Is it a good practice to always create a .cpp for each .h in a C++ project?

Question

Some classes, like exceptions or templates, only need the header file (.h), often there is no .cpp related to them.

I have seen some projects were (for some classes) there aren't any .cpp files associated to the headers files, perhaps because the implementation is so short that it is done directly in the .h, or maybe for other reasons, such as template classes, where it is mandatory to include the implementation in the header.

What is your opinion, if a class is too short, sould I avoid creating a .cpp file and writing the code directly on the header file ? If the code is written in the header file, should I include an empty .cpp so the files in the project remains consistent ?

Answer 1

I wouldn't add unnecessary .cpp files. Each .cpp file you add must be compiled, which just slows down the build process.

In general, using your class will only require the header file anyways - I see no advantage to an "empty" .cpp file for consistency in the project.

Answer 2

There is one advantage to always creates a .cpp for each .h, even when the former would be empty: it enforces at compile time that header files are self-contained. This is related with the guideline of including foo.h first in foo.cpp:

With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of dir/foo.cpp will break. Thus, this rule ensures that build breaks show up first for the people working on these files, not for innocent people in other packages.