C++ Linkage Error using static vectors. shape example

Question

I have a linkage error in this code. can someone solve this problem and explain why does it happen?

Header file

#pragma once
#include <iostream>
#include <vector>

class Shape
{
public:
    static void test_shapes();

    // static becuase every shape that created added to this list
    static std::vector<Shape*> _shapes;

};

Source file

#include "Shape.h"

void Shape::test_shapes()
{
    _shapes.size();
}