Why is fastcall used for replacing thiscall functions in memory instead of cdecl?

Question

Assume I were to be reversing some game which processes all of it's movement client side and have determined some function to be of the type:

bool __thiscall Player::CanJump(Player *this)

that I have determined to be a member of the Player object's vtable. Now lets assume I wanted to edit that object's vtable to point to my own dll injected implementation so that I could jump whenever I wanted. I could declare it as

bool __fastcall CanJumpReplacement(Player *player) {
    return true;
}

and replace the Player's vtable entry with a pointer to this function. This works as expected, but why should I use the fastcall convention here? Fastcall is used almost exclusively for this purpose from what I can tell, but I was reading through the calling conventions and cdecl seems to be a much closer match to thiscall than fastcall. Both calling conventions succeeded in replacing the function.

Answer 1

This question is a bit confusing.

Both __fastcall and __thiscall share that they use ecx as the first storage point. So either you implicitly say the class pointer will be in ecx (__thiscall) or you say the function is not a member function but has one argument - which also gets passed in ecx when using __fastcall so the class pointer still ends up in the right register.

Also, both calling conventions use callee cleanup so no problem here, too.

However, this only works for no-argument functions. If your function had an argument, it would end up in edx for __fastcall, but on the stack for __thiscall and therefore not work anymore.