Is it good C++ to define classes in .cpp file?

Question

I am a beginner in C++. I have a question regarding C++ design.

My file contains A,B,C,D,E class definitions. Class A contains the API which is used by other applications. I have defined this in a .h file. Classes B,C,D & E define concrete classes by inheriting an abstract class which is provided by some library. These definitions are not used by any external application, but only used by class A.

I have seen online that all the class definitions are provided in an .h file and the function implementations in a .cpp file. My question here is, even though class B,C,D & E definitions are not used externally by anyone, should I define them in the .h file? If I do define them there anyway, I cannot expose them to other applications, right?

You can declare a class in a Cpp file. You cannot use such class outside of that file. — DYZ, Sep 10 '20 at 17:59
I echo HolyBlackCat. Don't expose something you don't need to be part of the API. Oh, and wrap those classes in an [unnamed namespace](https://en.cppreference.com/w/cpp/language/namespace#Unnamed_namespaces), so that everything gets proper internal linkage. — StoryTeller - Unslander Monica, Sep 10 '20 at 18:07
@StoryTeller-UnslanderMonica You mean class A,B,C,D,E should be wrapped in an unnamed namespace or just class B,C,D,E — Punit Salian, Sep 10 '20 at 18:31

score 3 · Answer 1 · answered Sep 10 '20 at 18:30

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If a class is only used locally in one module, you can declare it in the .cpp file. This is actually good practice; don't expose more than necessary.

In a case where you need to define a class (or function, etc.) in a header (for example, to share it between several related .cpp file) but you do not want to expose it publicly, you can have a separate, private header file which is only included in the relevant places, but is not made public. This can be hinted at by appending "private" to the header file name.

answered Sep 10 '20 at 18:30

Basya

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Another techniques to hide implementation details (even from the linker) is to use [anonymous / unnamed namespaces](https://stackoverflow.com/questions/357404/why-are-unnamed-namespaces-used-and-what-are-their-benefits) (better than "private header"). May be worth mentioning to improve your answer. – πάντα ῥεῖ Sep 10 '20 at 19:02
@πάνταῥεῖ -- if I understand correctly, anonymous namespaces will limit access to one translation unit -- how does that replace the private header, which limits use to several, related, translation units? – Basya Sep 10 '20 at 20:53
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Seems I didn't read properly that you proposed more than one unit. In that case anonymous namespaces won't help. You're right about that. – πάντα ῥεῖ Sep 10 '20 at 21:18

Is it good C++ to define classes in .cpp file?

1 Answers1