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libstdc++: New _IotaArray utility to simplify index pack creation

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This patch introduces the internal helper type _IotaArray to simplify
defining a pack of indices via a structured binding declaration:
constexpr auto [...__is] = _IotaArray<N>;

_IotaArray is a C-array for lowest overhead in terms of template
instantiations. Non-GCC compilers that do not implement
__integer_pack have a slightly higher overhead.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (_IotaArray): Define.
	* testsuite/ext/iotaarray.cc: New test.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
This commit is contained in:
Matthias Kretz
2025-10-24 13:28:29 +02:00
parent 3ce0b19b87
commit ed8b9eeeee
2 changed files with 36 additions and 0 deletions
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16
libstdc++-v3/include/bits/utility.h
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@@ -172,6 +172,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
using index_sequence_for = make_index_sequence<sizeof...(_Types)>;
#endif // __glibcxx_integer_sequence
#if __cpp_structured_bindings >= 202411L
#if __has_builtin(__integer_pack)
template <auto _Num, typename _Tp = decltype(_Num)>
inline constexpr _Tp
_IotaArray[_Num] = {__integer_pack(_Tp(_Num))...};
#elif defined __glibcxx_integer_sequence
template <auto _Num, typename _Tp = decltype(_Num), typename = make_integer_sequence<_Tp, _Num>>
inline constexpr _Tp
_IotaArray[_Num];
template <auto _Num, typename _Tp, _Tp... _Is>
inline constexpr _Tp
_IotaArray<_Num, _Tp, integer_sequence<_Tp, _Is...>>[_Num] = {_Is...};
#endif // __integer_pack
#endif // __cpp_structured_bindings >= 202411L
#if __cplusplus >= 201703L
struct in_place_t {

20
libstdc++-v3/testsuite/ext/iotaarray.cc Normal file
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@@ -0,0 +1,20 @@
// { dg-do compile { target c++26 } }
#include <utility>
#include <type_traits>
template<auto N>
void test()
{
constexpr auto [id0, ...ids] = std::_IotaArray<N>;
static_assert( std::is_same_v<decltype(id0), const decltype(N)> );
static_assert( sizeof...(ids) == N - 1 );
static_assert( (id0 + ... + ids) == N*(N-1)/2 );
}
int main()
{
test<1>();
test<4u>();
test<8ull>();
}