Header and cpp or just cpp files - best practise?

Question

Looking around at different code bases I see a variety of styles:

Class "interfaces" defined in header file and the actual impl in a cpp file. In this approach the headers look well defined and easy to read but the cpp files look confusing as it's just a list of methods.
The second approach i see is just to put everything in a single class cpp file. These class files contain the definition and actual method impls in the body of the class definition. This approach looks better to me (more like Java and c#).

Which style should I be using?

How are you going to access your class from another CPP file with the second? — James M, Feb 04 '12 at 11:53
Exactly. The second approach works only if you put everything in one gigantic cpp file. This is not a good idea. — Mr Lister, Feb 04 '12 at 11:55

score 8 · Accepted Answer · answered Feb 04 '12 at 11:57

For all but the simplest programs, style #2 is simply impossible. If you #include a .cpp file with function definitions from multiple other .cpp files, the definitions get added to multiple object files (.o / .obj) and the linker will complain about clashing symbols.

Use style #1 and learn to live with the confusion.

score 4 · Answer 2 · edited May 23 '17 at 12:12

4

The former - interfaces in header files and class bodies in implementation files. You'll find this causes you fewer problems when working on large systems.

In C++ why have header files and cpp files?

edited May 23 '17 at 12:12

Community

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answered Feb 04 '12 at 11:57

Trevor Tippins

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Peter · Answer 3 · 2013-01-09T17:15:59.893

Header (HPP):
The header includes the declarations of your code, particularly function declarations. Technically speaking classes are defined in header-files, but again, the member functions are just declared.

Code in other files will include just this header and retain all necessary information from there.

Implementation (CPP):
The implementation includes the definition of functions, member-functions and variables.

Rationale:

Header-files gives a developer (a external user of your code) a plain overview and just offers the external available code (i.e. easy to read, only the information necessary for users).
Header-files allow the compiler to check the implementation for correctness
Header-files allow the compiler to check external code for correctness
Header-files allow for seperate-compilation. You need to keep in mind. that in former times, computers doesn't have enough resources to keep everything in main-memory during a compilation process. Header files are small, while implementation files are big.

Use #style 1, even for simple programs. So you can learn easily to work with. That maybe look outated today, especially in background of modern Multi-Pass-Compilers. But seperate header-files are even today beneficial. Rumours about the next C++-Standard appeared, as far as I know something like symbol export ( Java or C#) will be possible. But don't nail me down on this!

Notes:
- member-functions which are defined inside a class are by default inline, normally you don't want this
- use always defined guards

score 0 · Answer 4 · edited Mar 21 '13 at 10:32

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If you are developing large project, you'll find the first approach helps you a lot. The second approach may help you in small project. As your project becomes larger, management of complexity is a big issue of software development, and the first approach turns out to be a better choice.

edited Mar 21 '13 at 10:32

K.C.

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answered Mar 21 '13 at 10:08

lilde90

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score 0 · Answer 5 · answered Feb 21 '15 at 09:59

What I do is:

write .cpp files, with the method names prefixed with the class name
in the .h file, create an empty class, with the appropriate name, then use a cogapp generator script, cog_addheaders.py, to insert the declarations, eg:
- .cpp file: WeightsPersister.cpp
- .h file: WeightsPersister.h

This way I get:

fast compilation (just needs to recompile the .cpp file, unless I change the class interface)
few issues with circular declarations
acceptably low tedious mindless manual work :-)

Vinayak Garg · Answer 6 · 2012-02-04T12:13:33.083

Since you tagged with c++, go for first style. I don't find it confusing, for a Java programmer, it may seem different, but in C++, you are always going to use this approach.

In fact in my favorite IDE (MSVS), I open header file, and cpp file side by side. Makes looking up prototypes, and class declaration easy.

And when you have a dozen classes; a dozen .h files, and another dozen .cpp file, will make your work simpler. Because, when you want just to see, what a class does, you just open relevant .h file, and take a look at class members, and maybe short comments. You don't need to wade through several lines deep code.

Conclusion : The style options you gave, are option only for a small code, typically single file, with very few methods etc. Otherwise, it is not even a option. (@Thomas has given the reason why #2 is not even a option)

score 0 · Answer 7 · answered Feb 04 '12 at 12:01

C++ doesn't use "interfaces" they use classes - base/derived classes. I use one file to define class/and its implementation methods if the project is small and separate files if the project is large. In java, I pack them up into one package then import it once in need.

Header and cpp or just cpp files - best practise?

7 Answers7