why constructors "return" this pointer? - c++

Question

I noticed that the constructor will move this to eax before returning. This is a return value or something else?
class CTest {
int val_;
public:
CTest() {
0093F700 push ebp
0093F701 mov ebp,esp
0093F703 sub esp,0CCh
0093F709 push ebx
0093F70A push esi
0093F70B push edi
0093F70C push ecx
0093F70D lea edi,[ebp-0CCh]
0093F713 mov ecx,33h
0093F718 mov eax,0CCCCCCCCh
0093F71D rep stos dword ptr es:[edi]
0093F71F pop ecx
0093F720 mov dword ptr [this],ecx
val_ = 1;
0093F723 mov eax,dword ptr [this]
0093F726 mov dword ptr [eax],1
}
0093F72C mov eax,dword ptr [this]
0093F72F pop edi
0093F730 pop esi
0093F731 pop ebx
0093F732 mov esp,ebp
0093F734 pop ebp
0093F735 ret
VS2012 debug mode
I found that new will use its "return value". Seems like if(operator new() == 0) return 0; else return constructor();
class CTest {
int val_;
public:
CTest() {
val_ = 1;
__asm {
mov eax, 0x12345678
pop edi
pop esi
pop ebx
mov esp,ebp
pop ebp
ret
}
}
};
int main() {
CTest *test = new CTest; // test == 0x12345678
return 0;
}

Answer 1

Your second question disagrees with your first. How can new use if ( operator new() == 0 ) return 0; else return constructor(); if constructor() is producing the condition result?
Anyway…
What the compiler does with registers is the compiler's business. Registers tend to hold whatever information is immediately useful, and if the compiler is written with the belief that every time the constructor is used, the object is used immediately afterwards, it may reasonably choose to put this in a register.
An ABI may require constructors to do this, but I doubt any do. Anyway, such protocols only apply to things exported from libraries, not strictly within programs.
Any new expression does check the result of operator new against 0 before proceeding to initialize an object. operator new may signal failure by returning nullptr (or NULL, etc.).
This can actually be a problem with placement new expressions, because it represents unavoidable runtime overhead as the given pointer is generally already known to be non-null.

Answer 2

This can be a feature by design, in C++ and other languages, returning a reference to a given instance allows a more "idiomatic" use of the features offered by the object itself, in short it's the Named parameter Idiom .
But this is just 1 option, it can be useful sometimes, especially if you are able to design your library in a way that it only "takes actions" without having the need to pass a significant amount of parameters, so the chain of method calls stays readable.