Implement N4502, the C++ Detection Idiom.
* doc/xml/manual/status_cxx2017.xml: Update status table. * include/experimental/type_traits (void_t, is_detected, is_detected_v, detected_t, detected_or, detected_or_t, is_detected_exact, is_detected_exact_v, is_detected_convertible, is_detected_convertible_v): Define. * include/std/type_traits (__detector, __detected_or, __detected_or_t, __detected_or_t_): Define. * testsuite/experimental/type_traits/detection.cc: New. From-SVN: r225242
This commit is contained in:
Jonathan Wakely
Jonathan Wakely
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@ -1,3 +1,14 @@
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2015-07-01 Jonathan Wakely <jwakely@redhat.com>
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* doc/xml/manual/status_cxx2017.xml: Update status table.
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* include/experimental/type_traits (void_t, is_detected,
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is_detected_v, detected_t, detected_or, detected_or_t,
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is_detected_exact, is_detected_exact_v, is_detected_convertible,
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is_detected_convertible_v): Define.
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* include/std/type_traits (__detector, __detected_or, __detected_or_t,
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__detected_or_t_): Define.
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* testsuite/experimental/type_traits/detection.cc: New.
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2015-06-30 Jonathan Wakely <jwakely@redhat.com>
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* doc/Makefile.am (stamp-pdf-doxygen): Grep for LaTeX errors in log.
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@ -328,14 +328,13 @@ not in any particular release.
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</row>
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<row>
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<?dbhtml bgcolor="#C8B0B0" ?>
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<entry>
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<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4502.pdf">
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N4502
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</link>
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</entry>
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<entry> Support for the C++ Detection Idiom, V2 </entry>
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<entry>N</entry>
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<entry>Y</entry>
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<entry>Library Fundamentals 2 TS</entry>
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</row>
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@ -220,6 +220,59 @@ template <typename _From, typename _To>
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// raw_invocation_type_t (still unimplemented)
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_GLIBCXX_END_NAMESPACE_VERSION
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} // namespace fundamentals_v1
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inline namespace fundamentals_v2
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{
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_GLIBCXX_BEGIN_NAMESPACE_VERSION
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#define __cpp_lib_experimental_detect 201505
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// [meta.detect]
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template<typename...> using void_t = void;
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struct nonesuch
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{
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nonesuch() = delete;
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~nonesuch() = delete;
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nonesuch(nonesuch const&) = delete;
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void operator=(nonesuch const&) = delete;
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};
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template<template<typename...> class _Op, typename... _Args>
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using is_detected
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= typename std::__detector<nonesuch, void, _Op, _Args...>::value_t;
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template<template<typename...> class _Op, typename... _Args>
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constexpr bool is_detected_v = is_detected<_Op, _Args...>::value;
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template<template<typename...> class _Op, typename... _Args>
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using detected_t
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= typename std::__detector<nonesuch, void, _Op, _Args...>::type;
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template<typename _Default, template<typename...> class _Op, typename... _Args>
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using detected_or = std::__detected_or<_Default, _Op, _Args...>;
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template<typename _Default, template<typename...> class _Op, typename... _Args>
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using detected_or_t = typename detected_or<_Default, _Op, _Args...>::type;
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template<typename Expected, template<typename...> class _Op, typename... _Args>
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using is_detected_exact = is_same<Expected, detected_t<_Op, _Args...>>;
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template<typename Expected, template<typename...> class _Op, typename... _Args>
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constexpr bool is_detected_exact_v
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= is_detected_exact<Expected, _Op, _Args...>::value;
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template<typename _To, template<typename...> class _Op, typename... _Args>
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using is_detected_convertible
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= is_convertible<detected_t<_Op, _Args...>, _To>;
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template<typename _To, template<typename...> class _Op, typename... _Args>
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constexpr bool is_detected_convertible_v
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= is_detected_convertible<_To, _Op, _Args...>::value;
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_GLIBCXX_END_NAMESPACE_VERSION
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} // namespace fundamentals_v2
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} // namespace experimental
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} // namespace std
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@ -2417,6 +2417,41 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
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template<typename...> using void_t = void;
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#endif
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/// Implementation of the detection idiom (negative case).
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template<typename _Default, typename _AlwaysVoid,
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template<typename...> class _Op, typename... _Args>
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struct __detector
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{
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using value_t = false_type;
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using type = _Default;
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};
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/// Implementation of the detection idiom (positive case).
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template<typename _Default, template<typename...> class _Op,
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typename... _Args>
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struct __detector<_Default, __void_t<_Op<_Args...>>, _Op, _Args...>
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{
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using value_t = true_type;
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using type = _Op<_Args...>;
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};
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// Detect whether _Op<_Args...> is a valid type, use _Default if not.
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template<typename _Default, template<typename...> class _Op,
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typename... _Args>
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using __detected_or = __detector<_Default, void, _Op, _Args...>;
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// _Op<_Args...> if that is a valid type, otherwise _Default.
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template<typename _Default, template<typename...> class _Op,
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typename... _Args>
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using __detected_or_t
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= typename __detected_or<_Default, _Op, _Args...>::type;
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// _Op<_Args...> if that is a valid type, otherwise _Default<_Args...>.
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template<template<typename...> class _Default,
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template<typename...> class _Op, typename... _Args>
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using __detected_or_t_ =
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__detected_or_t<_Default<_Args...>, _Op, _Args...>;
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/// @} group metaprogramming
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/**
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@ -0,0 +1,85 @@
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// Copyright (C) 2015 Free Software Foundation, Inc.
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//
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// This file is part of the GNU ISO C++ Library. This library is free
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// software; you can redistribute it and/or modify it under the
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// terms of the GNU General Public License as published by the
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// Free Software Foundation; either version 3, or (at your option)
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// any later version.
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// This library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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// You should have received a copy of the GNU General Public License along
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// with this library; see the file COPYING3. If not see
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// <http://www.gnu.org/licenses/>.
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// { dg-options "-std=gnu++14" }
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// { dg-do compile }
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#include <experimental/type_traits>
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using std::declval;
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using std::ptrdiff_t;
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using std::experimental::is_detected;
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using std::experimental::is_detected_exact;
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using std::experimental::detected_or_t;
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using std::experimental::is_same_v;
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// Examples taken from N4502
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// archetypal helper alias for a copy assignment operation:
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template <class T>
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using copy_assign_t = decltype(declval<T&>() = declval<T const &>());
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// plausible implementation for the is_assignable type trait:
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template <class T>
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using is_copy_assignable = is_detected<copy_assign_t, T>;
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// plausible implementation for an augmented is_assignable type trait
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// that also checks the return type:
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template <class T>
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using is_canonical_copy_assignable = is_detected_exact<T&, copy_assign_t, T>;
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struct A { };
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struct B { B& operator=(const B&); };
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struct C { void operator=(const C&); };
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struct D { D& operator=(D&); };
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struct E { E& operator=(E&&); };
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static_assert( is_copy_assignable<A>::value, "A is copy assignable" );
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static_assert( is_copy_assignable<B>::value, "B is copy assignable" );
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static_assert( is_copy_assignable<C>::value, "C is copy assignable" );
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static_assert( !is_copy_assignable<D>::value, "D is not copy assignable" );
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static_assert( !is_copy_assignable<E>::value, "E is not copy assignable" );
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static_assert( is_canonical_copy_assignable<A>::value,
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"A has canonical copy assignment" );
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static_assert( is_canonical_copy_assignable<B>::value,
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"B has canonical copy assignment" );
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static_assert( !is_canonical_copy_assignable<C>::value,
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"C does not have canonical copy assignment" );
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static_assert( !is_canonical_copy_assignable<D>::value,
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"D does not have canonical copy assignment" );
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static_assert( !is_canonical_copy_assignable<E>::value,
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"E does not have canonical copy assignment" );
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// archetypal helper alias for a particular type member:
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template <class T>
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using diff_t = typename T::difference_type;
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// alias the type member, if it exists, otherwise alias ptrdiff_t:
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template <class Ptr>
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using difference_type = detected_or_t<ptrdiff_t, diff_t, Ptr>;
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struct has { using difference_type = char; };
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struct has_not { };
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struct inherits : has { };
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struct hides : private has { };
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struct reveals : private has { using has::difference_type; };
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static_assert( is_same_v<difference_type<has>, char>, "has" );
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static_assert( is_same_v<difference_type<has_not>, ptrdiff_t>, "has not" );
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static_assert( is_same_v<difference_type<inherits>, char>, "inherits" );
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static_assert( is_same_v<difference_type<hides>, ptrdiff_t>, "hides" );
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static_assert( is_same_v<difference_type<reveals>, char>, "reveals" );
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