C ++：包含头文件的最佳位置

Question

Let's say we have four files: ah, a.cpp, b1.h, and b2.h. And, we need to include b1.h and b2.h in either ah or a.cpp. Where should I include b1 and b2? Let's say only a.cpp needs b1 and b2.

Answer 1

If it's not needed for the header file then include it in the cpp.

This reduces compilation time and helps to understand the dependencies between the modules.

Answer 2

一般规则是，应避免在不使用其定义的标头中包含标头。

Answer 3

Only include what is needed in the header.

Do the class definitions and function declarations of ah require b1.h or b2.h? Then include what is needed.

Otherwise only include in the .cpp.

Just remember, each time you include a file your compilation takes that much longer.

Here are a couple hints of when things are needed:

• Return values or parameters do not need to be included. For example std::string blahFunc(std::string a); does not need <string> included in the header file(still needs a forward declaration though)

• Pointer types do not need to be included, they just need to be forward declared. For example randomType * f(); does not need to include randomType's header in it's header. All you have to do is forward declare with class randomType;

• References can also just be forward declared.

Answer 4

If b1.h and/or b2.h have definitions like struct or typedef , etc. that are actually used in ah (in a function prototype as parameters or return type, for example), then you should include them in the top of the header.

Otherwise, if b1.h/b2.h only provide definitions that are used internally to a.cpp (private members, etc), then include it at the top of a.cpp.

You should try to only include in a file what is actually needed for the compiler to understand that file. (Unlike windows, with the monstrosity that is <Windows.h> .)

Answer 5

I think the include directives should always be in your .h** files. The only include you should put in the a.cpp file should be

#include "a.hpp"

To understand why, suppose Bob uses your code inside his program, and your code is not disclosed (ie, you provide it only as a library). Then all he can see are your headers. If you include everything you need in your headers, he will still be able to check what are the dependencies of your code and make sure he has everything needed by your code. If you put the include directives in the .c** files, then unless your code is open source (ie he has access to the .c** files) he won't be able to see what are the packages he must make sure to have installed.