CODEWITHSUNDEEP
c++pointersreference
Prasoon Saurav
Prasoon Saurav

Reputation: 92942

Dereference and return by reference

Consider a small unit test case

struct A
{
   virtual void func(){}
   A& foo()
   {
     A *obj = reinterpret_cast<A*>(0xdeadbeef);
     return *obj; //1
   }
};

int main()
{
   A obj = obj.foo();
}

At line 1 is it implementation defined/unspecified that the deference would not happen as we are returning by reference and the program would not crash if an explicit access to the pointed to object is not made?

I had argument with one of my colleagues wherein he mentioned that the compiler, in most cases, would optimize the dereference of obj as we are returning it by reference and this code would not crash?

Thanks

Upvotes: 5

Views: 179

Answers (1)

MIKU_LINK
MIKU_LINK

Reputation: 192

I disassemble the code in MS VC8.0, and find things more interesting:

006D48D6  mov         dword ptr [ebp-8],ecx 
    A *obj = reinterpret_cast<A*>(0xdeadbeef);
006D48D9  mov         dword ptr [obj],0DEADBEEFh 
        return *obj; //1
006D48E0  mov         eax,dword ptr [obj] //2 
    }
006D48E3  mov         esp,ebp 
006D48E5  pop         ebp  
006D48E6  ret

//2 shows that just put the address of obj to the eax register as return value.

006D39FC  lea         ecx,[ebp-190h] 
006D3A02  call        A::foo (6A8C12h) 
006D3A07  push        eax  //3
006D3A08  lea         ecx,[ebp-190h] 
006D3A0E  call        A::A (6B89BEh)

eax is 0xdeadbeef, and push to stack as temporary local variable does. Than we call the copy constructor(which is trivial) . All these actions are just pass the address(which is illegal but program do not care).Because the A struct does not have any member which is binded to specific object, and program will not try to find specific object by dereference if it doesn' t need.So this bomb have not been fired.

        A *obj = reinterpret_cast<A*>(0xdeadbeef);
        A tmp ;
         temp = *obj;

Even this is OK because operator= is also pass by reference which in fact pass the address.If you add a member variable, it will fail because it will try to find and copy the member.

Upvotes: 4

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