inputrange

Inputrange is a term used in the C++ ranges library to describe a range whose iterator models the InputIterator concept. Such ranges support single-pass iteration: you can traverse the sequence from beginning to end by repeatedly advancing the iterator, but they do not guarantee that a second, independent pass will reproduce the same sequence without reinitializing the range.

A range is considered an input range if its begin and end (or a sentinel) are compatible and the iterator type satisfies the requirements of an InputIterator. In practice, this means the range can be iterated sequentially, but it may not support multi-pass iteration or repeated dereferences of the same position. The end of the range may be defined by a sentinel type rather than a contrasting iterator type.

Examples of input ranges include adapters over input streams, such as reading from standard input or other stream sources, where data is consumed in a single forward pass. While many standard library components can present input ranges, they do not guarantee that the same value will remain accessible after the iterator has advanced.

Input ranges contrast with forward ranges, which guarantee multi-pass iteration: a program can restart the traversal and produce the same results from the beginning. Consequently, algorithms that require multiple passes or re-access to previously seen elements must either operate on a forward (or stronger) range or be designed to work with single-pass input ranges.

In modern C++ (C++20 and later), std::ranges::input_range is a concept that classifies ranges whose iterator models InputIterator, providing a formal, standard way to express this capability in templates and in library code.