C++ 头文件和源文件 - 包含哪些内容和顺序

Question

I just started learning C++ and have trouble understanding the concept of header and source files, specifically which ones I'm supposed to include where.

Suppose I have the classes A and B, each of whose contents are seperated into a header file and a source file, and I have a file in which I want to use those classes.

├ A.h
├ A.cpp
├ B.h
├ B.cpp
└ Main.cpp

Which file do I need to include where, and is it possible to compile/link all files with a single command?

Answer 1

Which file do I need to include where

#include directive in fact is very simple preprocessor directive, which just adds content of the specified file into the target file. Conventionally you keep declaration of functions in a header file and definition of the said functions in the corresponding cpp file, and thus you at least want to have the header included there (in your case A.cpp includes Ah and B.cpp includes Bh ). Additionally you include headers in any file where you use the functions declared in the headers (eg if you use declarations of Ah and Bh in Main.cpp you include those files as well).

PS You, however, can define everything right in the header file, next to declarations, but as I said earlier preprocessor doesn't do anything fancy - it just adds the content of the include in the target file, and you usually don't want to have all definitions in place, because each translation unit which has it included will have the same definitions repeated over and over again.

Answer 2

You include what you need.

If A.cpp needs Ah then it should include it. Same for B.cpp probably needing to include Bh .

If Main.cpp uses A and B then it should include Ah and Bh .

Main.cpp should not rely on indirect includes. For example of Ah does include Bh then Main.cpp using both A and B should still include Ah and Bh even if the code would compile if Main.cpp would only directly include Ah (and by that also indirectly including Bh .)

In Which order?

If you follow the rule of always including what you use (rather than relying on indirectly including a header or rely on some other header being included before) then the order of #include directives should not matter. Changing order of includes breaking a build hints on a problem with includes in one of the headers that should be fixed. If you do things right then order of includes is just a matter of style.