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libstdc++: Prefer const T to std::add_const_t<T>

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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.
This commit is contained in:
Jonathan Wakely 2022-07-07 12:27:08 +01:00
parent 3723aedaad
commit 8be17e2ac7
4 changed files with 14 additions and 18 deletions
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11
libstdc++-v3/include/bits/stl_iterator.h
View file

@ -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

6
libstdc++-v3/include/bits/utility.h
View file

@ -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

9
libstdc++-v3/include/std/type_traits
View file

@ -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

6
libstdc++-v3/include/std/variant
View file

@ -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;