20 Memory management library [mem]

20.2 Memory [memory]

20.2.3 Pointer traits [pointer.traits]

20.2.3.1 General [pointer.traits.general]

The class template pointer_traits supplies a uniform interface to certain attributes of pointer-like types.

namespace std { template<class Ptr> struct pointer_traits { see below; }; template<class T> struct pointer_traits<T*> { using pointer = T*; using element_type = T; using difference_type = ptrdiff_t; template<class U> using rebind = U*; static constexpr pointer pointer_to(see below r) noexcept; }; }

20.2.3.2 Member types [pointer.traits.types]

The definitions in this subclause make use of the following exposition-only class template and concept: template<class T> struct ptr-traits-elem // exposition only { }; template<class T> requires requires { typename T::element_type; } struct ptr-traits-elem<T> { using type = typename T::element_type; }; template<template<class...> class SomePointer, class T, class... Args> requires (!requires { typename SomePointer<T, Args...>::element_type; }) struct ptr-traits-elem<SomePointer<T, Args...>> { using type = T; }; template<class Ptr> concept has-elem-type = // exposition only requires { typename ptr-traits-elem<Ptr>::type; }

If Ptr satisfies has-elem-type, a specialization pointer_traits<Ptr> generated from the pointer_traits primary template has the following members as well as those described in [pointer.traits.functions]; otherwise, such a specialization has no members by any of those names.

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using pointer = see below;

Type: Ptr.

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using element_type = see below;

Type: typename ptr-traits-elem<Ptr>::type.

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using difference_type = see below;

Type: Ptr::difference_type if the qualified-id Ptr::difference_type is valid and denotes a type ([temp.deduct]); otherwise, ptrdiff_t.

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template<class U> using rebind = see below;

Alias template: Ptr::rebind<U> if the qualified-id Ptr::rebind<U> is valid and denotes a type ([temp.deduct]); otherwise, SomePointer<U, Args> if Ptr is a class template instantiation of the form SomePointer<T, Args>, where Args is zero or more type arguments; otherwise, the instantiation of rebind is ill-formed.

20.2.3.3 Member functions [pointer.traits.functions]

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static pointer pointer_traits::pointer_to(see below r);
static constexpr pointer pointer_traits<T*>::pointer_to(see below r) noexcept;

Mandates: For the first member function, Ptr::pointer_to(r) is well-formed.

Preconditions: For the first member function, Ptr::pointer_to(r) returns a pointer to r through which indirection is valid.

Returns: The first member function returns Ptr::pointer_to(r).

The second member function returns addressof(r).

Remarks: If element_type is cv void, the type of r is unspecified; otherwise, it is element_type&.

20.2.3.4 Optional members [pointer.traits.optmem]

Specializations of pointer_traits may define the member declared in this subclause to customize the behavior of the standard library.

A specialization generated from the pointer_traits primary template has no member by this name.

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static element_type* to_address(pointer p) noexcept;

Returns: A pointer of type element_type* that references the same location as the argument p.

[Note 1:

This function is intended to be the inverse of pointer_to.

If defined, it customizes the behavior of the non-member function to_address ([pointer.conversion]).

— end note]