为什么我不需要在头文件中包含library.cpp？

Question

I have a question about libraries. When I write a library I have 2 files: library.h and library.cpp .

The first one contains the prototypes of the functions and the second one contains the definitions. Well, in library.cpp I include #include "library.h" , so they are connected in one way, but what happens with the header? Why don't I have to write #include "library.cpp" in the header?

When I use the library in the main file, I write #include "library.h" , which includes the prototypes, but how does the compiler know where the definitions are?

Answer 1

Explained briefly:

(1) Your library.cpp file is sent to the preprocessor.

(2) The preprocessor reads the line #include "library.h" and goes and finds the file library.h .

(3) The contents of library.h are literally copied into the library.cpp file.

(4) The library.cpp file is compiled and linked with the main file.

Thus, all of the "prototypes" in the header are copied into the top of the implementation file. .cpp files are compiled and linked. The header files themselves are not compiled -- their contents are copied into the .cpp files.

Answer 2

The preprocessor pulls the header file into the CPP file, so the compiler sees the prototypes and the definitions together.

If you were to pull the CPP file into the header file, you would either send the preprocessor into an infinite loop, or by using

#ifndef __FOOBAR__  
#define __FOOBAR__  
(code file)  
#endif

around the header AND source files you would read the file just once.

Answer 3

There is a tool called linker which is responsible for link your generated object files. You should look for compilation process to understand it better.

Answer 4

All the cpp files are compiled seperately and the code is accumulated in a big pile somewhere. The linker collects all the symbols in your code and assigns them an address within this pile. So although your file doesn't see library.cpp directly, it knows the symbols from library.h and the assigned addresses. It can then zoom directly to the required code in the big pile. Not the most technical answer I know..

Answer 5

It was my understanding that when you type #include "library.h" in your code the compiler is set to also load library.cpp. I was taught in school that there isn't really any way to break this connection unless you were to manually compile and link each file either using compiler commands or a makefile appropriate for the system you're using (Windows, *nix, MacOS, etc.)

You have to type #include "library.h" in the library.cpp file because that is where the function prototypes are stored. You can "cheat" and put the function prototypes in the .cpp file and just type #include "library.cpp" but this is bad practice and is generally discouraged in the programming community.