Defined in header `<iterator>` | ||
---|---|---|

template<class I> concept weakly_incrementable = std::movable<I> && requires(I i) { typename std::iter_difference_t<I>; requires /*is-signed-integer-like*/<std::iter_difference_t<I>>; { ++i } -> std::same_as<I&>; // not required to be equality-preserving i++; // not required to be equality-preserving }; | (since C++20) |

where `/*is-signed-integer-like*/<I>`

is `true`

if and only if `I`

is a signed-integer-like type (see below).

This concept specifies requirements on types that can be incremented with the pre- and post-increment operators, but those increment operations are not necessarily equality-preserving, and the type itself is not required to be `std::equality_comparable`

.

For `std::weakly_incrementable`

types, `a == b`

does not imply that `++a == ++b`

. Algorithms on weakly incrementable types must be single-pass algorithms. These algorithms can be used with istreams as the source of the input data through `std::istream_iterator`

.

`I`

models `std::weakly_incrementable`

only if, for an object `i`

of type `I`

:

- The expressions
`++i`

and`i++`

have the same domain, - If
`i`

is incrementable, then both`++i`

and`i++`

advance`i`

, and - If
`i`

is incrementable, then`std::addressof(++i) == std::addressof(i)`

.

An integer-like type is an (possibly cv-qualified) integer type (except for *cv* `bool`

) or an implementation-provided (not user-provided) class that behaves like an integer type, including all operators, implicit conversions, and `std::numeric_limits`

specializations. If an integer-like type represents only non-negative values, it is unsigned-integer-like, otherwise it is signed-integer-like.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|

LWG 3467 | C++20 | `bool` was considered as an integer-like type | excluded |

P2325R3 | C++20 | `default_initializable` was required | not required |

(C++20) | specifies that the increment operation on a type is equality-preserving and that the type is `weakly_incrementable` `equality_comparable` (concept) |

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