g++ weird behavoir with values returned by copy.

[Leandro Lucarella <llucar at mecon dot gov dot ar>]

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Hi, I was tring to figure out how g++ deals with return values by copy.
First I found that it doesn't return a copy, at least when I'm returning
a local variable created in my function (or method).
If I do something like this:
class A;
A test() {
    A tmp;
    return A;
}

Is tmp who is returned, not a copy. This works fine on most cases, but
I've found a case where the behavoir is much more weird (at least for
me).

and I found a very weird case, in wich with cause an object to be
destroyed when it's still in use using a reference:

#include <iostream>

using namespace std;

struct A {
        A(void) {}
        A(const A& a) {}
        ~A(void) {}
        A& operator=(const A& a) {
                return *this;
        }
        A& print(void) {
                return *this;
        }
};

A test(void) {
        A tmp;
        cout << "Doing something." << endl;
        return tmp;
}

int main(void) {
        A& ar = test().print();
	// Now ar points to a destroyed objec
        ar.print();
        return 0;
}


Here's a gdb session resumen transcription:
Breakpoint 1, main () at test_copia_return_simple.cpp:24
24              A& ar = test().print();
test() () at test_copia_return_simple.cpp:18
18              A tmp;
A (this=0xbffff880) at test_copia_return_simple.cpp:6    <---- tmp points to 0xbffff880
6               A(void) {}
test() () at test_copia_return_simple.cpp:19
19              cout << "Doing something." << endl;
Doing something.
20              return tmp;
A::print() (this=0xbffff880) at test_copia_return_simple.cpp:13 <---- ar points to 0xbffff880
13                      return *this;
~A (this=0xbffff880) at test_copia_return_simple.cpp:8   <---- tmp (0xbffff880) is destroyed
8               ~A(void) {}
main () at test_copia_return_simple.cpp:26
26              ar.print();
A::print() (this=0xbffff880) at test_copia_return_simple.cpp:13 <---- 0xbffff880 is used (ar.print())
13                      return *this;
14              }
main () at test_copia_return_simple.cpp:27
27              return 0;

If I try to do A& ar = test(); I get a compile time error.
I compiled this tests with g++ (GCC) 3.3.2 (Debian) using -Wall and -g3 flags only.

I don't know if this behavoir is normal since I'm a relative newbie C++ programmer,
but I found it odd so I thought it could be some kind of bug. If it's not a bug, I'll
appreciate if someone can explain me why copy returned values are not really returned
by copy.

Thanks a lot, and please Cc me because I'm not subscribed to the list.

- -- 
Leandro Lucarella @ MEcon                       .-----------------------------.
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