How to fix unresolved external symbol issue?

[Jenky]

Having issues fixing these issues:Error 2 error LNK2019: unresolved external symbol "public: __thiscall grid::grid(void)" (??0grid@@QAE@XZ) referenced in function "void __cdecl `dynamic initializer for 'GridObject''(void)" (??__EGridObject@@YAXXZ) F:\Dev103\thermonuclear war game\thermonuclear war game\Grid.obj


Error 3 error LNK1120: 1 unresolved externals F:\Dev103\thermonuclear war game\Debug\thermonuclear war game.exe 1

//header file
#include <iostream>
using namespace std;
class grid{

	int r, c;

public:
	grid();
	void GridFunction();
	int getR(){return r;}
	int getC(){return c;}
	void setR(int a){a = r;}
	void setC(int b){b = c;}

};
//create object
extern grid GridObject; //has something to do with this, when commented out fixes that error but rest of my program has issues. 

#include <iostream>
#include "Grid.h"
#include "Cities.h"

using namespace std;

grid GridObject;//has something to do with this

void grid::GridFunction(){
	
	//randomly generate grid size
	//set rows
	GridObject.setR(rand()%16+7);
	//set columns
	GridObject.setC(rand()%16+7);

	//display x co-ordinates
	for(int i=0;i <=GridObject.getC(); i++){
	
		cout << "  " << i << "  ";
	}

	int counter = 0;
	for(int s=1;s< GridObject.getC()*GridObject.getR();s++){

		cout << "+--+";


		if(!(s % GridObject.getC())){
			
			cout << endl;
				
				for(int s=0;s<GridObject.getC();s++){

					
					cout << "| << CityObject.getX() CityObject.getY()<< |";
				}
		}
		if(!(s%GridObject.getC())){
		//generate x co-ordinates
		counter++;
		cout <<" "<<counter;
		cout << endl;
		}
	}

}

It has more to the program but the error is in this part. Any help will be highly appreciated.

[Script Coder]

Try not creating the object in the header

[Jenky]

That wasn't the issue. Found out of someone else. I had made the grid constructor but had not given the definition in the .cpp file.