Defining a pointer to an OpenGL Vertex Array Object

Question

Consider the following OpenGL code snippet:

GLuint vao;
glCreateVertexArrays(1, glBindVertexArray(&vao));

I'm under the impression that what this code does is

Creates a pointer, of type GLuint, that will eventually point to an OpenGL Vertex Array Object. The name of this pointer is vao.
Indirectly allocates memory through OpenGL for the actual Vertex Array Object (i.e., no use of malloc in C or new in C++). The way that this is done is through the function glCreateVertexArrays. This function also makes the pointer vao point to the Vertex Array Object.

Is my understanding correct? If so, in (1), why is the type of the pointer GLuint and not explicitly defined as a pointer type (for example with * in C/C++ or with the OpenGL type GLintptr)?

Yakov Galka · Accepted Answer · 2016-10-28 17:33:48Z

6

This code is invalid, glBindVertexArray returns void so you cannot pass its return value as a parameter to glCreateVertexArrays, because there is none.

The correct way to create a VAO, as per OpenGL 4.5 profile, is:

GLuint vao;
glCreateVertexArrays(1, &vao);

As per the meaning of vao and why its type is GLuint: it is not a pointer. It is an opaque value that is used internally to locate the VAO inside the OpenGL implementation tables. The driver may allocate memory on both, the CPU and/or GPU for storing the state associated with the VAO. It is a complex state with quite a lot of book-keeping, which is much more involved than malloc or new. (In particular malloc can't be used to allocate GPU memory.)

answered Oct 28, 2016 at 17:33

Yakov Galka

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2 Comments

George Over a year ago

So I should think of vao as something like an (unsigned integer) key in a hash table lookup (that can get me to a vertex array object)?

Nicol Bolas Over a year ago

@George: Pretty much. That's how all OpenGL Objects work.

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