Annex D (normative) Compatibility features [depr]

A call wrapper may have a weak result type. If it does, the type of its member type result_­type is based on the type T of the wrapper's target object:

• (1.1)

  if T is a pointer to function type, result_­type shall be a synonym for the return type of T;

• (1.2)

  if T is a pointer to member function, result_­type shall be a synonym for the return type of T;

• (1.3)

  if T is a class type and the qualified-id T​::​result_­type is valid and denotes a type ([temp.deduct]), then result_­type shall be a synonym for T​::​result_­type;

• (1.4)

  otherwise result_­type shall not be defined.

To enable old function adaptors to manipulate function objects that take one or two arguments, many of the function objects in this International Standard correspondingly provide typedef-names argument_­type and result_­type for function objects that take one argument and first_­argument_­type, second_­argument_­type, and result_­type for function objects that take two arguments.

The following member names are defined in addition to names specified in Clause [function.objects]:

namespace std {
  template<class T> struct owner_less<shared_ptr<T>> {
    using result_type          = bool;
    using first_argument_type  = shared_ptr<T>;
    using second_argument_type = shared_ptr<T>;
  };

  template<class T> struct owner_less<weak_ptr<T>> {
    using result_type          = bool;
    using first_argument_type  = weak_ptr<T>;
    using second_argument_type = weak_ptr<T>;
  };

  template <class T> class reference_wrapper {
  public :
    using result_type          = see below; // not always defined
    using argument_type        = see below; // not always defined
    using first_argument_type  = see below; // not always defined
    using second_argument_type = see below; // not always defined
  };

  template <class T> struct plus {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = T;
  };

  template <class T> struct minus {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = T;
  };

  template <class T> struct multiplies {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = T;
  };

  template <class T> struct divides {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = T;
  };

  template <class T> struct modulus {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = T;
  };

  template <class T> struct negate {
    using argument_type = T;
    using result_type   = T;
  };

  template <class T> struct equal_to {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = bool;
  };

  template <class T> struct not_equal_to {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = bool;
  };

  template <class T> struct greater {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = bool;
  };

  template <class T> struct less {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = bool;
  };

  template <class T> struct greater_equal {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = bool;
  };

  template <class T> struct less_equal {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = bool;
  };

  template <class T> struct logical_and {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = bool;
  };

  template <class T> struct logical_or {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = bool;
  };

  template <class T> struct logical_not {
    using argument_type = T;
    using result_type   = bool;
  };

  template <class T> struct bit_and {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = T;
  };

  template <class T> struct bit_or {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = T;
  };

  template <class T> struct bit_xor {
    using first_argument_type  = T;
    using second_argument_type = T;
    using result_type          = T;
  };

  template <class T> struct bit_not {
    using argument_type = T;
    using result_type   = T;
  };

  template<class R, class T1>
  class function<R(T1)> {
  public:
    using argument_type = T1;
  };

  template<class R, class T1, class T2>
  class function<R(T1, T2)> {
  public:
    using first_argument_type  = T1;
    using second_argument_type = T2;
  };
}

reference_­wrapper<T> has a weak result type. If T is a function type, result_­type shall be a synonym for the return type of T.

The template specialization reference_­wrapper<T> shall define a nested type named argument_­type as a synonym for T1 only if the type T is any of the following:

• (4.1)

  a function type or a pointer to function type taking one argument of type T1

• (4.2)

  a pointer to member function R T0​::​f() cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*

• (4.3)

  a class type where the qualified-id T​::​argument_­type is valid and denotes a type ([temp.deduct]); the type T1 is T​::​argument_­type.

The template instantiation reference_­wrapper<T> shall define two nested types named first_­argument_­type and second_­argument_­type as synonyms for T1 and T2, respectively, only if the type T is any of the following:

• (5.1)

  a function type or a pointer to function type taking two arguments of types T1 and T2

• (5.2)

  a pointer to member function R T0​::​f(T2) cv (where cv represents the member function's cv-qualifiers); the type T1 is cv T0*

• (5.3)

  a class type where the qualified-ids T​::​first_­argument_­type and T​::​second_­argument_­type are both valid and both denote types ([temp.deduct]); the type T1 is T​::​first_­argument_­type and the type T2 is T​::​second_­argument_­type.

All enabled specializations hash<Key> of hash ([unord.hash]) provide two nested types, result_­type and argument_­type, which shall be synonyms for size_­t and Key, respectively.

The forwarding call wrapper g returned by a call to bind(f, bound_­args...) ([func.bind.bind]) shall have a weak result type.

The forwarding call wrapper g returned by a call to bind<R>(f, bound_­args...) ([func.bind.bind]) shall have a nested type result_­type defined as a synonym for R.

The simple call wrapper returned from a call to mem_­fn(pm) shall have a nested type result_­type that is a synonym for the return type of pm when pm is a pointer to member function.

The simple call wrapper returned from a call to mem_­fn(pm) shall define two nested types named argument_­type and result_­type as synonyms for cv T* and Ret, respectively, when pm is a pointer to member function with cv-qualifier cv and taking no arguments, where Ret is pm's return type.

The simple call wrapper returned from a call to mem_­fn(pm) shall define three nested types named first_­argument_­type, second_­argument_­type, and result_­type as synonyms for cv T*, T1, and Ret, respectively, when pm is a pointer to member function with cv-qualifier cv and taking one argument of type T1, where Ret is pm's return type.

The following member names are defined in addition to names specified in Clause [containers]:

namespace std {
  template <class Key, class T, class Compare, class Allocator>
  class map<Key, T, Compare, Allocator>::value_compare {
  public:
    using result_type          = bool;
    using first_argument_type  = value_type;
    using second_argument_type = value_type;
  };

  template <class Key, class T, class Compare, class Allocator>
  class multimap<Key, T, Compare, Allocator>::value_compare {
  public:
    using result_type          = bool;
    using first_argument_type  = value_type;
    using second_argument_type = value_type;
  };
}

The header <functional> has the following additions:

namespace std {
  template <class Predicate> class unary_negate;
  template <class Predicate>
    constexpr unary_negate<Predicate> not1(const Predicate&);
  template <class Predicate> class binary_negate;
  template <class Predicate>
    constexpr binary_negate<Predicate> not2(const Predicate&);
}

Negators not1 and not2 take a unary and a binary predicate, respectively, and return their logical negations ([expr.unary.op]).

template <class Predicate>
class unary_negate {
public:
  constexpr explicit unary_negate(const Predicate& pred);
  constexpr bool operator()(const typename Predicate::argument_type& x) const;
  using argument_type = typename Predicate::argument_type;
  using result_type   = bool;
};