C++ static variable in header file getting LNK error 2001 - c++

Question

I have myself a very basic program. I am trying to create a static vector in a class, which can then be accessed around my whole program which just creating an instance of that class which has the stored static vector.
here is my test program
testClass.h
#pragma once
#include <vector>
class TestClass
{
public:
TestClass();
~TestClass();
static void AddNumber(int number);
static std::vector<int> Numbers;
};
testClass.cpp
#include "TestClass.h"
TestClass::TestClass()
{
Numbers.push_back(1);
Numbers.push_back(2);
Numbers.push_back(3);
}
TestClass::~TestClass()
{
}
void TestClass::AddNumber(int number)
{
Numbers.push_back(number);
}
main.cpp
#include "TestClass.h"
#include <iostream>
int main()
{
TestClass testClass;
testClass.AddNumber(4);
for (int i = 0; i < testClass.Numbers.size(); i++)
{
std::cout << "Number at " << i << ", is : " << testClass.Numbers.at(i) << std::endl;
}
std::cin.get();
}
When I compile I am getting
Error 2 error LNK2001: unresolved external symbol "public: static class std::vector<int,class std::allocator<int> > TestClass::Numbers" (?Numbers#TestClass##2V?$vector#HV?$allocator#H#std###std##A) c:\Users\mainUser\documents\visual studio 2013\Projects\C++StaticClass\C++StaticClass\main.obj C++StaticClass
Error 3 error LNK2001: unresolved external symbol "public: static class std::vector<int,class std::allocator<int> > TestClass::Numbers" (?Numbers#TestClass##2V?$vector#HV?$allocator#H#std###std##A) c:\Users\mainUser\documents\visual studio 2013\Projects\C++StaticClass\C++StaticClass\TestClass.obj C++StaticClass
Error 4 error LNK1120: 1 unresolved externals c:\users\mainUser\documents\visual studio 2013\Projects\C++StaticClass\Debug\C++StaticClass.exe 1 1 C++StaticClass
Can anyone point me in the right direction on how to fix my problem.

Answer 1

You need to initialize static variable in cpp file. Add following line in testClass.cpp file
std::vector<int> TestClass::Numbers;