Re: reference lifetimes...

From:
James Kanze <james.kanze@gmail.com>
Newsgroups:
comp.lang.c++
Date:
Thu, 12 Nov 2009 00:34:37 -0800 (PST)
Message-ID:
<4a6914a1-ed0f-4aaf-a008-759c554aa731@t2g2000yqn.googlegroups.com>
On Nov 11, 7:31 pm, Victor Bazarov <v.Abaza...@comAcast.net> wrote:

James Kanze wrote:

    [...]

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.


I think you're contradicting yourself here. If 'T' is an
aggregate, and the form

     T t = { <initialiser list> };

does not perform copy-initialisation (from some temporary
aggregate into the 't' object), then there is no
copy-initialisation of member 'm_ref' anywhere, it's direct
initialisation of the reference with a temporary, which we
know as "binding to a temporary".


According to =A78.5/12:

    The initialization that occurs in argument passing,
    function return, throwing an exception (15.1), handling
    an exception (15.3), AND BRACE-ENCLOSED INITIALIZER
    LISTS (8.5.1) is called copy-initialization and is
    equivalent to the form

        T x = a;

In other words, it's copy initialization, because the standard
says it's copy initialization. (Don't ask me why the standard
makes this choice. In most cases where the syntax requires one,
and only one, initializer, it's copy initialization, but this
isn't an absolute: static_cast requires one, and only one,
initializer, but is direct initialization.)

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'.


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.)


Not a copy of 'foo', a copy of 'foo_holder',


That's not at all what the standard says. The standard quite
clearly treats aggregate initialization as a separate case (in a
separate section), and talks of initializing each of the
elements. It's the same as if you had an array.

which has no destructor, but it's irrelevant. It's possible
that GC++ creators followed the same faulty logic I did, and
came to the conclusion that they need to create a temporary of
type 'foo_holder', which then allowed destruction of the
temporary 'foo'.


Or that they have some other bug.

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


There is no foo_holder temporary.


Well, how do you know that the compiler doesn't create it?


The standard doesn't allow it.

Unless the OP looks at the generated assembly code, there is
no way to tell, is there? I mean, in real life. Perhaps
you're saying that the Standard requires that there shan't be
any copies of 'foo_holder' objects created during
initialisation (brace-enclosed), IOW copying is prohibited.
Then you're right and the GC++ has a bug.


You do raise an interesting point. What is g++ doing exactly.
I defined a second class, bar, exactly identical to foo (and
added tracing of the copy constructor to both as well), and
changed foo_holder to:

    struct foo_holder { foo const& m_ref; bar m_val; };

I then get the following output from g++:

    0x22ff40->foo::foo()
    0x22ff54->bar::bar()
    0x22ff40->foo::~foo()
    okay
    0x22ff54->bar::~bar()

    0x22ff40->foo::foo()
    okay
    0x22ff40->foo::~foo()

Very inconsistent, if you ask me.

--
James Kanze

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