You can ask the compiler to answer your calling convention questions

Raymond Chen

February 20th, 20130 0

If you want to figure out some quirks of a callingconvention,you can always ask the compiler to do it for you,on the not unreasonable assumption that the compilerunderstands calling conventions.

“When a __stdcallfunction returns a large structure by value,there is a hidden first parameter that specifies theaddress the return value should be stored.But if the function is a C++ instance method,then there is also a hidden thisparameter.Which goes first, the return value parameteror the this pointer?”

This is another case ofYou don’t need to ask me a question the compiler can answer more accurately.

struct LargeStructure
{
 char x[256];
};


class Something
{
public:
 LargeStructure __stdcall TestMe();
};


void foo(Something *something)
{
 LargeStructure x = something->TestMe();
}

You could compile this into a program andthen look in the debugger,or just ask the compiler to generate an assemblylisting.I prefer the assembly listing, since it saves a fewsteps,and the compiler provides helpful symbolic names.

  00015 mov     eax, DWORD PTR _something$[ebp]


; LargeStructure x = something->TestMe();


  00018 lea     ecx, DWORD PTR _x$[ebp]
  0001e push    ecx
  0001f push    eax
  00020 call    ?TestMe@Something@@
                QAG?AULargeStructure@@XZ
                ; Something::TestMe

We see that the last thing pushed onto the stack(and therefore the top parameter on the stackat the point of the call)is the something parameter,which is the thisfor the function.

Conclusion:The this pointer goes ahead of theoutput structure pointer.

Raymond Chen