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Groups > comp.lang.c++.moderated > #1388

Re: codecvt problem

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On 2011-06-07 11:07, Ulrich Eckhardt wrote:
> Daniel Krügler wrote:
>> Am 06.06.2011 20:50, schrieb Jeff Flinn:
>>> Vaclav Haisman wrote:
>>>> Rafał Moniuszko wrote, On 5.6.2011 15:25:
>>>> [...]
>>>> Add #include<cstring>  and do
>>>> std::memset (&mystate, 0, sizeof(mystate));:
>>>
>>> Why not mbstate_t mystate = {0};
>>
>> Or rely on aggregate-initialization with an empty initializer:
>>
>> mbstate_t mystate = {};
>
> Just wondering is that the same or are there subtle differences?

It is not guaranteed to be portable in C++03, because empty-list
initialization is only specified for aggregates, not for other types
(like scalar types), and mbstate_t is just specified to be a non-array
object type, therefore it may be an aggregate or not. It is portable in
C++0x because of the new concept of "list-initialization" and because of
a subtle change of a zero-initialization border case, see below.

In response to your actual question I would say that it should have the
same effects, assuming that mbstate_t is a C compatible object type (and
I don't think that anything else makes sense). In this case {} has the
effect of value-initializing all components of an aggregate and it has
the effect of value-initializing an object of scalar type.
Initializing an aggregate or a scalar with {0} is equivalent to the
effects that value-initialization has for such types, because in the end
we have a zero-initialization of all scalar sub-parts and this has the
effects of "set to the value of 0 (zero) converted to T;". This is near
to identical to memset, except for (to the best of my knowledge)

a) padding bytes before C++0x, but the FDIS has approved the core issue

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#694

fixing this subtle difference and

b) pointer types, because memset does not guarantee to have the same
effects as zero-initializing. In the latter case it is guaranteed to
have a null pointer value, but memset applied to a pointer value may be
something different.

I would say that especially the second difference is more a pro argument
to rely on value-initialization instead of using memset.

> Also, what
> about this C++ solution:
>
>    mbstate_t mystate = mbstate_t();

I would say that this is the best compromise if you want to support both
C++03 and C++0x code targets. It has the effect of value-initialization,
so it is equivalent to {} in C++0x and {0} in C++03 (for the subset of
types that I mentioned one or two special cases mentioned above.

HTH & Greetings from Bremen,

Daniel Krügler



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Thread

codecvt problem  Rafał Moniuszko <rafal.lists@gmail.com> - 2011-06-05 07:25 -0600
  Re: codecvt problem Vaclav Haisman <v.haisman@sh.cvut.cz> - 2011-06-05 21:41 -0600
    Re: codecvt problem  Jeff Flinn <TriumphSprint2000@hotmail.com> - 2011-06-06 12:50 -0600
      Re: codecvt problem Vaclav Haisman <v.haisman@sh.cvut.cz> - 2011-06-06 15:31 -0600
        Re: codecvt problem  Martin Bonner <martinfrompi@yahoo.co.uk> - 2011-06-08 02:06 -0600
          Re: codecvt problem  Vaclav Haisman <v.haisman@sh.cvut.cz> - 2011-06-08 17:24 -0600
            Re: codecvt problem  Seungbeom Kim <musiphil@bawi.org> - 2011-06-09 07:02 -0600
              Re: codecvt problem Vaclav Haisman <v.haisman@sh.cvut.cz> - 2011-06-16 17:04 -0600
                Re: codecvt problem Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2011-06-17 06:42 -0600
                Re: codecvt problem Seungbeom Kim <musiphil@bawi.org> - 2011-06-18 07:38 -0600
                Re: codecvt problem Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2011-06-20 13:16 -0600
                Re: codecvt problem Seungbeom Kim <musiphil@bawi.org> - 2011-06-23 14:42 -0600
        Re: codecvt problem  Seungbeom Kim <musiphil@bawi.org> - 2011-06-09 07:01 -0600
      Re: codecvt problem Daniel Krügler <daniel.kruegler@googlemail.com> - 2011-06-06 15:31 -0600
        Re: codecvt problem Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2011-06-07 03:07 -0600
          Re: codecvt problem  Daniel Krügler <daniel.kruegler@googlemail.com> - 2011-06-07 10:31 -0600
  Re: codecvt problem  Ulrich Eckhardt <ulrich.eckhardt@dominolaser.com> - 2011-06-06 12:47 -0600

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