std::ranges::views::single, std::ranges::single_view

Since C++23

Header: <ranges>

Produces a view that contains exactly one element of a specified value.

# Declarations

template< std::copy_constructible T >
requires std::is_object_v<T>
class single_view
: public ranges::view_interface<single_view<T>>

(since C++20) (until C++23)

template< std::move_constructible T >
requires std::is_object_v<T>
class single_view
: public ranges::view_interface<single_view<T>>

(since C++23)

namespace views {
inline constexpr /* unspecified */ single = /* unspecified */;
}

(since C++20)

Call signature

template< class T >
requires /* see below */
constexpr /* see below */ single( T&& t );

(since C++20)

# Notes

For a single_view, the inherited empty member function always returns false, and the inherited operator bool conversion function always returns true.

# Example

#include <iomanip>
#include <iostream>
#include <ranges>
#include <string>
#include <tuple>
 
int main()
{
    constexpr std::ranges::single_view sv1{3.1415}; // uses (const T&) constructor
    static_assert(sv1);
    static_assert(not sv1.empty());
 
    std::cout << "1) *sv1.data(): " << *sv1.data() << '\n'
              << "2) *sv1.begin(): " << *sv1.begin() << '\n'
              << "3)  sv1.size(): " << sv1.size() << '\n'
              << "4)  distance: " << std::distance(sv1.begin(), sv1.end()) << '\n';
 
    std::string str{"C++20"};
    std::cout << "5)  str = " << std::quoted(str) << '\n';
    std::ranges::single_view sv2{std::move(str)}; // uses (T&&) constructor
    std::cout << "6) *sv2.data(): " << std::quoted(*sv2.data()) << '\n'
              << "7)  str = " << std::quoted(str) << '\n';
 
    std::ranges::single_view<std::tuple<int, double, std::string>>
        sv3{std::in_place, 42, 3.14, "😄"}; // uses (std::in_place_t, Args&&... args)
 
    std::cout << "8)  sv3 holds a tuple: { "
              << std::get<0>(sv3[0]) << ", "
              << std::get<1>(sv3[0]) << ", "
              << std::get<2>(sv3[0]) << " }\n";
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 3428	C++20	single_view was convertible from std::in_place_t	the constructor is made explicit
LWG 4035	C++20	single_view did not provide the member function empty()	provides empty()
P2367R0	C++20	deduction guides for single_view failed to decay the argument;views::single copied but not wrapped a single_view	a decaying guide provided;made always wrapping