I'm trying to work out a simple, functioning game engine, and I'm starting out as basic as I think I can get.

I have a virtual base class, Entity; the only child class I have for it currently is the Block class. I have a SceneManager class that contains a vector of Entity pointers, and with the Init() function, I attempt to populate it with Block objects, using an overloaded constructor. At first I thought it was because I did not have a corresponding overloaded constructor in the base class, but even when I added it, I got the same error:

-

LNK2019: unresolved external symbol "public: __thiscall Entity::Entity(void)" referenced in function "public: __thiscall Block::Block(short,short)"

-

This is my code:

Entity.hpp

[spoiler]

#pragma once

/* Entity.hpp
   - base class for all entities
   - also contains flag enumerations specifically for general entity and enemy-like types
   */

#include "Collision.hpp"
#include "Vector.hpp"
#include <Windows.h>

enum EnemyFlags
{
	enInit    = 0x01,
	enAlive   = 0x02,
	enScared  = 0x04,
	enHiding  = 0x08,
	enBeaming = 0x10
};

enum EntityIDs
{
	eIDBlock = 0x01,
	eIDRed   = 0x02,
	eIDBlu   = 0x04,
	eIDGrn   = 0x08
};

class Entity
{
	static unsigned short Count;

public:
	Entity();
	virtual void Init()  = 0;
	virtual void Act()   = 0;
	virtual bool Alive() = 0;
	virtual void Draw(const HANDLE)  = 0;
	virtual Collision Collide(Entity*) = 0;
	const COORD GetPos() { return Position; };
	const __int32 ID;

protected:
	virtual void Move() = 0;
	virtual void Die()  = 0;

	COORD* Beacon;
	COORD  Position;
	Vector Velocity;
	Vector Dimensions;
	char   Flags;
	char   Frames[4];
	short CurFrame, EndFrame;
	short FrameDelay, FrameTimer;
	unsigned short Timer;
};

[/spoiler]

Block.hpp

[spoiler]

#pragma once

/* Block.hpp
   - stores information about block objects
   - contains a position and dimensions
   */

#include "Entity.hpp"

class Block : public Entity
{
public:
	Block();
	Block(short, short);

	void Init();
	void Act();
	bool Alive();
	void Draw(const HANDLE);
	Collision Collide(Entity*);
	const __int32 ID;

protected:
	void Move();
	void Die();
	
	COORD  Position;
	Vector Dimensions;
	Vector Velocity;
};

[/spoiler]

Block.cpp

[spoiler]

// Block.cpp

#include "Block.hpp"
#include <iostream>

Block::Block() :
	ID (eIDBlock)
{
	// empty for now
}

Block::Block(short x, short y) :
	ID (eIDBlock)
{
	Position.X = x;
	Position.Y = y;
	Dimensions.X = 16;
	Dimensions.Y = 16;
	Velocity.X = 0;
	Velocity.Y = 0;
}

void Block::Draw(const HANDLE backbuffer)
{
	SetConsoleCursorPosition(backbuffer, Position);
	std::cout << char(219);
}

Collision Block::Collide(Entity* other)
{
	// blank for now

	Collision info;
	info.Contact = false;
	info.Intersection.X = 0;
	info.Intersection.Y = 0;
	info.Intersection.Z = 0;
	info.OtherID = 0;
	info.Velocity.X = 0;
	info.Velocity.Y = 0;
	info.Velocity.Z = 0;

	return info;
}

void Block::Move()
{
	// empty for now
}

void Block::Init()
{
	// empty for now
}

void Block::Act()
{
	// empty for now
}

bool Block::Alive()
{
	return true;
}

void Block::Die()
{
	// empty for now
}

[/spoiler]

SceneManager.cpp

[spoiler]

// SceneManager.cpp

#include <fstream>
#include "SceneManager.hpp"
#include "Block.hpp"

SceneManager::SceneManager():
	hOut(GetStdHandle(STD_OUTPUT_HANDLE))
{
	// empty for now
}

void SceneManager::Init(__int32 stateID, short info)
{
	// depending on which state ID is given, it loads from an information file depending on info
	for (short col = 0; col < 80; col++)
		for (short row = 0; row < 25; row++)
			Entities.push_back(new Block(col, row));
}

void SceneManager::Quit()
{
	// empty for now
}

void SceneManager::Tick()
{
	for (unsigned short i = 0; i < Entities.size(); i++)
		Entities[i]->Draw(hOut);
}

void SceneManager::Save()
{
	// empty for now
}

void SceneManager::Load()
{
	// empty for now
}

[/spoiler]

You need to supply a body for your Entity constructor if you declare it in the class definition.

If I insert

-

Entity::Entity():
	ID (eIDBlock)
{
	// empty for now
};

-

into the code, I get these errors:

-

LNK2005: "public: __thiscall Entity::Entity(void)" already defined in Block.obj (GamePlay.obj)
LNK2005: "public: __thiscall Entity::Entity(void)" already defined in Block.obj (SceneManager.obj)
LNK2005: "public: __thiscall Entity::Entity(void)" already defined in Block.obj (StateFactory.obj)

Are you putting that in a header file? Function definitions in a header file need to be defined as inline. If you don't want to make it an inline function put the function definition in a source file.

Oh, there we go. I thought I'd just do a quick fix and put the source in the header file, but now I've learned why that can mess things up.

Problem solved. Thanks!