In OpenGL, why does glVertexAttribPointer require the “pointer” argument to be passed in as void*?

Question

The specification for the glVertexAttribPointer is as follows:

void glVertexAttribPointer( GLuint index,
    GLint size,
    GLenum type,
    GLbool         


        

Answer 1

One way of looking at it is that the last argument is always a pointer:

• If no VBO is bound, it's a pointer relative to base address 0. Which is a regular memory address, just the way pointers are normally used in C/C++.
• If a VBO bound, it's a pointer relative to the base address of the buffer.

At least that's the only logical explanation I could ever find.

Personally, I think that overloading the entry point this way was a very unfortunate decision, for a number of reasons:

• It confuses people to no end.
• It requires ugly type casts in C/C++.
• It does not work at all in more type safe languages, like Java.

In languages like Java, you typically end up with overloaded versions of the function that accept different types. As a somewhat curious historical note, the overloaded version with an int argument was missing in the initial version of the GLES20 bindings in Android, which meant that you could not use VBOs from Java. So this has tripped up more that just the occasional casual OpenGL programmer.

Answer 2

Legacy.

That argument had a different meaning before VBOs: you'd keep the vertex data in client memory and pass the address of the array (see glEnableClientState and such).

Now the last parameter can have 2 meanings (offset for buffer objects, address for client state arrays). Khronos did not provide a separate version for gl*Pointer functions for buffer objects, so you need to do this awkward cast.