thiscall

Thiscall is a calling convention used for non-static C++ member functions on 32-bit x86 platforms, most commonly with Microsoft Visual C++ on Windows. It defines how the this pointer (the object instance) and function arguments are passed and how the stack is cleaned up when a member function is called.

In this convention, the implicit this pointer is passed in the ECX register. The explicit function arguments are passed on the stack in right-to-left order. The callee (the called function) is responsible for cleaning up the stack, making thiscall similar to stdcall in terms of stack cleanup, but with the this pointer supplied in ECX rather than on the stack. The function’s return value is placed in EAX, as with other 32-bit integer and floating-point returns.

Thiscall is mainly associated with the Microsoft Visual C++ compiler on 32-bit Windows. Some other Windows toolchains, such as MinGW GCC, implement __thiscall to emulate MSVC behavior. On 64-bit Windows, there is no separate __thiscall; the Windows x64 ABI uses a single convention for member functions, with the this pointer passed in a general-purpose register (such as RCX) according to the platform’s 64-bit rules.

Portability considerations apply when interfacing with code compiled under different ABIs or on different architectures. Because thiscall relies on a register for this and a specific stack-cleanup model, code compiled with this convention may not be portable to non-Microsoft or non-32-bit environments without proper interop measures.