< cpp‎ | experimental‎ | ranges
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template< class I >
struct difference_type {};
template< class T >
struct difference_type<T*>;
template< class T >
struct difference_type<const T> : difference_type<std::decay_t<T>> {};
template< class T >

    requires requires { typename T::difference_type; }

struct difference_type<T>;
template< class T >

    requires !requires { typename T::difference_type; } &&
             requires(const T& a, const T& b) { {a - b} -> Integral; }

struct difference_type<T>;

Computes the associated difference type of the type I, if any. Users may specialize difference_type for a program-defined type.

1) Primary template is an empty struct.
2) Specialization for pointers. If T is an object type, provides a member type type equal to std::ptrdiff_t. Otherwise, there is no member type.
3) Specialization for const-qualified types.
4) Specialization for types that define a public and accessible member type difference_type. Provides a member type type equal to T::difference_type.
5) Specialization for types that do not define a public and accessible member type difference_type but do support subtraction. Provides a member type type equal to std::make_signed_t<decltype(std::declval<T>() - std::declval<T>())>.

[edit] Helper alias template

template< class T >
using difference_type_t = typename ranges::difference_type<T>::type;
(ranges TS)

[edit] Example

[edit] See also

specifies that a Semiregular type can be incremented with pre- and post-increment operators
(concept) [edit]
compatibility traits class that collects an iterator’s associated types
(alias template)[edit]