Don't understand why I am getting an unresolved external symbol? - c++

Question

I don't understand why I am getting an unresolved external symbol in this specific example.
I have a Rectangle struct defined in a header file.
I have a Level class which has a pointer to a Player class as the Level creates a new player.
I my Player class, the constructor takes in a pointer to it's current level.
So since my Level includes Player.h I forward declare Level in my Player class to prevent a circular reference.
However, when I try and add a Rectangle member variable to my Player class - I get the unresolved external symbol error, but I can't see why - since the Player class includes the header - and the definition is in the header. It works fine in my Level class.
Also - if I change the member variable to a pointer it links fine.
Can someone explain to me why this is the case and how I can fix the issue?
// Rectangle.h
#pragma once
namespace Test
{
struct Rectangle
{
public:
Rectangle();
Rectangle(
int x,
int y,
int width,
int height) :
X(x),
Y(y),
Width(width),
Height(height)
{
}
int X;
int Y;
int Width;
int Height;
};
}
// Level.h
#pragma once
#include "Player.h"
#include <memory>
namespace Test
{
class Level
{
public:
Level()
{
_player.reset(new Player(this));
};
private:
std::unique_ptr<Player> _player;
};
}
// Player.h
#pragma once
#include "Rectangle.h"
namespace Test
{
class Level;
class Player
{
public:
Player(
Level* currentLevel) :
_currentLevel(currentLevel)
{
}
private:
Level* _currentLevel;
// This line causes the issue
Rectangle _localBounds;
};
}

Answer 1

You forgot to add braces on your Rectangle constructor. Make it like this:
Rectangle() {}
So the problem is you have the definition but not the implementation. You can indeed declare a function without braces but it is like you make a promise to the compiler that you are going to add the function's body somewhere else. That's why separating classes to .hpp and .cpp files works.
Note: Retired Ninja gave the answer first in a comment below the post.