This 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 on the x86 architecture. The thiscall calling convention cannot be explicitly specified in a program, because thiscall is 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.