How do I get buffers from another class for CreateBuffer

Question

I believe this is not strictly specific to DirectX but just to pure c++ language problem.

I have a class (simplified):

class A {
public:
    ID3D11Buffer* getVBuffer() const {
        return m_vbuffer;
    }

    // I will make this public for this example
    ID3D11Buffer* m_vbuffer;
};

and then in some other place I am creating vertex buffer:

device->CreateBuffer( ..., &instanceOfA->m_vbuffer );

it works ok, but why with the getter method doesn't?

ID3D11Buffer* _vertexBuffer = instanceOfA->getVBuffer();
device->CreateBuffer( ..., &_vertexBuffer  );

it crashes at Map function with access violation, but I'd like to have getter method used.

// EDIT

when I wrote like this:

device->CreateBuffer( ..., &instanceOfA->getVBuffer()  );

the compiler (VS2012) told me:

error C2102: '&' requires l-value

Is it then even possible to use getters in this situation?

Answer 1

It's a pointer misunderstanding.

Consider this, even more simplified example:

ID3D11Buffer *original_buffer = NULL;
ID3D11Buffer *returned_buffer = original_buffer;

Okay, so you have two pointers that both point to the same location. This is what happens when you call the getter. The only thing they have in common is that they point to the same address.

device->CreateBuffer(&returned_buffer);

Now, your "returned_buffer" points to the newly created buffer. The "original_buffer" still points to its initial value NULL.

device->MapBuffer(original_buffer);

... and you try to map NULL. Boom!