On Thu, 7 Jul 2022 at 17:55, Jonathan Wakely via Libstdc++
<libstd...@gcc.gnu.org> wrote:
>
> Does anybody see a problem with this change? The point is to avoid
> unnecessary class template instantiations.
>
> Tested aarch64-linux.
Pushed to trunk now.
>
> -- >8 --
>
> For any typedef-name or template parameter, T, add_const_t<T> is
> equivalent to T const, so we can avoid instantiating the std::add_const
> class template and just say T const (or const T).
>
> This isn't true for a non-typedef like int&, where int& const would be
> ill-formed, but we shouldn't be using add_const_t<int&> anyway, because
> we know what that type is.
>
> The only place we need to continue using std::add_const is in the
> std::bind implementation where it's used as a template template
> parameter to be applied as a metafunction elsewhere.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/stl_iterator.h (__iter_to_alloc_t): Replace
> add_const_t with const-qualifier.
> * include/bits/utility.h (tuple_element<N, cv T>): Likewise for
> all cv-qualifiers.
> * include/std/type_traits (add_const, add_volatile): Replace
> typedef-declaration with using-declaration.
> (add_cv): Replace add_const and add_volatile with cv-qualifiers.
> * include/std/variant (variant_alternative<N, cv T>): Replace
> add_const_t, add_volatile_t and add_cv_t etc. with cv-qualifiers.
> ---
> libstdc++-v3/include/bits/stl_iterator.h | 11 +++++------
> libstdc++-v3/include/bits/utility.h | 6 +++---
> libstdc++-v3/include/std/type_traits | 9 +++------
> libstdc++-v3/include/std/variant | 6 +++---
> 4 files changed, 14 insertions(+), 18 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/stl_iterator.h
> b/libstdc++-v3/include/bits/stl_iterator.h
> index 12a89ab229f..049cb02a4c4 100644
> --- a/libstdc++-v3/include/bits/stl_iterator.h
> +++ b/libstdc++-v3/include/bits/stl_iterator.h
> @@ -2536,19 +2536,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // of associative containers.
> template<typename _InputIterator>
> using __iter_key_t = remove_const_t<
> - typename iterator_traits<_InputIterator>::value_type::first_type>;
> + typename iterator_traits<_InputIterator>::value_type::first_type>;
>
> template<typename _InputIterator>
> - using __iter_val_t =
> - typename iterator_traits<_InputIterator>::value_type::second_type;
> + using __iter_val_t
> + = typename iterator_traits<_InputIterator>::value_type::second_type;
>
> template<typename _T1, typename _T2>
> struct pair;
>
> template<typename _InputIterator>
> - using __iter_to_alloc_t =
> - pair<add_const_t<__iter_key_t<_InputIterator>>,
> - __iter_val_t<_InputIterator>>;
> + using __iter_to_alloc_t
> + = pair<const __iter_key_t<_InputIterator>,
> __iter_val_t<_InputIterator>>;
> #endif // __cpp_deduction_guides
>
> _GLIBCXX_END_NAMESPACE_VERSION
> diff --git a/libstdc++-v3/include/bits/utility.h
> b/libstdc++-v3/include/bits/utility.h
> index e0e40309a6d..6a192e27836 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -86,19 +86,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> template<size_t __i, typename _Tp>
> struct tuple_element<__i, const _Tp>
> {
> - typedef typename add_const<__tuple_element_t<__i, _Tp>>::type type;
> + using type = const __tuple_element_t<__i, _Tp>;
> };
>
> template<size_t __i, typename _Tp>
> struct tuple_element<__i, volatile _Tp>
> {
> - typedef typename add_volatile<__tuple_element_t<__i, _Tp>>::type type;
> + using type = volatile __tuple_element_t<__i, _Tp>;
> };
>
> template<size_t __i, typename _Tp>
> struct tuple_element<__i, const volatile _Tp>
> {
> - typedef typename add_cv<__tuple_element_t<__i, _Tp>>::type type;
> + using type = const volatile __tuple_element_t<__i, _Tp>;
> };
>
> #if __cplusplus >= 201402L
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index 2572d8edd69..e5f58bc2e3f 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -1577,20 +1577,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> /// add_const
> template<typename _Tp>
> struct add_const
> - { typedef _Tp const type; };
> + { using type = _Tp const; };
>
> /// add_volatile
> template<typename _Tp>
> struct add_volatile
> - { typedef _Tp volatile type; };
> + { using type = _Tp volatile; };
>
> /// add_cv
> template<typename _Tp>
> struct add_cv
> - {
> - typedef typename
> - add_const<typename add_volatile<_Tp>::type>::type type;
> - };
> + { using type = _Tp const volatile; };
>
> #if __cplusplus > 201103L
>
> diff --git a/libstdc++-v3/include/std/variant
> b/libstdc++-v3/include/std/variant
> index 5ff1e3edcdf..f8f15665433 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -107,15 +107,15 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> template<size_t _Np, typename _Variant>
> struct variant_alternative<_Np, const _Variant>
> - { using type = add_const_t<variant_alternative_t<_Np, _Variant>>; };
> + { using type = const variant_alternative_t<_Np, _Variant>; };
>
> template<size_t _Np, typename _Variant>
> struct variant_alternative<_Np, volatile _Variant>
> - { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
> + { using type = volatile variant_alternative_t<_Np, _Variant>; };
>
> template<size_t _Np, typename _Variant>
> struct variant_alternative<_Np, const volatile _Variant>
> - { using type = add_cv_t<variant_alternative_t<_Np, _Variant>>; };
> + { using type = const volatile variant_alternative_t<_Np, _Variant>; };
>
> inline constexpr size_t variant_npos = -1;
>
> --
> 2.36.1
>
