如何在这里摆脱dynamic_cast？

Question

I'm making a simple graphics engine for my game.

This is interface part:

class Texture { ... };

class DrawContext 
{ 
    virtual void set_texture(Texture* texture) = 0; 
}

This is implementation part:

class GLTexture : public Texture
{
public:
    ...
    GLuint handle;
};

void GLDrawContext::set_texture(Texture* texture)
{
    // Check if it's GLTexture, otherwise do nothing.
    if (GLTexture* gl_texture = dynamic_cast<GLTexture*>(texture))
    {
        glBindTexture(GL_TEXTURE_2D, gl_texture->handle);
    }
}

Does it make sense to use dynamic_cast here? Is there a way to avoid it?

Answer 1

Could you try reversing the concern?

class Texture
{
public:
    virtual void set_texture() = 0;
};

class GLTexture : public Texture
{
public:
    virtual void set_texture();
    GLuint handle;
};

void GLTexture::set_texture()
{
    glBindTexture(GL_TEXTURE_2D, handle);
}

---

class DrawContext 
{ 
    virtual void set_texture(Texture* texture) = 0; 
};

class GLDrawContext : public DrawContext
{
    virtual void set_texture(Texture* texture);
};

void GLDrawContext::set_texture(Texture* texture)
{
    texture->set_texture();
}

Answer 2

Sure, use static_cast instead, though you will lose some error handling if you pass in a bogus pointer. We use an assert_cast idea to dynamic on debug builds and static for release to get around the RTTI for this sort of thing.

Answer 3

I think the standard way to avoid a dynamic_cast would be to add a virtual method to the Texture class:

virtual int get_texture_handle() const {return -1;}

Then override the method only in your GLTexture class:

virtual int get_texture_handle() const {return gl_texture->handle;}

Then your calling code would look like this:

int handle = texture->get_texture_handle();
if (handle >= 0) glBindTexture(GL_TEXTURE_2D, handle);

Answer 4

A slightly different approach that involves modifying the Texture class.

class Texture 
{
    virtual void bind_texture(){} 
};
class GLTexture : public Texture 
{
    virtual void bind_texture(); 
};
void GLTexture::bind_texture()
{
    glBindTexture(GL_TEXTURE_2D, handle);
}
class DrawContext 
{ 
    virtual void set_texture(Texture* texture) = 0; 
};
class GLDrawContext : public DrawContext
{
    virtual void set_texture(Texture* texture);
};
void GLDrawContext::set_texture(Texture* texture)
{
    if( texture )
        texture->bind_texture();
}

Answer 5

As an alternative you could try using generics to get away from dynamic cast. Generics will let you catch errors at compile time (you can never pass a DirectX texture to a GL DrawContext). Additionally, there will be no cost for dynamic dispatch and the compiler should be able to do inlining.

namespace GL_impl {

struct Texture {
    GLuint handle;
};

struct DrawContext {
    void set_texture(Texture* texture)
    {
        glBindTexture(GL_TEXTURE_2D, texture->handle);
    }
};

} // GL_impl


struct use_GL {
    typedef GL_impl::Texture Texture;
    typedef GL_impl::DrawContext DrawContext;
};

template <class use_impl>
void f()
{
    typedef typename use_impl::Texture Texture;
    typedef typename use_impl::DrawContext DrawContext;

    Texture t;
    DrawContext ctx;
    ctx.set_texture(&t);
}

void call_f_with_gl()
{
    f<use_GL>();
}