std::ranges::range

template< class T >
concept range = requires( T& t ) {
  ranges::begin(t); // equality-preserving for forward iterators
  ranges::end  (t);
};

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).

Note: In the definition above, the required expressions ranges::begin(t) and ranges::end(t) do not require implicit expression variations.

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