To do this, you have interfaces for all your API-specific objects (renderer, mesh, texture, etc.). Program all your high-level code (like models, lights, materials) to these interfaces. Now, write concrete implementations for these interfaces, one implementation per API.

To do this, you have interfaces for all your API-specific objects (renderer, mesh, texture, etc.).Program all your high-level code (like models, lights, materials) to these interfaces.

// The renderer interface.
// Your game code should work with this and not the concrete implementations.
class IRenderer
{
public:
    virtual void Render() = 0;
};

Now, write concrete implementations for these interfaces, one implementation per API.

// The Direct3D 11 renderer.
class D3DRenderer : public IRenderer
{
public:
    virtual void Render()
    {
        // Clears the screen.
        context->ClearRenderTargetView(renderTargetView, { 0.0f, 0.5f, 1.0f, 1.0f });
        
        // Render the rest of the scene...
    }

private:
    ID3D11DeviceContext *context;
    ID3D11RenderTargetView *renderTargetView;
    // Other API-specific data...
};

// The OpenGL renderer.
class OpenGLRenderer : public IRenderer
{
public:
    virtual void Render()
    {
        // Clears the screen
        glClearColor(0.0f, 0.5f, 1.0f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        
        // Render the rest of the scene...
    }
};

// The abstract factory.
class IEngineFactory
{
    virtual IRenderer *CreateRenderer() = 0;
};

// The D3D11 factory.
class D3DEngineFactory
{
    virtual IRenderer *CreateRenderer()
    {
        return new D3DRenderer();
    }
};

// The OpenGL factory.
class OpenGLEngineFactory
{
    virtual IRenderer *CreateRenderer()
    {
        return new OpenGLRenderer();
    }
};