Difference between revisions of "cpp/ranges/range"

Revision as of 17:51, 6 November 2023

The range concept defines the requirements of a type that allows iteration over its elements by providing an iterator and sentinel that denote the elements of the range.

Semantic requirements

Given an expression E such that decltype((E)) is T, T models range only if

• [ranges::begin(E), ranges::end(E)) denotes a range, and
• both ranges::begin(E) and ranges::end(E) are amortized constant time and do not alter the value of E in a manner observable to equality-preserving expressions, and
• if the type of ranges::begin(E) models forward_iterator, ranges::begin(E) is equality-preserving (in other words, forward iterators support multi-pass algorithms).

Notes

A typical range class only needs to provide two functions:

1. A member function begin() whose return type models input_or_output_iterator.
2. A member function end() whose return type models sentinel_for<It>, where It is the return type of begin().

Alternatively, they can be non-member functions, to be found by argument-dependent lookup.

Example

#include <iostream>
#include <ranges>
#include <vector>
 
template<typename T>
struct range_t : private T
{
    using T::begin, T::end; /* ... */
};
static_assert(std::ranges::range<range_t<std::vector<int>>>);
 
template<typename T>
struct scalar_t
{
    T t {}; /* no begin/end */
};
static_assert(not std::ranges::range<scalar_t<int>>);
 
int main()
{
    if constexpr (range_t<std::vector<int>> r; std::ranges::range<decltype(r)>)
        std::cout << "r is a range\n";
 
    if constexpr (scalar_t<int> s; not std::ranges::range<decltype(s)>)
        std::cout << "s is not a range\n";
}

r is a range
s is not a range

Defect reports

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