引用对象内部的指针

Question

I asked a question similar to this awhile ago which answered my question perfectly, but I have a new dilemma which I don't fully understand and would appreciate it if someone could help. I have the following variable in a class:

ID3D11Buffer* variable

I need to de-reference this variable in my draw call for DirectX so I create a function to do so:

const ID3D11Buffer* GetBuffer() const { return &variable; }

But then I get this error which I don't really understand and it's kinda hard to google:

error C2440: 'return' : cannot convert from 'ID3D11Buffer *const *' to 'const ID3D11Buffer *'

Could a wise C++ guru enlighten me please? :D

Edit: With the answer provided I get a new error, this was why I was trying to add the & at the start, how would I make this line valid?

The draw call line:

md3dImmediateContext->IASetVertexBuffers(0, 1, cube.GetBuffer(), &stride, &offset);

The answer below creates this new error:

error C2664: 'ID3D11DeviceContext::IASetVertexBuffers' : cannot convert parameter 3 from 'const ID3D11Buffer *' to 'ID3D11Buffer *const *'

The original code I had to make this work was

md3dImmediateContext->IASetVertexBuffers(0, 1, &buffer, &stride, &offset);

Edit # 2: Thanks for the help, your suggestion made the get function valid, but the IASetVertexBuffers function still didn't like it. It returns this error with the new suggestion:

error C2664: 'ID3D11DeviceContext::IASetVertexBuffers' : cannot convert parameter 3 from 'const ID3D11Buffer *const *' to 'ID3D11Buffer *const *'

Answer 1

GetBuffer should return a pointer to the buffer - not a pointer (to pointer) to it. Since variable is already defined as a pointer, you can simply return that:

const ID3D11Buffer* GetBuffer() const { return variable; }

&variable is of type ID3D11Buffer** which doesn't match a pointer of type ID3D11Buffer* , hence the error.

Edit -

After reading the edit made to the OP's post, it is clear that GetBuffer is supposed to return a pointer (to pointer) to the buffer. So what was in fact wrong was the return type , not the return value. So he simply needs to change the return type signature to:

const ID3D11Buffer* const* GetBuffer const { return &variable; }
//                ^^^^^^^^

and return what he originally did, which was the address of the pointer variable .

Answer 2

variable is already of the type ID3D11Buffer* . So returning return variable; should suffice.

Answer 3

look at const X * const

If you draw a line at the *

The const on the left refers to the pointer, so you can't change the pointer value (where it points)

the const on the right refers to the object being pointed to, so you can't change the contents

Answer 4

A little trick to find out which is const (pointer or variable) you can read it backwards.

• const type* X; -> Pointer to a variable of type 'type' which is const (the data is const)
• type * const X; -> const pointer to a variable of type 'type' (the pointer is const)
• const type * const X; -> const pointer to a variable of type 'type' which is const (data and pointer are const)