Re: reference lifetimes...

From:

Victor Bazarov <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 11 Nov 2009 08:58:48 -0500

Message-ID:

<hdeftt$632$1@news.datemas.de>

James wrote:

"Victor Bazarov" <v.Abazarov@comAcast.net> wrote in message
news:hdee11$3ui$1@news.datemas.de...

James wrote:

[...]

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.

I am a bit confused. So, are you saying that the output:

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

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

OR

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

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

is perfectly fine because of undefined behavior? James Kanze threw me
off when he said it could be compiler bug.

darn.

Yep. Essentially it's the same as

    class foo { ... }; // just like yours

    class HasARef {
       const foo& m_ref;
    public:
       HasARef(const foo& r) : m_ref(r) {}
    };

    #include <iostream>
    int main() {
       HasARef h(( foo() )); // need double parens
       std::cout << "okay\n";
    }

What happens here is the reference 'm_ref' is *not* bound directly to
the temporary, but instead goes through a "copying" process - the
argument to the constructor. *The argument* is bound to the temporary.
  The 'm_ref' member isn't because there is no direct initialisation of
it with the expression that yields the temporary.

It's not a compiler bug, AFAICT.

V
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