Compiler error LKN2019: Unresolved external symbol when dealing with inheritance - c++

Question

I have the following Files in C++:
Object.h:
class Object{
public:
Object(void);
~Object(void);
GLfloat getLeft();
GLfloat getRight();
GLfloat getBottom();
GLfloat getTop();
};
Player.h:
class Player : public Object{
public:
Player(void);
~Player(void);
GLfloat getLeft();
GLfloat getRight();
GLfloat getBottom();
GLfloat getTop();
};
Platform.h:
class Platform : public Object{
public:
Platform(void);
~Platform(void);
GLfloat getLeft();
GLfloat getRight();
GLfloat getBottom();
GLfloat getTop();
};
Pillar.h:
class Pillar : public Object{
public:
Pillar(void);
~Pillar(void);
GLfloat getLeft();
GLfloat getRight();
GLfloat getBottom();
GLfloat getTop();
};
Collision.h:
#include "Player.h"
#include "Platform.h"
#include "Pillar.h"
class Collision
{
public:
Collision(void);
~Collision(void);
bool CollisionDetect(Object* obj1, Object* obj2);
};
Collision.cpp:
bool Collision::CollisionDetect(Object* obj1, Object* obj2){
return !(obj2->getLeft() > obj1->getRight()
|| obj2->getRight() < obj1->getLeft()
|| obj2->getTop() > obj1->getBottom()
|| obj2->getBottom() < obj1->getTop());
}
CollisionDetect will be comparing whether 2 different Objects have collided and to do this I need specific x and y values however, the types of object may vary depending on different circumstances, and each type of object will have different ways to find their x and y values. This means that I have had to re-define the relevant get Methods in their subclasses, but when I run this code I end up with the errors:
1>Collision.obj : error LNK2019: unresolved external symbol "public: float __thiscall Object::getLeft(void)" (?getLeft#Object##QAEMXZ) referenced in function "public: bool __thiscall Collision::CollisionDetect(class Object *,class Object *)" (?CollisionDetect#Collision##QAE_NPAVObject##0#Z)
1>Collision.obj : error LNK2019: unresolved external symbol "public: float __thiscall Object::getRight(void)" (?getRight#Object##QAEMXZ) referenced in function "public: bool __thiscall Collision::CollisionDetect(class Object *,class Object *)" (?CollisionDetect#Collision##QAE_NPAVObject##0#Z)
1>Collision.obj : error LNK2019: unresolved external symbol "public: float __thiscall Object::getBottom(void)" (?getBottom#Object##QAEMXZ) referenced in function "public: bool __thiscall Collision::CollisionDetect(class Object *,class Object *)" (?CollisionDetect#Collision##QAE_NPAVObject##0#Z)
1>Collision.obj : error LNK2019: unresolved external symbol "public: float __thiscall Object::getTop(void)" (?getTop#Object##QAEMXZ) referenced in function "public: bool __thiscall Collision::CollisionDetect(class Object *,class Object *)" (?CollisionDetect#Collision##QAE_NPAVObject##0#Z)
1>C:\Users\Student\Dropbox\Work\Year 2\Graphics 1\Coursework\GraphicsCoursework\Debug\GraphicsCoursework.exe : fatal error LNK1120: 4 unresolved externals
Is it a problem with my inheritance that is causing this, or is it something else?

Answer 1

Exactly make the 4 functions in the object class "virtual"
Infact i would suggest make it pure virtual,
class Object{
public:
Object(void);
~Object(void);
virtual GLfloat getLeft() = 0;
virtual GLfloat getRight() = 0;
virtual GLfloat getBottom() = 0;
virtual GLfloat getTop() = 0;
};
This will force you to override those four function in the derived class, and provide a definition..

Answer 2

Each of your classes, Derived as well as Base need to define those 4 member functions.
Only pure virtual functions in C++ are allowed to exist without a definition.
Also, the redeclaration and definition of these 4 member functions in derived classes HIDE the functions in the Base class. From your usage it seems you want to use dynamic polymorphism through function overidding and for that you need to use the keyword virtual.

Answer 3

seems like you need virtual functions.
class Object{
public:
Object(void);
~Object(void);
virtual GLfloat getLeft()=0;
virtual GLfloat getRight()=0;
virtual GLfloat getBottom()=0;
virtual GLfloat getTop()=0;
};