在多个cpp文件C ++中使用类/ struct / union [英] Using a class/struct/union over multiple cpp files C++

问题描述

我试图在C ++中创建一个类，并且能够在多个C ++文件中访问该类的元素。我已经尝试超过7可能的方案来解决错误，但已经失败。

  //资源（我可能是错误的） .h 
 class Jam {
 public：
 int age; 
} jam; 
 
 //functions.cpp 
 #includeresources.h
 void printme（）{
 std :: cout<堵塞
} 
 
 //main.cpp 
 #includeresources.h
 int main（）{
 printme（）; 
 std :: cout<<堵塞
} 
  

错误1错误LNK2005：class Jam jam （？jam @@ 3VJam @@ A）已在stdafx.obj中定义

错误2错误LNK1169：one或更多的多重定义符号

我理解错误是多重定义，因为我包括 resources.h 。如何解决这个问题？我试着在CPP文件中声明 class Jam ，然后为每个CPP文件声明 extern class Jam jam; 需要访问类。我也试过声明指向类的指针，但我没有成功。谢谢！

解决方案

变量 jam ，并且包含在多个CPP类中，这是一个问题。

变量不应该在H文件中声明，以避免这种情况。在H文件中保留类定义，但在CPP文件的一个中定义变量（如果您需要全局访问它，请将其定义为 extern

  // resources.h 
 class Jam {
 public：
 int age; 
}; 
 extern果酱酱//包含此标题的所有文件都会知道它
 
 //functions.cpp 
 #includeresources.h
 Jam jam; //链接器将链接所有对jam的引用
 void printme（）{
 std :: cout<堵塞
} 
 
 //main.cpp 
 #includeresources.h
 int main（）{
 printme（）; 
 std :: cout<<堵塞
} 
  




I am trying to create a class in C++ and be able to access elements of that class in more than one C++ file. I have tried over 7 possible senarios to resolve the error but have been unsuccessful. I have looked into class forward declaration which doesen't seem to be the answer (I could be wrong).

//resources.h
class Jam{
public:
int age;
}jam;

//functions.cpp
#include "resources.h"
void printme(){
std::cout << jam.age;
}

//main.cpp
#include "resources.h"
int main(){
printme();
std::cout << jam.age;
}

Error 1 error LNK2005: "class Jam jam" (?jam@@3VJam@@A) already defined in stdafx.obj

Error 2 error LNK1169: one or more multiply defined symbols found

I understand the error is a multiple definiton because I am including resources.h in both CPP files. How can I fix this? I have tried declaring the class Jam in a CPP file and then declaring extern class Jam jam; for each CPP file that needed to access the class. I have also tried declaring pointers to the class, but I have been unsuccessful. Thank you!

解决方案

The variable jam is defined in the H file, and included in multiple CPP classes, which is a problem.

Variables shouldn't be declared in H files, in order to avoid precisely that. Leave the class definition in the H file, but define the variable in one of the CPP files (and if you need to access it globally - define it as extern in all the rest).

For example:

//resources.h
class Jam{
public:
int age;
};
extern Jam jam; // all the files that include this header will know about it

//functions.cpp
#include "resources.h"
Jam jam; // the linker will link all the references to jam to this one
void printme(){
std::cout << jam.age;
}

//main.cpp
#include "resources.h"
int main(){
printme();
std::cout << jam.age;
}

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