include stdafx.h in header or source file?

Question

I have a header file called stdafx.h and this one is precompiled of course. I've read that I should include these files into my .cpp files, but some of these statements are already needed in the header file coming with that.

Should I add the stdafx into my header or into my cpp? I thought it was good practise to put it into the header, but I seem to be obliged to put it into the header instead.

Example:

stdafx contains freeglut.
my class header file has an attribute of GLenum.

Should I include the stdafx into the .h of the class?

Answer 1

stdafx.h should be the first include in EVERY cpp file in your project.

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Consider that C++ doesn't compile header files, just Cpp files.

Therefore if the stdafx is the first include in the cpp file, then the compiler will have everything the header file needs, when it hits the header-file in the Cpp file.

e.g.

You have A.cpp & A.h.
A.h needs std:string.

you have B.cpp & B.h
B.h needs A.h therefore B.h needs std::string too.

Because it's good practice, you put #include <string> in stdafx.h.

Your build fails because nothing can see std::string

Now put stafx.h as the first include in A.cpp and B.cpp.
When the compiler hits A.cpp, it picks up the include for <string>, then picks up A.h, and everything is happy because we know what std::string is.

The compiler now hits B.cpp, again it includes stdafx first, which brings <string>, then hits B.h, which brings A.h which is again happy because std::string has already been included.

Hope this helps.

Answer 2

• Only include things in your precompiled header which should be there
• Your precompiled header file must be the first include in every .cpp
• I would avoid including it in another header in favor of forward declaration

Ask yourself these two questions before including something in stdafx.h

1. Will this header never be changed by me?
2. Do I need this included in every multiple source file?

If the answer is "No" to either of those then don't include it.