在多个 cpp 文件 C++ 上使用类/结构/联合
我正在尝试在 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.我已经尝试了超过 7 种可能的解决方案来解决错误，但都没有成功.我研究了类前向声明??，这似乎不是答案(我可能是错的).
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) 已在 stdafx.obj 中定义
错误 2 错误 LNK1169:找到一个或多个多重定义符号
我知道错误是多重定义，因为我在两个 CPP 文件中都包含 resources.h.我怎样才能解决这个问题?我尝试在 CPP 文件中声明 class Jam，然后为需要访问该类的每个 CPP 文件声明 extern class Jam jam;.我也尝试过声明指向该类的指针，但没有成功.谢谢！
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!
推荐答案
变量jam定义在H文件中，包含在多个CPP类中，有问题.
The variable jam is defined in the H file, and included in multiple CPP classes, which is a problem.
不应在 H 文件中声明变量，以避免发生这种情况.将类定义保留在 H 文件中，但在 CPP 文件的 一个 中定义变量(如果您需要全局访问它 - 在所有休息).
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).
例如:
//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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