奇怪的链接器错误与静态std :: map

问题描述

为什么当我尝试在Visual Studio 2008中编译时会出现链接器错误

  c>  c> >  
已经声明了静态成员 _myMap ，但是没有定义它。添加此行 int main（）：
  std :: map< int，int> MyClass :: _ myMap; 
  
想象它是一个已声明但未在任何.cpp文件中定义的函数如果您使用链接器错误。
 
Why do I get linker error when I try to compile this in Visual Studio 2008
#include <stdafx.h>
#include <iostream>
#include <map>
#include <string>

class MyClass
{
public:
 MyClass () { };
 virtual ~MyClass() {};

 static std::string niceString (std::map<int, int> mappp) { _myMap = mappp; return "nice string"; };

private:
 static std::map<int, int> getMap ( ) { return _myMap; };

 static std::map<int, int> _myMap;
};

int main(){

 std::map<int, int> mappp;

 mappp[1] = 1;

 std::cout << MyClass::niceString(mappp);

}
error is:
Error 1 error LNK2001: unresolved external symbol "private: static class std::map<int,int,struct std::less<int>,class std::allocator<struct std::pair<int const ,int> > > MyClass::_myMap" (?_myMap@MyClass@@0V?$map@HHU?$less@H@std@@V?$allocator@U?$pair@$$CBHH@std@@@2@@std@@A) test22.obj test22
 解决方案 
You've declared the static member _myMap, but not defined it.  Add this line just above int main():
std::map<int, int> MyClass::_myMap;
Think of it like a function that has been declared but not defined in any .cpp file - you get a linker error if you use it.
                        
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