Linux C++编译报错："multiple definition of" / "does not name a type"

最近编译larbin_daemon爬虫服务器端管理程序时，总是会报如下的错误：

g++    -c -o editConf.o editConf.cpp
editConf.cpp:49:8: error: redefinition of ‘std::string projectG’
editConf.h:34:8: error: ‘std::string projectG’ previously declared here
editConf.cpp:50:8: error: redefinition of ‘std::string ipG’
editConf.h:35:8: error: ‘std::string ipG’ previously declared here
editConf.cpp:51:8: error: redefinition of ‘std::string httpPortG’
editConf.h:36:8: error: ‘std::string httpPortG’ previously declared here
editConf.cpp:52:8: error: redefinition of ‘std::string pagesConnexionsG’
editConf.h:37:8: error: ‘std::string pagesConnexionsG’ previously declared here
editConf.cpp:53:8: error: redefinition of ‘std::string dnsConnexionsG’
editConf.h:38:8: error: ‘std::string dnsConnexionsG’ previously declared here
editConf.cpp:54:8: error: redefinition of ‘std::string depthInSiteG’
editConf.h:39:8: error: ‘std::string depthInSiteG’ previously declared here
editConf.cpp:55:8: error: redefinition of ‘std::string waitDurationG’
editConf.h:40:8: error: ‘std::string waitDurationG’ previously declared here
editConf.cpp:56:8: error: redefinition of ‘std::string timeoutPageG’
editConf.h:41:8: error: ‘std::string timeoutPageG’ previously declared here
editConf.cpp:57:8: error: redefinition of ‘std::string timeoutIncrG’
editConf.h:42:8: error: ‘std::string timeoutIncrG’ previously declared here
make: *** [editConf.o] Error 1

十分困惑，明明只定义了一次嘛！

其实，

editConf.h中，有如下代码片段：

string projectG;//G代表global,类中也有对应的私有变量
string ipG;
string httpPortG;
string pagesConnexionsG;
string dnsConnexionsG;
string depthInSiteG;
string waitDurationG;
string timeoutPageG;
string timeoutIncrG;

然后在editConf.cpp中是直接用了（赋值什么的）如：

httpPortG = r[6];

在editConf.cpp和main.cpp中都包含了editConf.h头文件，这就问题所在。

当你在头文件中声明一个变量，就像我在editConf.h中写的，每个包含了该头文件的源文件，无论是直接的还是间接的，都会获得一个它自己对于该变量的拷贝，那么当你link所有的.o文件到一起时候，linker就会认为该变量在多个.o文件中被实例化了。

解决的方法是：在头文件中（如editConf.h）用extern声明变量，在对应的源文件中（如editConf.cpp）实例化变量。

参考资料：

http://stackoverflow.com/questions/18763270/c-linking-error-with-multiple-definition-on-linux