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.
using pointer = see below;
Type: Ptr.
using element_type = see below;
Type: typename ptr-traits-elem<Ptr>​::​type.
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.
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]

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.
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]