__thiscall
The Microsoft-specific __thiscall calling convention is used on C++ class member functions on the x86 architecture. It's the default calling convention used by member functions that don't use variable arguments (vararg functions).
Under __thiscall, the callee cleans the stack, which is impossible for vararg functions. Arguments are pushed on the stack from right to left. The this pointer is passed via register ECX, and not on the stack.
On ARM, ARM64, and x64 machines, __thiscall is accepted and ignored by the compiler. That's because they use a register-based calling convention by default.
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. The /clr:pure and /clr:safe compiler options are deprecated in Visual Studio 2015 and unsupported in Visual Studio 2017.
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++, it doesn't have a C name decoration scheme.
When you define a non-static class member function out-of-line, specify the calling convention modifier only in the declaration. You don't have to specify it again on the out-of-line definition. The compiler uses the calling convention specified during declaration at the point of definition.