Calling __thiscall functions is easier with the __fastcall trick 🎓

Avoid inline assembly when calling __thiscall functions. This tutorial covers the theory of registers and the technique for passing the this pointer through ECX using __fastcall.

👉 https://www.youtube.com/watch?v=hZQzJkzmPG4

Video Game Reverse Engineering: Calling __thiscall Functions

When you are digging into a game's engine, you are going to run into member functions all the time. The problem is that these functions use the __thiscall calling convention, which expects the this pointer to be sitting in the ECX register. Standard C++ doesn't give you a direct way to just point at an address and say "call this as a member function" without some extra work. Instead of messing around with messy inline assembly or complicated shellcode, you can use a clever trick by defining the function as __fastcall. Since __fastcall passes the first argument in ECX and the second in EDX, you can pass your this pointer as the first argument and just use a dummy placeholder for the second. This makes the compiler do the heavy lifting for you and keeps your code much cleaner. You should watch this video on calling member functions using the fastcall trick to see the register management in action. This is the most efficient way to trigger game logic—like firing a weapon or opening a menu—by calling the internal function address directly.

The Fundamentals of Game Reversing: Understanding Calling Conventions

You can't really get far in game reversing if you don't understand how the CPU handles data being passed between functions. Every time a game calls a function, there is a "contract" between the caller and the callee known as a calling convention. This determines whether arguments are pushed onto the stack or placed in registers like EAX, ECX, or EDX. If you try to call a function using the wrong convention, you’ll end up with a stack imbalance or a null pointer reference that will crash the game immediately. We have a deep dive on calling conventions and why you need to know them that explains the differences between cdecl, stdcall, and fastcall in detail. Understanding this is the difference between a hack that works and one that just closes the game. It is a core part of video game reverse engineering because once you know where the data is supposed to be, you can manipulate it safely. Whether you are using a tool like Cheat Engine or writing a full internal DLL, the register state is the most important thing you need to monitor.

Advanced Game Reversing: Member Function Hooking and Redirection

Once you know how to call these functions, the next logical step is learning how to intercept them. Hooking a member function is tricky because you have to preserve the this pointer so the game can continue its original execution after your code runs. By using the same __fastcall trick mentioned earlier, you can create a hook that catches the this pointer in ECX and lets you inspect or modify the object's data before the real function ever sees it. We have a complete guide on how to call __thiscall functions by address and another specifically for hooking __thiscall member functions. This is how people create things like "Auto-Loot" or "ESP" by tapping into the game's own update loops and object managers. By mastering these register-level tricks, you aren't just changing numbers in memory; you are actually hijacking the game's internal logic and making it work for you. This is a massive part of game reversing because it allows for the most powerful features without needing to rewrite half the engine from scratch. At guidedhacking.com, we show you how to do this correctly so you can build stable, professional tools.