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

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.

Data members

Member functions

Deduction guides

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";
}

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