Header files and include best practice

Question

I have a quick question regarding header files, include statements, and good coding style. Suppose I have 2 classes with associated source and header files, and then a fina

Answer 1

Include all you need in every file, but do not include any file that you do not need. Normally, it is the job of the included file to make sure it is not included twice, using precompiler flags, etc...

For example if is needed by Foo.cpp, but not by Foo.h, include it in Foo.cpp not in Foo.h. If required in both, include in both.

Tangentially, as a best practice, never use using directives in a header file. If you need you can use using directives in implementation files (.cpp).