(its interface). Each .c module includes its own .h file and usually also other .h files that specifies what it needs from other modules in order to implement the services advertised in its interface. This physical organization corresponds to the logical organization of a module. The multiple header approach makes it easy to determine the dependencies. The single header approach forces us to look at every declarations used by any module and decide if its relevant. The simple fact is that maintenance of a code is invariably done with incomplete information and from a local perspective. The better localization leads to less information to compile a module and thus faster compilation..

回答8:

I like putting interfaces into header files and implementation in cpp files. I don't like writing C++ where I need to add member variables and prototypes to the header and then the method again in the C++. I prefer something like:

module.h

struct IModuleInterface : public IUnknown
{
    virtual void SomeMethod () = 0;
}

module.cpp

class ModuleImpl : public IModuleInterface,
                   public CObject // a common object to do the reference
                                              // counting stuff for IUnknown (so we
                                              // can stick this object in a smart 
                                              // pointer).
{
    ModuleImpl () : m_MemberVariable (0)
    {
    }

    int m_MemberVariable;

    void SomeInternalMethod ()
    {
        // some internal code that doesn't need to be in the interface
    }

    void SomeMethod ()
    {
        // implementation for the method in the interface
    }

    // whatever else we need
};

I find this is a really clean way of separating implementation and interface.

回答9:

There is no better approach, only common and less common cases.

The more common case is when you have a class/function interface to declare/define. It's better to have only one .cpp/.c with the definitions, and one header for the declarations. Giving them the same name makes easy to understand that they are directly related.

But that's not a "rule", that's the common way and the most efficient in almost all cases.

Now in some cases( like template classes or some tiny struct definition ) you'll not need any .c/.cpp file, just the header. We often have some virtual class interface definition in only a header file for example, with only virtual pure functions or trivial functions.

And in other rare cases (like an hypothetical main.c/.cpp file) if wouldn't be always required to allow code from external compilation unit to call the function of a given compilation unit. The main function is an example (no header/declaration needed), but there are others, mostly when it's code that "connect all the other parts together" and is not called by other parts of the application. That's very rare but in this case a header make no sense.

回答10:

It's all about what code needs to be aware of what other code. You want to reduce the amount other files are aware of to the bare minimum for them to do their jobs.

They need to know that a function exists, what types they need to pass into it, and what types it will return, but not what it's doing internally. Note that it's also important from the programmers point of view to know what those types actually mean. (e.g which int is the row, and which is the column) but the code itself doesn't care. This is why naming the function and parameters sensibly is worthwhile.

As others have said, if there's nothing in a cpp file worth exposing to other parts of the code, as is normally the case with main.c, then there's no need for a header file.

It's occasionally worth putting everything you want to expose in a single header file (e.g, Func1and2and3.h), so that anything that knows about Func1 knows about Func2 as well, but I'm personally not keen on this, as it means that you tend to load a hell of a lot of junk along with the stuff you actually want.

Summary: Imagine that you trust that someone can write code and that their algorithms, design, etc. are all good. You want to use code they've written. All you need to know is what to give them to get something to happen, what you should give it to, and what you'll get back. That's what needs to go in the header files.

回答11:

If your file exposes an interface - that is, if it has functions which will be called from other files - then it should have a header file. Otherwise, it shouldn't.

回答12:

As already noted, generally, there will be one header (.h) file for each source (.c or .cpp) file.

However, you should look at the cohesiveness of the files. If the various source files provide separate, individually reusable sets of functions - an ideal organization - then you should certainly have one header per file. If, however, the three source files provide a composite set of functions (that is too big to fit into one file), then you would use a more complex organization. There would be one header for the external services used by the main program - and that would be used by other programs needing the same services. There would also be a second header used by the cooperating source files that provides 'internal' definitions shared by those files.

(Also noted by Pax): The main program does not normally need its own header - no other source code should be using the services it provides; it uses the services provided by other files.

回答13:

If you want your compiled code to be used from another compilation unit you will need the header files. There are some situations for which you do now need/want to have a headers.

The first such case are main.c/cpp files. This class is not meant to be included and as such there is no need for a header file.

In some cases you can have a header file that defines behavior of a set of different implementations that are loaded through a dll that is loaded at runtime. There will be different set of .c/.cpp files that implement variations of the same header. This can be common in plugin systems.

回答14:

Generally cpp/c files are for implementation and h/hpp (hpp are not used often) files are for header files (prototypes and declarations only). Cpp files don't always have to have a header file associated with it but it usually does as the header file acts like a bridge between cpp files so each cpp file can use code from another cpp file.

One thing that should be strongly enforced is the no use of code within a header file! There's been too many times where header files break compiles in any size project because of redefinitions. And that's simply when you include the header file in 2 different cpp files. Header files should always be designed to be included multiple times as well. Cpp files should never be included.

回答15:

In general, I don't think there is any explicit relationship between .h and .c files. In many cases (probably most), a unit of code is a library of functionality with a public interface (.h) and an opaque implementation (.c). Sometimes a number of symbols are needed, like enums or macros, and you get a .h with no corresponding .c and in a few circumstances, you will have a lump of code with no public interface and no corresponding .h

in particular, there are a number of times when, for the sake of readability, the headers or implementations (seldom both) are so big and hairy that they end up being broken into many smaller files, for the sake of the programmer's sanity.

来源：https://stackoverflow.com/questions/608631/should-every-c-or-c-file-have-an-associated-header-file