Re: member functions returning class type

"Jim Langston" <>
Fri, 28 Dec 2007 08:24:57 -0800
Kira Yamato wrote:

On 2007-12-27 02:09:02 -0500, Jalen <> said:

On Dec 27, 10:34 am, eastern_strider <> wrote:

Very good insight; it makes sense to me.

Thanks very much.

On Dec 26, 9:28 pm, Kira Yamato <> wrote:

On 2007-12-26 20:49:58 -0500, eastern_strider
<> said:


In the following code snippet, if I comment out line b, the output


whereas if I comment out line c, the output becomes:

copy constructor

Any explanations regarding what is causing this difference is

#include <iostream>
using namespace std;

class A {
A() { cout << "constructor" << endl; }
A(const A& obj) { cout << "copy constructor" << endl; }
A foo() { return A(); }

int main()
A a;
A b(a); // <--- line b
A c(; // <--- line c

return 0;

Interesting find on this C++ behavior.

I'm guessing it's a compiler optimization.

In line b, you're declaring another object that is different from
a. So, the compiler had to invoke the copy constructor to create
another object (b).

However, in line c, the call returns a temporary object,
call it t. This object t is already another object that is different
from a. So, the compiler most likely just aliased the variable c to t,
to avoid an unnecessary object copying.

The scope of t is now the body of main and not line c anymore. That is,
it won't be cleaned up at the end of line c, but instead, it
will be cleaned up at the end of main.



This problem can't be reproduced on Microsoft Visual C++ 6.0.
Line c will invoke the copy constructor.

It is certainly reproducbile in g++ 4.0.1.

I guess that just say MSVC++ sucks. :)

Or maybe there is some check buttons you need to click before enabling
certain optimizations that are apparently not on by default.

MSVC++ .net 2003 reproduces the OPs output. VSVC++ 6.0 sucks, it was

Jim Langston

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