c++中静态成员变量要在类外部再定义，否则产生link2001错误

class testClass
{
public:
static int m_i;
};

//类外部定义，若不写会产生error LNK2001: unresolved external symbol "public: static int testClass::m_i" (?m_i@testClass@@2HA)
int testClass::m_i;

int main(int argc, char* argv[])
{
printf("%d\n",testClass::m_i);
printf("\n");
return 0;
}