Monday, April 24, 2017

Digest for comp.lang.c++@googlegroups.com - 12 updates in 6 topics

"Chris M. Thomasson" <invalid@invalid.invalid>: Apr 24 02:55PM -0700

On 4/20/2017 9:28 PM, Alf P. Steinbach wrote:
> On 21-Apr-17 6:01 AM, rami17 wrote:
[...]
 
> better feedback, at least in the clc++ newsgroup, if you present these
> things as your exploration of issues, not as your gifts to other
> programmers, and ask for feedback as a student.
 
This is sage advise Alf!
"Chris M. Thomasson" <invalid@invalid.invalid>: Apr 24 02:50PM -0700

On 4/19/2017 4:28 PM, Chris M. Thomasson wrote:
> }
 
> mb_fence(mb_seq_cst);
> }
[...]
> _______________________________
 
> Will have normal std c++ in a day or two. Posted here, for all of the
> wonderful contributors and readers of this fine group.
 
Take a look at the wrlock function with that damn annoying seq_cst
barrier. Well, I managed to make it conditional wrt:
____________________________
void wrlock()
{
if (m_wrstate.fetch_sub(1, mb_relaxed) < 1)
{
m_wrlockset.sub(1);
}
 
int count = m_count.fetch_add(-INT_MAX, mb_relaxed);
 
if (count < INT_MAX)
{
if (m_rdwake.fetch_add(INT_MAX - count, mb_relaxed) + INT_MAX -
count)
{
m_wrwset.sub(1);
}
 
mb_fence(mb_seq_cst);
}
 
else
{
mb_fence(mb_acquire);
}
}
____________________________
 
Big difference: Especially for x86 platforms!
 
Yeah!
Christiano <christiano@engineer.com>: Apr 24 06:01AM -0300

I'm reading a book which has the following code:
 
---Book-----------------------------------
struct Day {
vector<double> hour {vector<double>(24, -7777)};
}
------------------------------------------
 
And he explains:
 
--Book------------------------------------
Why didn't we write
 
struct Day {
vector<double> hour {24, -7777};
};
 
?
 
That would have been simpler, but unfortunately, we would have gotten a
vector of two elements (24 and -1). When we want to specify the number
of elements for a vector for which an integer can be converted to the
element type,
we unfortunately have to use the () initializer syntax.
------------------------------------------
 
Ok, I understand what he is saying. But is not the following code better?
 
struct Day {
vector<double> hour(24, -7777);
}
 
Is there any special reason why he chose to write in this way rather
than in the simplest form?
 
 
 
Book ISBN 978-0-321-99278-9
Christiano <christiano@engineer.com>: Apr 24 06:15AM -0300

On 04/24/17 06:01, Christiano wrote:
 
 
> Is there any special reason why he chose to write in this way rather
> than in the simplest form?
 
Clarifying
 
"in this way" means:
 
struct Day {
vector<double> hour {vector<double>(24, -7777)};
}
 
"simplest form" means:
 
struct Day {
vector<double> hour(24, -7777);
}
"Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Apr 24 02:45PM +0200

On 24-Apr-17 11:01 AM, Christiano wrote:
 
> struct Day {
> vector<double> hour(24, -7777);
> }
 
It would be simpler if that syntax were supported.
 
Initializers for data members were added as an afterthought in C++11, to
avoid the redundancy of specifying the same initialization in each
constructor.
 
The standardization committee had to work within the existing awkward
syntax where round parenteses have just too many meanings, e.g. yielding
the so called "most vexing parse" of C++. They chose to support only
curly braces initializers, in two forms: direct initialization `T
m{...}` and copy initialization `T m = {...}`.
 
 
> Is there any special reason why he chose to write in this way rather
> than in the simplest form?
 
I imagine it's for pedagogical purposes, to illustrate the point above.
 
Otherwise, just write
 
struct Day
{
array<double, 24> hour{};
};
 
with the values initialized to 0.
 
IMHO that's roughly the simplest form, except that the purpose of this
struct is unclear.
 
 
Cheers & hth.,
 
- Alf
"Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com>: Apr 24 02:50PM +0200

On 24-Apr-17 2:45 PM, Alf P. Steinbach wrote:
> the so called "most vexing parse" of C++. They chose to support only
> curly braces initializers, in two forms: direct initialization `T
> m{...}` and copy initialization `T m = {...}`.
 
Sorry, that was an awkward and misleading formulation.
 
You /can/ write
 
struct S
{
int x = 1;
};
 
and you can write
 
struct S
{
int x{ 1 };
};
 
but not -- there's no syntax for it --
 
 
struct S
{
int x( 1 );
};
 
 
Cheers!,
 
- Alf (now heading for coffee)
Christiano <christiano@engineer.com>: Apr 24 06:37PM -0300

On 04/24/17 09:50, Alf P. Steinbach wrote:
> };
 
> Cheers!,
 
> - Alf (now heading for coffee)
 
My mistake. I assumed it would work without taking the test before.
 
The code bellow doesn't work:
struct X
{
vector<int> a(20, -7777);
};
 
And the two codes bellow work:
 
struct X
{
vector<int> a = vector<int>(20, -7777);
};
 
struct X
{
vector<int> a {vector<int>(20, -7777)};
};
 
But using clang version 3.8.0 only works when using -std=c++11.
 
Thank you, Alf and Stefan.
Jeff-Relf.Me @.: Apr 24 11:40AM -0700

ram@zedat.fu-berlin.de (Stefan Ram): Apr 24 09:29AM

Christiano <christiano@engineer.com> did NOT write exactly this:
|vector< double >hour{ vector< double >( 24, -77 )};
|vector< double >hour{ 24, -77 };
|vector< double >hour( 24, -77 );
 
No, the last line has about the same effect as the
first line but is clearer (shorter).
 
The first could be used if there is a style rule active
that requires every declared identifier to followed by
a brace or it could be a preparation for a use of auto:
 
auto hour{ vector< double >( 24, -77 )};
 
.
udpbot <bleachbot@httrack.com>: Apr 24 04:35PM +0200

ram@zedat.fu-berlin.de (Stefan Ram): Apr 24 06:01PM

>|vector< double >hour( 24, -77 );
>No, the last line has about the same effect as the
>first line but is clearer (shorter).
 
After reading Alf, I have to add that I have failed to take
into account that the initialization with parentheses is not
allowed when a member of class is to be initialized at the
place of its declaration in the class specifier.
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