Question

I asked a while ago how to use virtual classes in c++, and to my dismay i learned that you can’t. But one user,(namely “Emilio Garavaglia” thanks a bunch), posted a way to get something similar to virtual classes, just with some extra code. however, i’m having some trouble getting what i’m doing to compile. here’s the code:

global_defs.h

#define Interface class

#define abstract_class class

#define implements : public 

I_Graphics.h

#ifndef I_GRAPHICS_H
#define I_GRAPHICS_H

#include <string>
#include "global_defs.h"

Interface I_Graphics
{
public:
    virtual ~I_Graphics() {};

    virtual void Initialize() = 0;
    virtual void Frame() = 0;
    virtual void Shutdown() = 0;

    class I_Model;

    virtual I_Model * CreateModel() = 0;

};

Interface I_Graphics::I_Model
{
public:
    virtual ~I_Model() {}

    virtual void Initialize(std::string const & filename, std::string const & textureFilename) = 0;
    virtual void * GetVertexBuffer() = 0;
    virtual void * GetIndexBuffer() = 0;
};


#endif

Graphics.h

#ifndef GRAPHICS_H
#define GRAPHICS_H

#include "global_defs.h"

#include <map>
#include <string>
#include <memory>
#include "I_Graphics.h"

class Graphics implements I_Graphics
{
public:
    Graphics();
    ~Graphics();

    void Initialize();
    void Frame();
    void Shutdown();

    class Model;

    I_Model * CreateModel() {return new Model;}   // <--- compile error here

private:
    std::map <std::string, I_Model *> m_ModelList;
};

class Graphics::Model implements I_Graphics::I_Model
{
public:
    Model();
    ~Model();

    void Initialize(std::string filename, std::string textureFilename);
    void * GetVertexBuffer();
    void * GetIndexBuffer();
};

#endif

Graphics.cpp
nothing going here, havn’t really started working on the hard part yet, just trying to get the model instantiation to work.

#include "Graphics.h"

Graphics::Graphics()
{

}

Graphics::~Graphics()
{
}

void Graphics::Initialize()
{

}

void Graphics::Frame()
{

}

void Graphics::Shutdown()
{

}

Graphics::Model::Model()
{

}

Graphics::Model::~Model()
{
}

void Graphics::Model::Initialize(std::string filename, std::string textureFilename)
{


}

void * Graphics::Model::GetVertexBuffer()
{
    return NULL;
}

void * Graphics::Model::GetIndexBuffer()
{
    return NULL;
}

so, as the little comment says, i get an error there saying:

error C2512: 'Graphics::Model' : no appropriate default constructor available

when there obviously is a constructor for it in graphics.cpp . Can someone please explain what the compiler is complaining about here?

EDIT:
not sure if it means anything, but when mousing over the little red squiggle in MSVC, it says, “object of abstract class type Graphics::Model is not allowed” . …but it doesn’t have any pure virtual members, so it’s not abstract right?

EDIT:
On the suggestion of Castilho, i declare CreateModel in graphics.h like before, but then defined it in graphics.cpp, and it yielded a much more specific error, but i still don’t understand why.

error C2259: 'Graphics::Model' : cannot instantiate abstract class
1> due to following members:
1> 'void I_Graphics::I_Model::Initialize(const std::string &,const std::string &)' : is abstract
1> i_graphics.h(28) : see declaration of 'I_Graphics::I_Model::Initialize'

Answer 1

You’re using the Model class before it is defined. Define the function CreateModel in a separate CPP and it may work.