libstdc++-v3/testsuite/experimental/type_traits/detection.cc

   1 // Copyright (C) 2015-2019 Free Software Foundation, Inc.
   2 //
   3 // This file is part of the GNU ISO C++ Library.  This library is free
   4 // software; you can redistribute it and/or modify it under the
   5 // terms of the GNU General Public License as published by the
   6 // Free Software Foundation; either version 3, or (at your option)
   7 // any later version.
   8
   9 // This library is distributed in the hope that it will be useful,
  10 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  11 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12 // GNU General Public License for more details.
  13
  14 // You should have received a copy of the GNU General Public License along
  15 // with this library; see the file COPYING3.  If not see
  16 // <http://www.gnu.org/licenses/>.
  17
  18 // { dg-do compile { target c++14 } }
  19
  20 #include <experimental/type_traits>
  21
  22 using std::declval;
  23 using std::ptrdiff_t;
  24 using std::experimental::is_detected;
  25 using std::experimental::is_detected_exact;
  26 using std::experimental::detected_or_t;
  27 using std::experimental::is_same_v;
  28
  29 // Examples taken from N4502
  30
  31 // archetypal helper alias for a copy assignment operation:
  32 template <class T>
  33 using copy_assign_t = decltype(declval<T&>() = declval<T const &>());
  34
  35 // plausible implementation for the is_assignable type trait:
  36 template <class T>
  37   using is_copy_assignable = is_detected<copy_assign_t, T>;
  38
  39 // plausible implementation for an augmented is_assignable type trait
  40 // that also checks the return type:
  41 template <class T>
  42   using is_canonical_copy_assignable = is_detected_exact<T&, copy_assign_t, T>;
  43
  44 struct A { };
  45 struct B { B& operator=(const B&); };
  46 struct C { void operator=(const C&); };
  47 struct D { D& operator=(D&); };
  48 struct E { E& operator=(E&&); };
  49
  50 static_assert( is_copy_assignable<A>::value,  "A is copy assignable" );
  51 static_assert( is_copy_assignable<B>::value,  "B is copy assignable" );
  52 static_assert( is_copy_assignable<C>::value,  "C is copy assignable" );
  53 static_assert( !is_copy_assignable<D>::value, "D is not copy assignable" );
  54 static_assert( !is_copy_assignable<E>::value,  "E is not copy assignable" );
  55
  56 static_assert( is_canonical_copy_assignable<A>::value,
  57                "A has canonical copy assignment" );
  58 static_assert( is_canonical_copy_assignable<B>::value,
  59                "B has canonical copy assignment" );
  60 static_assert( !is_canonical_copy_assignable<C>::value,
  61                "C does not have canonical copy assignment" );
  62 static_assert( !is_canonical_copy_assignable<D>::value,
  63                "D does not have canonical copy assignment" );
  64 static_assert( !is_canonical_copy_assignable<E>::value,
  65                "E does not have canonical copy assignment" );
  66
  67 // archetypal helper alias for a particular type member:
  68 template <class T>
  69   using diff_t = typename T::difference_type;
  70 // alias the type member, if it exists, otherwise alias ptrdiff_t:
  71 template <class Ptr>
  72   using difference_type = detected_or_t<ptrdiff_t, diff_t, Ptr>;
  73
  74 struct has { using difference_type = char; };
  75 struct has_not { };
  76 struct inherits : has { };
  77 struct hides : private has { };
  78 struct reveals : private has { using has::difference_type; };
  79
  80 static_assert( is_same_v<difference_type<has>,      char>,      "has" );
  81 static_assert( is_same_v<difference_type<has_not>,  ptrdiff_t>, "has not" );
  82 static_assert( is_same_v<difference_type<inherits>, char>,      "inherits" );
  83 static_assert( is_same_v<difference_type<hides>,    ptrdiff_t>, "hides" );
  84 static_assert( is_same_v<difference_type<reveals>,  char>,      "reveals" );