Virtual Method that returns <this>?

Question

In my vtable i found a method that simply returns ecx. Now im confused as to what this tries to accomplish ? Is this a known useful sequence ?

Answer 1

The C++ compiler of Visual Studio uses ecx as the default register for this pointer, a virtual method which returns ecx then actually returns this or *this. For example, you can test the following code:

class A
{
public:
  virtual A getmyself() { return *this; }
  virtual A* getmyselfpointer() { return this; }
}

The generated assembly code for getmyself (the same for getmyselfpointer) is

getmyself:
  mov     eax, ecx
  retn

This detail is not true for clang or gcc since they do not use ecx as default register for this.

Answer 2

It could be a method that at a high-level is intended to return a covariant object pointer.

Answer 3

If it would be ARM I'd say that it is constructor or destructor (see this answer for more details). As a part of more general pattern I had seen such a things in a multi-platform applications where the main code of the function was #ifdef'ed out and only return value was left for compatibility with other platforms/build configurations.

Did you see calls to this function?