# std::incrementable_traits

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
template< class I > struct incrementable_traits {}; | (1) | (since C++20)
template< class T > requires std::is_object_v <T> struct incrementable_traits<T*>; | (2) | (since C++20)
template< class T > struct incrementable_traits<const T> : incrementable_traits<T> {}; | (3) | (since C++20)
template< class T > requires requires { typename T::difference_type; } struct incrementable_traits<T>; | (4) | (since C++20)
template< class T > requires (!requires { typename T::difference_type; }) && requires(const T& a, const T& b) { { a - b } -> std::integral ; } struct incrementable_traits<T>; | (5) | (since C++20)

Computes the associated difference type of the type `I`, if any. A program may specialize `incrementable_traits` for a program-defined type.

1) Primary template is an empty struct.
2) Specialization for pointers.
Provides a member type `difference_type` same as std::ptrdiff_t.
3) Specialization for const-qualified types.
4) Specialization for types that define a public and accessible member type `difference_type`.
Provides a member type `difference_type` same as `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 `difference_type` same as std::make_signed_t <decltype( std::declval <T>() - std::declval <T>())>. The implicit expression variations rule (see below) applies to the expression a - b.

### Implicit expression variations

A requires expression that uses an expression that is non-modifying for some constant lvalue operand also requires implicit expression variations.

### Example

| This section is incomplete Reason: no example

### See also

weakly_incrementable (C++20) | specifies that a `semiregular` type can be incremented with pre- and post-increment operators (concept)
iter_value_titer_reference_titer_const_reference_titer_difference_titer_rvalue_reference_titer_common_reference_t (C++20)(C++20)(C++23)(C++20)(C++20)(C++20) | computes the associated types of an iterator (alias template)
iterator_traits | provides uniform interface to the properties of an iterator (class template)

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