2) The 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.
[edit] Example#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() {}
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