Meaning of #define THIS_

Question

For example, there is a function named CreateFrame like,

virtual STDMETHODIMP CreateFrame(THIS_ LPCSTR Name, 
LPD3DXFRAME*ppNewFrame);

I think STDMETHODIMP means It will return HResult and stdcall function, but I don't know what THIS_ in argument menas.

THIS_ is defined in combaseapi.h like this.

  #define PURE                    = 0
  #define THIS_
  #define THIS                    void

For more information, the function 'CreateFrame ' is called automatically when D3DXLoadMeshHierarchyFromX is called.

Answer 1

Those are just old-school COM macros. Don't worry about them, especially since you are looking at the deprecated D3DX header for legacy Direct3D 9 from 13+ years ago...

virtual STDMETHODIMP Func(THIS);

This is a function that takes ONE parameter: the implicit this pointer, returns an HRESULT, and is annotated for proper COM calling convention __stdcall.

virtual STDMETHODIMP Func(THIS_ LPCSTR Name, LPD3DXFRAME*ppNewFrame);

This is a function that takes THREE parameters: the implicit this pointer, a pointer to a string, and a pointer to a specific object), returns an HRESULT, and is annotated for proper COM calling convention.

To declare a function that returns a type other than HRESULT you'd use STDMETHOD_:

virtual STDMETHOD_(ULONG, Func) (THIS_ LPCSTR Name, LPD3DXFRAME*ppNewFrame);

These old headers often also supported use from C rather than C++, where this is not implicit. Legacy D3DX never bothered with the C call-paths, but many old system headers did use them. In fact, if you did a little further in combaseapi.h you'll see the C language section defines THIS_ and THIS as:

#define PURE
#define THIS_                   INTERFACE FAR* This,
#define THIS                    INTERFACE FAR* This

The more modern MIDL compiler generates code that's slightly less confusing:

virtual HRESULT STDMETHODCALLTYPE Func(void);

or

virtual void STDMETHODCALLTYPE Func(UINT value, LPCWSTR name);