I think I am having the problem mentioned in the title, but I can't be sure of it as well...
Got 3 classes, GameMode, Entity, Paddle.
Paddle Inherit from Entity, and both Paddle and Entity need to #include "GameMode.h" in order to pass down the constructor a GameMode* and store it inside of every Entity.
I pretty much forward declared everything inside everything else but stuff just won't compile, the error I am getting is C2504 "Entity: base class undefined" .
I'll paste below the .h and .cpp for the Entity class, and only .h for the other two. Can someone tell me what am I doing wrong? 
Entity.h
#pragma once
#include "SDL2\SDL.h"
#include "GameMode.h"
#include "Utility.h"
using namespace util;
class GameMode;
enum class PivotMode: Uint8 {CENTER,TOP_LEFT};
class Entity
{
protected://variables
float XCenter;
float YCenter;
SDL_Rect CollisionBox;
SDL_Texture* Sprite;
GameMode* Game;
SDL_Renderer* Renderer;
public://constructors
Entity(GameMode* gameRef, PivotMode inputMode, int x, int y, std::string path);
virtual ~Entity();
Entity(const Entity&) = delete;
Entity& operator=(const Entity&) = delete;
Entity(Entity&&) = delete;
Entity& operator=(Entity&&) = delete;
public://methods
virtual void Update(float deltaTime) = 0;
virtual void Draw(float interpolation) = 0;
private://methods
SDL_Texture* RequestTexture(std::string path)const;
void SetCollisionBox(int x, int y, PivotMode InputMode);
};
Entity.cpp
#include "Entity.h"
Entity::Entity(GameMode* gameRef, PivotMode inputMode, int x, int y, std::string path)
:Game{ gameRef }, XCenter{ static_cast<float>(x) }, YCenter{ static_cast<float>(y) }
{
if (Game) { Renderer = Game->GetRenderer(); }
Sprite = RequestTexture(path);
if (Sprite)
{ SetCollisionBox(x, y, inputMode); }
}
Entity::~Entity()
{
}
SDL_Texture* Entity::RequestTexture(std::string path)const
{
if (Game->IsRunning())
{
return Game->RequestTexture(path);
}
return nullptr;
}
void Entity::SetCollisionBox(int x, int y, PivotMode InputMode)
{
SDL_QueryTexture(Sprite, nullptr, nullptr, &CollisionBox.w, &CollisionBox.h);
switch (InputMode)
{
case PivotMode::CENTER:
{
CollisionBox.x = x - CollisionBox.w / 2;
CollisionBox.y = y - CollisionBox.h / 2;
XCenter = static_cast<float>(x);
YCenter = static_cast<float>(y);
}break;
case PivotMode::TOP_LEFT:
{
CollisionBox.x = x;
CollisionBox.y = y;
XCenter = static_cast<float>(x) + CollisionBox.w / 2;
YCenter = static_cast<float>(y) + CollisionBox.h / 2;
}break;
}
}Paddle.h
#pragma once
#include "Entity.h"
#include "GameMode.h"
class GameMode;
enum class PivotMode:Uint8;
class Paddle : public Entity
{
public://methods
Paddle(GameMode* gameRef, PivotMode inputMode, int x, int y, std::string path);
~Paddle();
virtual void Update(float deltaTime);
virtual void Draw(float interpolation);
};GameMode.h
#pragma once
#include "SDL2\SDL.h"
#include <string>
#include <vector>
#include <memory>
#include "App.h"
#include "Entity.h"
#include "Paddle.h"
class Entity;
class Paddle;
class GameMode
{
friend class App;
private://variables
bool Running;
SDL_Window* Window;
SDL_Renderer* Renderer;
App* AppRef;
public://constructors
GameMode(SDL_Window* Window, SDL_Renderer* Renderer, App* App);
~GameMode();
GameMode(const GameMode&) = delete;
GameMode& operator=(const GameMode&) = delete;
GameMode(GameMode&&) = delete;
GameMode& operator=(GameMode&&) = delete;
public://methods
SDL_Texture* RequestTexture(std::string path)const;
bool IsRunning()const;
SDL_Renderer* GetRenderer()const;
private://methods
void Update(float deltaTime);
void Draw(float interpolation);
};
) 
