Re: reference lifetimes...

James Kanze <>
Wed, 11 Nov 2009 10:30:03 -0800 (PST)
On Nov 11, 1:26 pm, Victor Bazarov <> wrote:

James wrote:

Here is my code:

#include <iostream>

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

       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



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

The form

     T t = { blah };

is copy-initialisation.

But not of T. In his case, he copy initializes the member m_ref
of foo_holder with a temporary.

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

I don't see anything in the standard which says that. He's
using aggregate initialization, which means that he's providing
a list of expressions to initialize each of the elements of the
aggregate. In his case, foo() is the expression which
initializes the foo_holder::m_ref, and not the foo_holder
itself. And the distinction between copy initialization and
direct initialization doesn't apply to either references or
aggregate initialization. (If you'll look at the points in
=A78.5/14, you'll see that the standard sends you to another
section for both aggregate initialization and reference
initialization, before considering whether it is copy
initialization or not.) The compiler can still make a copy (at
least at present), but then it is the copy which is bound to the
reference, and whose lifetime is extended. (And of course, if
the compiler did make a copy, we'd see two calls to the
destructor each time.)

The 'foo_holder' temporary is destroyed after initialising of
the 'fh' variable, causing the destruction of your foo'

There is no foo_holder 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

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

No. It's a question of aggregate initialization.

James Kanze

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