# std::experimental::ranges::distance

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< cpp‎ | experimental‎ | ranges

Defined in header `<experimental/ranges/iterator>` |  |
namespace { constexpr /* unspecified */ distance = /* unspecified */; } |  | (ranges TS) (customization point object)
Call signature |  |
template< Iterator I, Sentinel <I> S > constexpr ranges::difference_type_t<I> distance(I first, S last); | (1) |
template< Range R > constexpr ranges::difference_type_t<ranges::iterator_t<R>> distance(R&& r); | (2) |
template< SizedRange R > constexpr ranges::difference_type_t<ranges::iterator_t<R>> distance(R&& r); | (3) |

Returns the distance between `first` and `last`, or between the beginning and the end of the range `r`.

1) If SizedSentinel <S, I> is satisfied, equivalent to return last - first;. Otherwise, returns the number of increments needed to get from `first` to `last`. If `[first, last)` does not denote a range, then `I` and `S` must be the same type and must model `SizedSentinel`, and `[last, first)` must denote a range. Otherwise, the behavior is undefined.
2) Equivalent to return ranges::distance(ranges::begin(r), ranges::end(r));
3) Equivalent to return ranges::size(r);

Instantiating overloads (2-3) may be ill-formed if the header `<experimental/ranges/range>` is not included before the point of instantiation.

### Customization point objects

The name `ranges::distance` denotes a customization point object, which is a function object of a literal `Semiregular` class type (denoted, for exposition purposes, as `DistanceT`). All instances of `DistanceT` are equal. Thus, `ranges::distance` can be copied freely and its copies can be used interchangeably.

Given a set of types `Args...`, if std::declval <Args>()... meet the requirements for arguments to `ranges::distance` above, `DistanceT` will satisfy ranges:: Invocable <const DistanceT, Args...>. Otherwise, no function call operator of `DistanceT` participates in overload resolution.

In every translation unit in which `ranges::distance` is defined, it refers to the same instance of the customization point object. (This means that it can be used freely in things like inline functions and function templates without violating the one-definition rule.)

### Return value

The distance between `first` and `last`, or between the beginning and the end of the range `r`.

### Example

| This section is incomplete Reason: no example

### See also

distance | returns the distance between two iterators (function template)
advance | advances an iterator by given distance (function template)
next | increment an iterator (function template)
prev | decrement an iterator (function template)
size | obtains the size of a range whose size can be calculated in constant time (customization point object)

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