链接具有相同定义的两个目标文件时出现多定义错误

Question

Basically my prob is this :

I have a header file - foo.h with a structure pointer human *person = NULL in it. The definition for structure human is in another header included within foo.h . I am creating a shared object file game.so using foo.h and few other headers and cpps.

Now , I have two different cpp files - a.cpp and b.cpp which include the header foo.h. I am creating an object file ao separately and bo separately. I am linking both the object file and game.so for creating another shared object file tennis.so and while doing so , am getting multiple definition for the "person". I am aware that the multiple definition error is because ao contains the structure definition for person , so does bo .

I have used #pragma once in foo.h already. ao is getting compiled separately and bo is getting compiled separately. so i dont think #pragma once or ifdef will be useful here because both a.cpp and b.cpp includes foo.h

I cant change the structure defintion in foo. h to any other cpp file due to some dependencies while creating game.so

Is there any other way to resolve the multiple definition error I get while creating tennis.so ?

Answer 1

You need to make sure the person variable is only defined once. To do that, you need to only declare that variable in foo.h :

extern human *person;

Then, in foo.cpp , you define it:

human *person = NULL;

You then include foo.o in the object files that make up game.so .

Answer 2

You appear to be confusing DECLARATION with DEFINITION. Your header should DECLARE the structure, your C/C++ will then use such declaration to define variables or refer to instances of such definition[s].