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How toi get array reference from std::array
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Marcel Mueller
Feb 7, 2019, 11:46:53 AM2/7/19
to
If I have a array variable
std::array<int,5> data;
and a function taking an array reference
void foo(int (&arr)[5]);
how to pass the content of data to this function without a hard cast?
All the methods of std::array return only raw pointers without the
required size in the type.
OK, I can write a template helper function to do a safe conversion from
std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
of the cast.
Why does std::array<T,N>::data not return T(&)[N] which implicitly
converts to T* if the length information is not needed? The internal
storage is of type T[N] anyway.
Marcel
std::array<int,5> data;
and a function taking an array reference
void foo(int (&arr)[5]);
how to pass the content of data to this function without a hard cast?
All the methods of std::array return only raw pointers without the
required size in the type.
OK, I can write a template helper function to do a safe conversion from
std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
of the cast.
Why does std::array<T,N>::data not return T(&)[N] which implicitly
converts to T* if the length information is not needed? The internal
storage is of type T[N] anyway.
Marcel
Öö Tiib
Feb 8, 2019, 3:51:11 AM2/8/19
to
On Thursday, 7 February 2019 18:46:53 UTC+2, Marcel Mueller wrote:
> If I have a array variable
>
> std::array<int,5> data;
>
> and a function taking an array reference
>
> void foo(int (&arr)[5]);
>
> how to pass the content of data to this function without a hard cast?
>
> All the methods of std::array return only raw pointers without the
> required size in the type.
>
>
> OK, I can write a template helper function to do a safe conversion from
> std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
> of the cast.
Write helper function template that takes reference to
> If I have a array variable
>
> std::array<int,5> data;
>
> and a function taking an array reference
>
> void foo(int (&arr)[5]);
>
> how to pass the content of data to this function without a hard cast?
>
> All the methods of std::array return only raw pointers without the
> required size in the type.
>
>
> OK, I can write a template helper function to do a safe conversion from
> std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
> of the cast.
implementation-specific member (like "__elems_" in libcpp)
directly. If you target multiple platforms then choose what
it's name is with preprocessor. That member is required to
exist and to be public by standard.
There are only handful of viable standard library implementations
and these don't rename the member too often so it will take half
an hour to get majority covered. The only corner case is
when you have zero-sized std::arrays (those are allowed to lack
that member) but otherwise it is kind of piece of cake.
> Why does std::array<T,N>::data not return T(&)[N] which implicitly
> converts to T* if the length information is not needed? The internal
> storage is of type T[N] anyway.
https://cplusplus.github.io/LWG/issue930
So it won't change anytime soon and therefore I would go with what
I suggested above.
James Kuyper
Feb 8, 2019, 8:00:02 AM2/8/19
to
On 2/8/19 03:51, Öö Tiib wrote:
> On Thursday, 7 February 2019 18:46:53 UTC+2, Marcel Mueller wrote:
>> If I have a array variable
>>
>> std::array<int,5> data;
>>
>> and a function taking an array reference
>>
>> void foo(int (&arr)[5]);
>>
>> how to pass the content of data to this function without a hard cast?
>>
>> All the methods of std::array return only raw pointers without the
>> required size in the type.
>>
>>
>> OK, I can write a template helper function to do a safe conversion from
>> std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
>> of the cast.
>
> Write helper function template that takes reference to
> implementation-specific member (like "__elems_" in libcpp)
> directly. If you target multiple platforms then choose what
> it's name is with preprocessor. That member is required to
> exist and to be public by standard.
Citation, please? 26.3.7.1p3 doesn't list any public data members for
> On Thursday, 7 February 2019 18:46:53 UTC+2, Marcel Mueller wrote:
>> If I have a array variable
>>
>> std::array<int,5> data;
>>
>> and a function taking an array reference
>>
>> void foo(int (&arr)[5]);
>>
>> how to pass the content of data to this function without a hard cast?
>>
>> All the methods of std::array return only raw pointers without the
>> required size in the type.
>>
>>
>> OK, I can write a template helper function to do a safe conversion from
>> std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
>> of the cast.
>
> Write helper function template that takes reference to
> implementation-specific member (like "__elems_" in libcpp)
> directly. If you target multiple platforms then choose what
> it's name is with preprocessor. That member is required to
> exist and to be public by standard.
std::array<>, as far as I can see, and neither do any of the general
container requirements (tables 83, 84, 85, 86, and 87).
>> Why does std::array<T,N>::data not return T(&)[N] which implicitly
>> converts to T* if the length information is not needed? The internal
>> storage is of type T[N] anyway.
>
> It is issue 930 with status "not a defect".
> https://cplusplus.github.io/LWG/issue930
> So it won't change anytime soon and therefore I would go with what
> I suggested above.
only". "for exposition only" means precisely that - it does NOT mandate
that std::array be implemented in that manner, only that it have the
same observable behavior as if it did so. The identifier "elems" is not
present anywhere in the n4659.pdf, the latest draft version of the C++
standard that I have on my desktop machine, essentially equivalent to
C++2017. Perhaps the committee decided that it was misleading to explain
the behavior of std::array in terms of elems?
Öö Tiib
Feb 8, 2019, 9:16:54 AM2/8/19
to
On Friday, 8 February 2019 15:00:02 UTC+2, James Kuyper wrote:
> On 2/8/19 03:51, Öö Tiib wrote:
> > On Thursday, 7 February 2019 18:46:53 UTC+2, Marcel Mueller wrote:
> >> If I have a array variable
> >>
> >> std::array<int,5> data;
> >>
> >> and a function taking an array reference
> >>
> >> void foo(int (&arr)[5]);
> >>
> >> how to pass the content of data to this function without a hard cast?
> >>
> >> All the methods of std::array return only raw pointers without the
> >> required size in the type.
> >>
> >>
> >> OK, I can write a template helper function to do a safe conversion from
> >> std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
> >> of the cast.
> >
> > Write helper function template that takes reference to
> > implementation-specific member (like "__elems_" in libcpp)
> > directly. If you target multiple platforms then choose what
> > it's name is with preprocessor. That member is required to
> > exist and to be public by standard.
>
> Citation, please? 26.3.7.1p3 doesn't list any public data members for
> std::array<>, as far as I can see, and neither do any of the general
> container requirements (tables 83, 84, 85, 86, and 87).
The std::array is required to be aggregate and requirements of
> On 2/8/19 03:51, Öö Tiib wrote:
> > On Thursday, 7 February 2019 18:46:53 UTC+2, Marcel Mueller wrote:
> >> If I have a array variable
> >>
> >> std::array<int,5> data;
> >>
> >> and a function taking an array reference
> >>
> >> void foo(int (&arr)[5]);
> >>
> >> how to pass the content of data to this function without a hard cast?
> >>
> >> All the methods of std::array return only raw pointers without the
> >> required size in the type.
> >>
> >>
> >> OK, I can write a template helper function to do a safe conversion from
> >> std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
> >> of the cast.
> >
> > Write helper function template that takes reference to
> > implementation-specific member (like "__elems_" in libcpp)
> > directly. If you target multiple platforms then choose what
> > it's name is with preprocessor. That member is required to
> > exist and to be public by standard.
>
> Citation, please? 26.3.7.1p3 doesn't list any public data members for
> std::array<>, as far as I can see, and neither do any of the general
> container requirements (tables 83, 84, 85, 86, and 87).
aggregate forbid private or protected non-static data members.
Sure, in theory it may be purely magical thing turned on by
incantation "#include <array>" but such implementations
don't exist.
> >> Why does std::array<T,N>::data not return T(&)[N] which implicitly
> >> converts to T* if the length information is not needed? The internal
> >> storage is of type T[N] anyway.
> >
> > It is issue 930 with status "not a defect".
> > https://cplusplus.github.io/LWG/issue930
> > So it won't change anytime soon and therefore I would go with what
> > I suggested above.
>
> That issue mentions std::array::elems, which was marked "for exposition
> only". "for exposition only" means precisely that - it does NOT mandate
> that std::array be implemented in that manner, only that it have the
> same observable behavior as if it did so. The identifier "elems" is not
> present anywhere in the n4659.pdf, the latest draft version of the C++
> standard that I have on my desktop machine, essentially equivalent to
> C++2017. Perhaps the committee decided that it was misleading to explain
> the behavior of std::array in terms of elems?
initialize it so only thing to do is to look what it's name is up from library
code.
Marcel Mueller
Feb 8, 2019, 11:25:30 AM2/8/19
to
Am 08.02.19 um 09:51 schrieb Öö Tiib:
compiled on quite different platforms, at least ones that I cannot test.
For now I don't rely on constexpr, so I can live with the cast.
Marcel
>> OK, I can write a template helper function to do a safe conversion from
>> std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
>> of the cast.
>
> Write helper function template that takes reference to
> implementation-specific member (like "__elems_" in libcpp)
> directly. If you target multiple platforms then choose what
> it's name is with preprocessor. That member is required to
> exist and to be public by standard.
>
> There are only handful of viable standard library implementations
> and these don't rename the member too often so it will take half
> an hour to get majority covered. The only corner case is
> when you have zero-sized std::arrays (those are allowed to lack
> that member) but otherwise it is kind of piece of cake.
Hmm, not that pretty. Especially for open source software which may be
>> std::array<T,N>& to T(&)[N]. But this can no longer be constexpr because
>> of the cast.
>
> Write helper function template that takes reference to
> implementation-specific member (like "__elems_" in libcpp)
> directly. If you target multiple platforms then choose what
> it's name is with preprocessor. That member is required to
> exist and to be public by standard.
>
> There are only handful of viable standard library implementations
> and these don't rename the member too often so it will take half
> an hour to get majority covered. The only corner case is
> when you have zero-sized std::arrays (those are allowed to lack
> that member) but otherwise it is kind of piece of cake.
compiled on quite different platforms, at least ones that I cannot test.
For now I don't rely on constexpr, so I can live with the cast.
Marcel
Öö Tiib
Feb 11, 2019, 2:47:57 AM2/11/19
to
platform vendors *love* to be as incompatible as possible and that
won't change.
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