NAME¶

std::ranges::constant_range - std::ranges::constant_range

Synopsis¶

Defined in header <ranges>
template< class T >

concept constant_range = (1) (since C++23)
ranges::input_range<T> &&

/*constant-iterator*/<ranges::iterator_t<T>>;
Helper concepts
template< class T >

concept /*constant-iterator*/ =
std::input_iterator<T> && (2) (exposition only*)

std::same_as<std::iter_const_reference_t<T>,
std::iter_reference_t<T>>;

1) The constant_range concept is a refinement of range for which ranges::begin
returns a constant iterator.
2) The exposition-only concept /*constant-iterator*/<T> is satisfied when the result
of the indirection operation of the input iterator is its const reference type which
implies read-only.

Example¶

// Run this code

#include <ranges>
#include <span>
#include <string_view>
#include <vector>

// mechanisms for ensuring the parameter is a constant range
// 1) an overload set where the mutable one defers to the constant one
template<std::ranges::constant_range R>
void takes_any_range1(R&& r)
{
// R is definitely a constant range
}

template<std::ranges::range R>
void takes_any_range1(R&& r)
{
takes_any_range1(std::views::as_const(std::forward<R>(r)));
}

// 2) one function template that shadows its parameter
template<std::ranges::range R>
void takes_any_range2(R&& _r)
{
auto r = std::views::as_const(std::forward<R>(_r));

// r is definitely a constant range
// never use _r again
}

// 3) one function template that recursively invokes itself
template<std::ranges::range R>
void takes_any_range3(R&& r)
{
if constexpr (std::ranges::constant_range<R>)
{
// R is definitely a constant range
// put implementation here
}
else
takes_any_range3(std::views::as_const(std::forward<R>(r)));
}

static_assert
(
std::ranges::constant_range<const std::vector<int>> and
not std::ranges::constant_range<std::vector<int>> and
std::ranges::constant_range<std::string_view> and
not std::ranges::constant_range<std::span<int>> and
std::ranges::constant_range<std::span<const int>> and
not std::ranges::constant_range<const std::span<int>>
);

int main() {}