ID3DXFont::DrawText modifying the IDirect3DDevice9 vtable

I am mess开发者_运维问答ing around with Direct3D - modifying vtable entries for a device object so that my functions get called instead the provided ones. I noticed that despite this mostly working, at points something restores the IDirect3DDevice9 vtable, for example ID3DXFont::DrawText - the very first call; ulterior ones don't touch it.

Why does this happen?

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Modifying vtables is pretty much undefined behavior. Maybe there are caches or different copies of the vtable. It's all implementation dependent. You're lucky it works at all!