Wednesday, June 22, 2016

Digest for comp.lang.c++@googlegroups.com - 25 updates in 10 topics

Ralf Goertz <me@myprovider.invalid>: Jun 22 03:47PM +0200

What is the following warning given by g++ supposed to mean?
 
es_test.cc:6:32: warning: unknown escape sequence: '\)'
std::string legal="\(",illegal="\)";
^
In particular, if '\(' is a known escape sequence (for what?) why isn't
'\)'?
Christian Gollwitzer <auriocus@gmx.de>: Jun 22 04:10PM +0200

Am 22.06.16 um 15:47 schrieb Ralf Goertz:
 
> es_test.cc:6:32: warning: unknown escape sequence: '\)'
> std::string legal="\(",illegal="\)";
> ^
 
In general, you should double all literal backslashes in a string, in
order not to fall for something like "C:\newstuff" which gets
interpreted as a newline character. The backslash is treated as a
literal backslash if the character that follows does not give a vald
escape sequence; but even if it des not now, a future compiler miht
treat it differently.
 
> In particular, if '\(' is a known escape sequence (for what?) why isn't
> '\)'?
 
 
I don't know, but since you intend to write literal backslashes, you
should write it as "\\(" and "\\)"
 
Christian
Ralf Goertz <me@myprovider.invalid>: Jun 22 05:00PM +0200

Am Wed, 22 Jun 2016 16:10:36 +0200
> > isn't '\)'?
 
> I don't know, but since you intend to write literal backslashes, you
> should write it as "\\(" and "\\)"
 
No, I don't intend to write that. I came across that warning when I
accidentally omitted one backslash trying to write "\\\\)". I
experimented and discovered that "\(" didn't give the warning. Since I
had never seen the escape sequence '\(' I looked it up in the standard.
But there I only found: "Escape sequences in which the character
following the backslash is not listed in Table 7 are
conditionally-supported, with implementation-defined semantics." Table 7
doesn't list '\('. So I figured gcc might have defined it. But I found
nothing in the man pages. That's why I asked.
guinness.tony@gmail.com: Jun 22 09:19AM -0700

On Wednesday, 22 June 2016 14:47:34 UTC+1, Ralf Goertz wrote:
> ^
> In particular, if '\(' is a known escape sequence (for what?) why isn't
> '\)'?
 
'\(' isn't a known escape sequence and, as such, should be just as
"illegal" as '\)'.
 
The question really should be "why does g++ accept this non-standard
escape sequence?". Which, of course, only the authors, documentation
and/or source code comments of g++ can tell you.
 
Out of interest, I've tested for other such undocumented escape sequences.
Apart from the "legal" ones (as defined by The Standard), g++ also
recognises '\%', '\(', '\[' and '\{', which all translate to their
unescaped versions. Additionally, it recognises '\E' and '\e',
translating both to ASCII <ESC>.
legalize+jeeves@mail.xmission.com (Richard): Jun 22 05:41PM

[Please do not mail me a copy of your followup]
 
Christian Gollwitzer <auriocus@gmx.de> spake the secret code
>> std::string legal="\(",illegal="\)";
>> ^
 
>In general, you should double all literal backslashes in a string,
 
Good god, no.
 
Just use raw string literals.
 
C++11, ya know. It's already 5 years old.
--
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legalize+jeeves@mail.xmission.com (Richard): Jun 22 05:43PM

[Please do not mail me a copy of your followup]
 
Ralf Goertz <me@myprovider.invalid> spake the secret code
 
>No, I don't intend to write that. I came across that warning when I
>accidentally omitted one backslash trying to write "\\\\)".
 
VS 2015 can refactor such string literals to raw string literals and I
added a similar check to clang-tidy:
<http://clang.llvm.org/extra/clang-tidy/checks/modernize-raw-string-literal.html>
--
"The Direct3D Graphics Pipeline" free book <http://tinyurl.com/d3d-pipeline>
The Computer Graphics Museum <http://computergraphicsmuseum.org>
The Terminals Wiki <http://terminals.classiccmp.org>
Legalize Adulthood! (my blog) <http://legalizeadulthood.wordpress.com>
Victor Bazarov <v.bazarov@comcast.invalid>: Jun 22 01:51PM -0400

On 6/22/2016 1:41 PM, Richard wrote:
>[..]
> C++11, ya know. It's already 5 years old.
 
Can you trust a 5-year-old with your stuff? ;-)
 
V
--
I do not respond to top-posted replies, please don't ask
Wouter van Ooijen <wouter@voti.nl>: Jun 22 06:47PM +0200

Op 21-Jun-16 om 9:21 PM schreef Stefan Ram:
 
> Then make that
 
> char* p,
> q;
 
 
ROFL!
 
Wouter
ram@zedat.fu-berlin.de (Stefan Ram): Jun 22 03:16PM

>cout << func(x) << " " << x << endl;
 
C++ does not specify the order of the evaluation (the
»sequencing«) of the operands of the operator »<<«.
 
So, one implementation can legally evaluate »func( x )«
before »x«, while another implementation can legally
evaluate »x« before »func( x )«.
 
You can rewrite this as
 
int const a = func( x );
int const b = x;
::std::cout << a << " " << b << '\n';
 
or
 
int const b = x;
int const a = func( x );
::std::cout << a << " " << b << '\n';
 
to have it deterministically the one way or the other.
 
Or maybe to
 
cout << func(x) << " ";
cout << x << endl;
 
if that was what you wanted.
mehrdad ghassempoory <m-ghassempoory@ntlworld.com>: Jun 22 04:07PM +0100

I have a very simple program shown below, I use gnu c++ version 4.8.4.
I expect the program to print:
 
0 2
 
but I get
 
0 0
 
however, if I use clang++ version 3.5, I get what I expected:
 
0 2
 
Is this a bug in gnu c++ or maybe I am confused about the way <<
operator works?
++++++++++++++++++++++++++++++++++++++++++++++++++
#include <iostream>
 
using namespace std;
 
int func(int& x)
{
x=2;
return 0;
}
 
int main()
{
int x;
 
x=0;
cout << func(x) << " " << x << endl;
 
return 0;
}
---------------------------------------------------
Mr Flibble <flibble@i42.co.uk>: Jun 22 12:54AM +0100

On 21/06/2016 23:07, DecadentLinuxUserNumeroUno wrote:
>> crap here.
 
> then why not translate it in C++ or C if you have some brains to do so.
> Don't talk crap here - fucker.
 
Oh look, we have a new little fucktard make a post.
 
Why would I, mate, want to translate some random spammer's Pascal source
code into C++? You are in serious need of a clue.
 
/Flibble
Ian Collins <ian-news@hotmail.com>: Jun 22 04:11PM +1200

On 06/22/16 09:04 AM, Mr Flibble wrote:
 
<stuff that won't change anything>
 
Please don't respond to (and even worse quote) Ramine spam news servers
filter out!
 
--
Ian
Mr Flibble <flibble@i42.co.uk>: Jun 21 10:04PM +0100

On 21/06/2016 21:35, Ramine wrote:
 
> You can download my efficient Threadpool engine version 3.0 that scales
> well from:
 
> https://sites.google.com/site/aminer68/an-efficient-threadpool-engine-that-scales-well
 
This is a C++ newsgroup not a Pascal newsgroup so stop posting Pascal
crap here.
 
/Flibble
Stanimir Stamenkov <s7an10@netscape.net>: Jun 18 01:52AM +0300

Mon, 6 Jun 2016 08:42:02 -0400, /Jerry Stuckle/:
 
> Yes, by not following the RFC's and then complaining about others, you
> are behaving as an asshole.
 
> See https://www.ietf.org/rfc/rfc3676.txt section 4.3.
 
This doesn't mandate everyone should use this signature separator.
It has its uses mainly for signatures which include more than simple
name, and spread two or more lines. One may sign using any of:
 
- Name
/Name
-- Name
 
or another style. I think you're just being rude, lazy, or just
ignorant by not stripping it yourself (if the reader doesn't or
can't do it automatically for you), just like you should trim quotes
to a sane minimum, and that can't be automatic.
 
--
Stanimir
MikeCopeland <mrc2323@cox.net>: Jun 17 11:02AM -0700

I'm confused about string::reverse_iterator usage.
What I'm trying to do is scan the end of a string variable for all
numeric digits..and then convert those digits to an integer. For
example, I have;
std::string myData = "cldat89";
and I want to extract and convert the "89" to an integer variable.
Various explanations I've found via Google aren't making sense to me,
as they mostly assume I want to reverse the whole string data or such.
I just want to scan backwards (from last-to-first) until a character
_isn't_ a digit...and then be able to work with the substring of the
digit characters.
Although done this task "brute-force", the concept of
reverse_iterator seemed interesting, so I wanted to try it: so far, I
don't "get it". Please advise. TIA
 
 
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Victor Bazarov <v.bazarov@comcast.invalid>: Jun 17 02:29PM -0400

On 6/17/2016 2:02 PM, MikeCopeland wrote:
> Although done this task "brute-force", the concept of
> reverse_iterator seemed interesting, so I wanted to try it: so far, I
> don't "get it". Please advise. TIA
 
When you "scan backwards", remember that the characters come in the
reverse order. So, something like this is supposed to work:
 
int accumulator(0);
int multiplier(1), maxnumber(100000);
std::locale defloc;
for (auto it = myData.rbegin();
it != myData.rend() && multiplier < maxnumber;
++it, multiplier *= 10)
{
char c = *it;
if (std::isdigit(c, defloc))
accumulator += (c - '0') * multiplier;
else
break;
}
 
(I didn't actually test this, but it should give you an idea).
 
V
--
I do not respond to top-posted replies, please don't ask
legalize+jeeves@mail.xmission.com (Richard): Jun 17 06:23PM

[Please do not mail me a copy of your followup]
 
MikeCopeland <mrc2323@cox.net> spake the secret code
 
> std::string myData = "cldat89";
>and I want to extract and convert the "89" to an integer variable.
 
A reverse iterator is not needed:
 
1 #include <iostream>
2 #include <string>
3
4 int main()
5 {
6 std::string s{"cldata89"};
7 auto pos = s.find_last_not_of("0123456789");
8 int val = std::stoi(s.substr(pos + 1));
9 std::cout << val << '\n';
10 return 0;
11 }
 
> g++ --std=c++11 /tmp/a.cpp
> ./a.out
89
 
However, for the purposes of education, a reverse iterator simply
traverses a container in the opposite order of a forward iterator.
I think cppreference.com does a good job of explaining it:
<http://en.cppreference.com/w/cpp/iterator/reverse_iterator>
--
"The Direct3D Graphics Pipeline" free book <http://tinyurl.com/d3d-pipeline>
The Computer Graphics Museum <http://computergraphicsmuseum.org>
The Terminals Wiki <http://terminals.classiccmp.org>
Legalize Adulthood! (my blog) <http://legalizeadulthood.wordpress.com>
David Brown <david.brown@hesbynett.no>: Jun 17 09:51AM +0200

>> So, the question is, does the man wrong, or i did something wrong ?
 
> Here is what that man said:
 
> a[5] is the same as 5[a]
 
True (within the right context). But that was considered an unfortunate
quirk of the language and very bad style 20 years ago when this thread
was started - it is even more so now.
 
It is good to be helpful and try to answer questions on Usenet, but I
doubt that the OP here has been holding his breath for the last two
decades, waiting for an answer!
Robert Wessel <robertwessel2@yahoo.com>: Jun 16 11:29PM -0500

>> So, the question is, does the man wrong, or i did something wrong ?
 
>Here is what that man said:
 
>a[5] is the same as 5[a]
 
 
It's not true for declarations, so "int 10[a]" doesn't work. OTOH,
when you access an array, you can write:
 
a[10] = 3;
 
-or-
 
10[a] = 3;
 
and accomplish the same thing. That's because the array accessor
essentially decays into a pointer expression. IOW, the two above
lines are equivalent to:
 
*(a+10) = 3;
 
-and-
 
*(10+a) = 3;
 
The later form ("10[a]") is primarily used to confuse newbies.
 
But it's in no way a comment.
Real Troll <real.troll@trolls.com>: Jun 17 06:21PM +0100

On 17/06/2016 08:51, David Brown wrote:
> It is good to be helpful and try to answer questions on Usenet, but I
> doubt that the OP here has been holding his breath for the last two
> decades, waiting for an answer!
 
He must have died by now!!!!!!!!!!
Ian Collins <ian-news@hotmail.com>: Jun 17 10:47PM +1200

> On Friday, December 12, 1997 at 12:00:00 AM UTC-8, Alexey Maykov wrote:
 
Time travel again...
 
--
Ian
legalize+jeeves@mail.xmission.com (Richard): Jun 17 05:35PM

[Please do not mail me a copy of your followup]
 
As others have already mentioned swapping the name of an array and
it's index may be valid syntax, but it is not recommended. Just because
something is allowed doesn't mean it's a good idea.
 
maykov@gmail.com spake the secret code
 
>Here is what that man said:
 
>a[5] is the same as 5[a]
 
For expressions in the C language, yes.
 
1 #include <stdio.h>
2
3 int main()
4 {
5 int a[1] = { 0 };
6 0[a] = 10;
7 printf("%d\n", a[0]);
8 return 0;
9 }
 
> gcc /tmp/a.c
> ./a.out
10
 
For expressions in the C++ language, no.
 
1 #include <iostream>
2 #include <vector>
3
4 int main()
5 {
6 std::vector<int> a(1);
7 0[a] = 10;
8 std::cout << a[0] << '\n';
9 return 0;
10 }
 
> g++ /tmp/a.cpp -o a2.out
/tmp/a.cpp: In function 'int main()':
/tmp/a.cpp:7:4: error: no match for 'operator[]' (operand types are 'int' and 'std::vector<int>')
0[a] = 10;
^
--
"The Direct3D Graphics Pipeline" free book <http://tinyurl.com/d3d-pipeline>
The Computer Graphics Museum <http://computergraphicsmuseum.org>
The Terminals Wiki <http://terminals.classiccmp.org>
Legalize Adulthood! (my blog) <http://legalizeadulthood.wordpress.com>
Jorgen Grahn <grahn+nntp@snipabacken.se>: Jun 16 02:42AM

On Tue, 2016-06-14, Wouter van Ooijen wrote:
 
[ÖT]
> contains a verb like is, has, contains, includes, etc.
 
> The other function (the one that makes something empty) should probably
> be called make_empty().
 
But still, the C++ containers have a different convention:
 
bool empty() const;
void clear();
 
For better or worse, I've adopted that convention. If I write a
container-like class and need that feature, I'll use those names.
I probably would even if I objected to them more than I do.
 
/Jorgen
 
--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .
Jorgen Grahn <grahn+nntp@snipabacken.se>: Jun 16 02:32AM

On Tue, 2016-06-14, 嘱 Tiib wrote:
> What is correct by those "conventions"? Is it 'vector::get_allocator()' or
> 'vector::allocator()'? Is it 'vector::capacity()' or 'vector::get_capacity()'?
> Feels there are no such "conventions".
 
Let's say "in line with the /lack/ of naming conventions", then.
 
> programming language therefore usage of word that is both verb and
> adjective ('empty') for function name in meaning of that adjective is
> misleading. Frank way of naming such function would be 'is_empty'.
 
There doesn't seem to be a lot of is_adjective() in the standard
library, though. Except in <type_traits>.
 
...
> if (foobar.size() != 0) ... // the reality
 
> I suspect that it is so (at least partially) because 'empty' is such a
> bad name. ;-)
 
I think that if people write such code, it's because they usually want
the negation -- my "hard to spot the !" argument.
 
I don't think I've seen size() used that way. Maybe I've been
lucky. I have sometimes been tempted to write such code myself, but
after an incident with std::list (where size() at least /might/ be
O(n)) I hesitate to ask for more information than I'm really going to
need.
 
/Jorgen
 
--
// Jorgen Grahn <grahn@ Oo o. . .
\X/ snipabacken.se> O o .
Ralf Goertz <me@myprovider.invalid>: Jun 22 09:56AM +0200

Am Tue, 21 Jun 2016 09:19:16 -0700 (PDT)
> be platform-neutral so usually there will be several
> platform-specific implementation after something of it is
> standardized.
 
I am well aware of boost's role in the advancement of the standard. Of
course there can be modifications during that process. That is one of
the reasons why I try to get rid of boost functions as soon as there are
std alternatives. Not that I don't like boost. On the contrary, I am
very grateful for the work of its programmers.
 
> I have also read gossip that boost had some Perl-like features to that
> "ECMAScript" that std::regex is not required to contain but i'm
> unsure if it is about those backslashes.
 
It seems I hadn't looked hard enough. Boost clearly states that the
backslash is an escape character in all three variants of the format
string (Sed, Perl, Boost-Extended). What puzzled me was that it is also
(and in the Sed-case only) used for the `normal' escape sequences like
'\t' which means that the string literals "\t" and "\\t" give the same
result in boost::regex_replace when used as a replacement (format)
string.
 
> actually does. These two things are never exactly same and also both
> documentation and behavior will change over time. Also, what problem
> needs usage of both boost::regex and std::regex in mix?
 
None of course. But as I said in my initial post I am switching from
boost::regex to std::regex now that my upgrading gcc made the latter
available. During that process I discovered the problem.
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