Re: NRVO or I think so

From:
Victor Bazarov <v.Abazarov@comAcast.net>
Newsgroups:
microsoft.public.vc.language
Date:
Fri, 13 Mar 2009 10:12:21 -0400
Message-ID:
<gpdpk6$9s6$1@news.datemas.de>
K?r?at wrote:

Hi,

I have a very simple class :

class Foo
{
        private :
            int m_nVal;
        public:
            Foo () {cout << "Constructor..." << endl;}
            Foo (const Foo&) {cout << "Copy constructor..." << endl;}
            ~Foo () {cout << "Destructor..." << endl;}

            void setter (int nVal) {m_nVal = nVal;}
};

And some code using this class :

1 Foo getFoo ()
2 {
3 Foo foo_1;
4
5 foo_1.setter (11);
6
7 return foo_1;
8 }
9
10 int _tmain (int argc, _TCHAR* argv [])
11 {
12 Foo foo = getFoo ();
13 return 0;
14 }

I expeced this output :

Constructor... (For line 3)
Copy constructor... (For line 7, a temporary object is copy-constructed from
foo_1)
Destructor... (For foo_1)
Copy constructor... (For line 12, foo is copy-constructed from the temporary
object)
Destructor... (for foo)
Destructor... (for the temporary object)

but the real output is (using Visual Studio 2008 compiler with debug
configuration) :

Constructor...
Copy constructor...
Destructor...
Destructor...

When I checked the assemby out I see address of the foo pushed into the
stack and used directly in getFoo () rather than creating a new temporary.
This is the optimization named NRVO, isn't this?
But I am compiling in debug configuration and optimizations are disabled,
why the compiler apply NRVO and how can I deactivate NRVO?


The compiler is allowed to forgo creating another temporary in the case of

     T func(); // function that returns a T
     T o = func(); // initialisation of another T

and construct the return value directly into 'o' (see [class.copy]/15).
  It is not really *optimization*. It's just allowed to construct the
object directly into the other one. Since it's part of the language,
the implementation-specific optimization mechanisms probably do not
apply here. And besides, VC++ developers probably chose to always apply
those because they may have thought there was no sense in letting the
users control this. Unlike inlining, for example.

And why would you *want* to deactivate NRVO?

V
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