Re: reference lifetimes...

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 11 Nov 2009 08:26:21 -0500

Message-ID:

<hdee11$3ui$1@news.datemas.de>

James wrote:

Here is my code:

#include <iostream>

struct foo
{
   foo()
   {
       std::cout << this
                 << "->foo::foo()"
                 << std::endl;
   }

   ~foo()
   {
       std::cout << this
                 << "->foo::~foo()"
                 << std::endl;
   }
};

struct foo_holder
{
   foo const& m_ref;
};

int main()
{
   {
       foo_holder fh = { foo() };
       std::cout << "okay" << std::endl;
   }

   std::cout << std::endl;

   {
       foo const& ref = foo();
       std::cout << "okay" << std::endl;
   }

   return 0;
}

Why does the const reference not properly maintain its lifetime over the
call to 'cout' when the reference is contained within a POD
'foo_holder'? I get the following output:

0x22ff50->foo::foo()
0x22ff50->foo::~foo()
okay

0x22ff50->foo::foo()
okay
0x22ff50->foo::~foo()

Something seems terribly wrong here... Is there anyway to overcome this?

The form

T t = { blah };

is copy-initialisation. The compiler is free to create another
temporary (of your class 'foo_holder') before initializing 'fh' with it,
which makes the temporary's 'm_ref' member bound to the temporary 'foo'.
The 'foo_holder' temporary is destroyed after initialising of the 'fh'
variable, causing the destruction of your foo' temporary.

The references when initialised with the above form are treated
differently. A possible copy of the object is created and the reference
is then bound to it. That temporary (either another one or the original
one) survives as long as the reference.

Also, I don't think this has anything to do with PODs.

V
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