The __thiscall calling convention is used on member functions and is the default calling convention used by C++ member functions that do not use variable arguments. Under __thiscall, the callee cleans the stack, which is impossible for vararg functions. Arguments are pushed on the stack from right to left, with the this pointer being passed via register ECX, and not on the stack, on the x86 architecture.
One reason to use __thiscall is in classes whose member functions use __clrcall by default. In that case, you can use __thiscall to make individual member functions callable from native code.
When compiling with /clr:pure, all functions and function pointers are __clrcall unless specified otherwise.
In releases before Visual C++ 2005, the thiscall calling convention could not be explicitly specified in a program, because thiscall was not a keyword.
vararg member functions use the __cdecl calling convention. All function arguments are pushed on the stack, with the this pointer placed on the stack last
Because this calling convention applies only to C++, there is no C name decoration scheme.
On ARM and x64 machines, __thiscall is accepted and ignored by the compiler.
For non-static class functions, if the function is defined out-of-line, the calling convention modifier does not have to be specified on the out-of-line definition. That is, for class non-static member methods, the calling convention specified during declaration is assumed at the point of definition.