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

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

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

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

Typical implementations of single_view store a single member of type copyable-box<T> (until C++23)movable-box<T> (since C++23). For description purposes, this member is hereinafter called value_.

The lifetime of the element is bound to the parent single_view. Copying single_view makes a copy of the element.

Customization point objects

The name views::single denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __single_fn.

All instances of __single_fn are equal. The effects of invoking different instances of type __single_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, views::single can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to views::single above, __single_fn models
.

• std::invocable<__single_fn, Args...>,
• std::invocable<const __single_fn, Args...>,
• std::invocable<__single_fn&, Args...>, and
• std::invocable<const __single_fn&, Args...>.
  

Otherwise, no function call operator of __single_fn participates in overload resolution.

Member functions

single_view() requires std::default_initializable<T> = default;

constexpr explicit single_view( const T& t );

constexpr explicit single_view( T&& t );

template< class... Args >
  requires std::constructible_from<T, Args...>
constexpr explicit single_view( std::in_place_t, Args&&... args );

constexpr T* begin() noexcept;
constexpr const T* begin() const noexcept;

constexpr T* end() noexcept;
constexpr const T* end() const noexcept;

static constexpr std::size_t size() noexcept;

constexpr T* data() noexcept;
constexpr const T* data() const noexcept;

Deduction guides

template< class T >
single_view( T ) -> single_view<T>;

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 <iostream>
#include <iomanip>
#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(): " << quoted(*sv2.data()) << '\n'
              << "7)  str = " << 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";
}

1) *sv1.data(): 3.1415
2) *sv1.begin(): 3.1415
3)  sv1.size(): 1
4)  distance: 1
5)  str = "C++20"
6) *sv2.data(): "C++20"
7)  str = ""
8)  sv3 holds a tuple: { 42, 3.14, 😄 }

Defect reports

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

See also