Tuesday, June 9, 2015

Digest for comp.lang.c++@googlegroups.com - 15 updates in 9 topics

fl <rxjwg98@gmail.com>: Jun 09 02:44PM -0700

Hi,
 
There is an on-line post, which says that following code fails in case of
self-assignment. I run it on MSVC, but do not find any failure yet. I have
tried n=0 and n=10 two cases.
 
What is wrong with my comprehension on this code?
 
 
Thanks,
 
 
 
 
////////////
class array {
int tmp;
 
int *data;
size_t n;
public:
array(){
n=0;
data=new int[n];
}
array &array::operator =(const array &rhs);
};
 
array &array::operator =(const array &rhs)
{
delete[] data;
 
n = rhs.n;
data = new int[n];
std::copy_n(rhs.data, n, data);
 
return *this;
}
 
int main () {
array a0;
array b0;
 
a0=a0;
return 0;
}
Louis Krupp <lkrupp@nospam.pssw.com.invalid>: Jun 09 04:11PM -0600


>array &array::operator =(const array &rhs)
>{
> delete[] data;
 
At this point, you can't access anything data points to, so you might
as well set it to, say, the null pointer:
 
data = 0;
 
Try self- assignment now, and see what happens.
 
 
> a0=a0;
> return 0;
>}
 
Louis
fl <rxjwg98@gmail.com>: Jun 09 03:49PM -0700

On Tuesday, June 9, 2015 at 3:11:12 PM UTC-7, Louis Krupp wrote:
> > return 0;
> >}
 
> Louis
 
I find the failure by adding the following member function:
 
void array::array_set(int tmp, int number){
data[number]=tmp;
}
 
int main()
{
a0.array_set(3, 2);
a0=a0;
}
 
Then, a0.data array cannot keep its original value after self-assignment.
 
 
I am still new to C++. I noticed that data is its private data, and don't
know whether your insertion
 
data = 0;
 
is inside the member function or not. If you could give me more detail,
that is better to know other view of the problem.
 
Thanks,
"Chris M. Thomasson" <nospam@nospam.nospam>: Jun 09 03:41PM -0700

> "Doug Mika" wrote in message
> news:3d6437f0-f89e-4dcb-af75-82358e32fea4@googlegroups.com...
 
[...]
> Aborted (core dumped)
 
> Which begs the question, where is the setting to enable threading,
> and is there a flag that is needed in MinGW to enable threading?
 
Try using the following:
 
g++ -std=c++11 -pthread -o main *.cpp
 
witch compiles your example. It runs fine with:
 
http://www.tutorialspoint.com/compile_cpp11_online.php
 
;^)
legalize+jeeves@mail.xmission.com (Richard): Jun 09 10:37PM

[Please do not mail me a copy of your followup]
 
Victor Bazarov <v.bazarov@comcast.invalid> spake the secret code
 
>> If you want people to do your work for them, it is customary to pay them.
>. ^^^^^^^^^^
>I think you meant "for you". ;-)
 
Yeah, saw that after I posted, but cancelling an article on usenet was
never well supported when usenet was well maintained and it's virtually
impossible now. So I simply let the mistakes fly and if someone is
confused by what I said, I correct it.
 
But yes, you're correct in correcting me :-).
 
The bottom line is that it's generally considered rude to show up in
a forum and ask other people to do your work.
 
See <http://www.catb.org/esr/faqs/smart-questions.html> for general
behavior guidelines on asking questions.
--
"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>
legalize+jeeves@mail.xmission.com (Richard): Jun 09 10:31PM

[Please do not mail me a copy of your followup]
 
slp53@pacbell.net spake the secret code
 
>You know, C is still a subset of C++; and it always will be, purists
>like yourself notwithstanding.
 
Not really. nullptr is not a keyword in C, but it is a keyword in C++.
 
We're talking about POSIX, which is neither a part of the C standard nor
is it part of the C++ standard, so the whole idea of "C being a subset
of C++" is irrelevant.
--
"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>
fl <rxjwg98@gmail.com>: Jun 09 01:59PM -0700

Hi,
 
When I try to run a snippet code from the web, it has a Debug Assertion
Failed error in MSVC. Specifically, when it is stepped into
 
b0=a0;
 
it jumps to the assign function
 
array &array::operator =(const array &rhs);
 
At line:
delete[] data;
 
the Debug Assertion Error pops.
 
I do not understand why it has such an error. Could you explain it to me?
 
 
Thanks,
 
 
/////////////
class array {
int tmp;
 
int *data;
size_t n;
public:
array(){
data=&tmp;
}
array &array::operator =(const array &rhs);
};
 
array &array::operator =(const array &rhs)
{
delete[] data;
 
n = rhs.n;
data = new int[n];
std::copy_n(rhs.data, n, data);
 
return *this;
}
 
int main () {
array a0;
array b0;
 
b0=a0;
return 0;
}
fl <rxjwg98@gmail.com>: Jun 09 02:12PM -0700

On Tuesday, June 9, 2015 at 1:59:37 PM UTC-7, fl wrote:
 
> b0=a0;
> return 0;
> }
 
The original code snippet does not have definition
 
int tmp;
 
inside class array. I added it to make
 
delete[] data;
 
work.
 
It is possible that data shouldn't be used in such a way. Then, how to use
data in the class array?
 
 
Thanks,
fl <rxjwg98@gmail.com>: Jun 09 02:22PM -0700

On Tuesday, June 9, 2015 at 2:12:13 PM UTC-7, fl wrote:
 
> It is possible that data shouldn't be used in such a way. Then, how to use
> data in the class array?
 
> Thanks,
 
Oh, I find the answer. The constructor can be:
 
public:
array(){
n=0;
data=new int[n];
}
 
Thanks,
Paavo Helde <myfirstname@osa.pri.ee>: Jun 09 02:57PM -0500

Doug Mika <dougmmika@gmail.com> wrote in
 
> So what would be the rule, when do I need typename C and when do I
> just write C? It should be obvious that inside the <> brackets is
> always a type, what else could it be?
 
The last typename above applies to 'iterator', not to 'C'. The compiler
knows C is a type, but it does not have such knowledge about 'C::iterator'.
 
hth
Doug Mika <dougmmika@gmail.com>: Jun 09 01:05PM -0700

On Tuesday, June 9, 2015 at 2:57:20 PM UTC-5, Paavo Helde wrote:
 
> The last typename above applies to 'iterator', not to 'C'. The compiler
> knows C is a type, but it does not have such knowledge about 'C::iterator'.
 
> hth
 
Yes, I sort of figured that, it is nice however that someone confirms it.
(It's not explicitly written in my book)
Thanks
Marcel Mueller <news.5.maazl@spamgourmet.org>: Jun 09 10:51PM +0200

On 09.06.15 19.55, Doug Mika wrote:
> The following for example will not work: (but why do we need typename C here and just C previously?)
 
> //this won't compile, "typename" is needed, but why is it needed here but
> //not previously?
 
template<typename C> vector<C> create_container(int size) ...
 
Here C is replaced by the individual type at instantiation of the
template. C is always a type name because this is restricted in the
template pattern, maybe C evaluates to 'typename myclass'.
 
template<typename C, typename V>
vector<typename C::iterator> find_all(C& c, V v)
 
This is a different story. The 'typename' applies to the nested iterator
type rather than to C itself. And 'iterator' could be anything within
the type C.
 
 
Marcel
Chris Vine <chris@cvine--nospam--.freeserve.co.uk>: Jun 09 08:52PM +0100

On Tue, 09 Jun 2015 20:26:57 +0200
> class (Clause 9) declared using the class or struct class-key. "
 
> should tell us that, unlike enum and union, "either [...] class or
> struct" can be used to refer to a non-union class.
 
I have come across this outside VS when specializing std::hash for a
type which might be used as the key for an unordered container. The
standard does not specify whether the std::hash<T> class template type
is defined using the 'class' or 'struct' keyword. §20.8.12 of the C++11
standard uses the 'struct' keyword when specifying the specializations
required by the standard, but only requires that any given
specialization must comprise a 'class template'.
 
clang exhibits the annoying behaviour of emitting a bogus warning if
you provide a hash specialization using a different keyword from the
one which happens to be used by the C++ standard library in question
for std::hash<T>, which means that if you use clang with a recent
version of libstdc++ you get the warning when using the 'struct' keyword
instead of the 'class' keyword for a specialization. I am glad to say
that gcc does not do this.
 
Chris
drew@furrfu.invalid (Drew Lawson): Jun 09 03:38PM

In article <ml4pfp$4ne$1@dont-email.me>
>1) Getting data from a database that represents money
>2) Comes into C++ as a string
>3) Convert it double to do calculations
 
There is your problem. Never use floating point for money.
 
In addition to the practical problems you've seen, it is a conceptual
error. Monetary systems have discrete minimum values (frequently
a mil) and rules about how they are treated.
 
This is the sort of problem that is recreated every time programmers
try to replace centuries of accounting practices.
 
--
|Drew Lawson | Of all the things I've lost |
| | I miss my mind the most |
woodbrian77@gmail.com: Jun 09 08:06AM -0700

On Tuesday, June 9, 2015 at 12:01:00 AM UTC-5, Paavo Helde wrote:
 
> Beware that in real life the array-style new[] and delete[] have about zero
> usage cases. These come up almost only in silly learning examples like
> this.
 
I use them in my code -
 
http://webEbenezer.net/build_integration.html
 
 
Brian
Ebenezer Enterprises - "Free at last; free at last. Thank
G-d Almighty we are free at last." Martin Luther King Jr.
 
http://webEbenezer.net
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