Question

Suppose I have two .h files: A.h and B.h.
Moreover, A.h includes B.h itself:

B.h – defines class B.

class B {
  ...
};

A.h – defines class A, which uses class B.

#include B.h

class A {
  void SomeFunction(const B& b);
};

Now, I have some .cpp file, that uses both A and B classes (B class maybe used not only in A::SomeFunction(B))

What are the pluses to include both A.h and B.h (instead of only A.h) from the perspective of design-patterns and coding style.

Answer 1

Including both “A.h” and “B.h” makes the dependencies completely clear. There is no reason why “A.h” could not have forward-declared class B, and so including both headers prevents your “.cpp” file from breaking should the transitive include be changed to a forward declaration. In general, it is not a good idea to rely on transitive includes, and instead one should explicitly include all direct dependencies. (Note that this does not apply for “master include” headers that are intended to give transitive includes).