Error when calling static method c++ - function

Question

Consider the following code:
Automobile.h
class Automobile
{
static string m_stCityCode;
static bool CheckCityCode(const Automobile& obj);
};
Automobile.cpp
bool Automobile::CheckCityCode(const Automobile& obj)
{
return m_stCityCode == obj.m_stCityCode;
}
int main()
{
//do something
}
I get the following error
"Severity Code Description Project File Line Suppression State
Error LNK2001 unresolved external symbol "public: static class
std::basic_string<char,struct std::char_traits<char>,class
std::allocator<char> > Automobile::m_stCityCode"
(?m_stCityCode#Automobile##2V?$basic_string#DU?$char_traits#D#std##V?$allocator#D#2##std##A) myPro C:\Users\zhivko.rusev\Documents\Visual
Studio 2015\Projects\myPro\myPro\Calls.obj 1 "
I would appreciate every help for solving this problem. Thanks in advance!

Answer 1

The static member needs to be defined. The error message is the linker's way of telling you it isn't. Your code declares the static member but does not define it.
To define it, in a single compilation unit (i.e. a non-header source file) simply add a line at file scope after including the header file
#include "Automobile.h"
std::string Automobile::m_stCityCode = ""; // change the initialiser to whatever suits
Do this in exactly one compilation unit. One is necessary to define the symbols. Multiple definitions (e.g. in multiple source files within your project) will cause the linker to complain about symbols being defined multiple times.
There are other problems in your code as shown, beyond what you have asked about, but I'll assume that just reflects you having left information out.