std::ranges::view_interface<D>::operator bool
From cppreference.com
< cpp | ranges | view interface
C++
Feature test macros (C++20)
Concepts library (C++20)
Metaprogramming library (C++11)
Ranges library (C++20)
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Ranges library
(C++23)(C++23)
(C++26)(C++26)
(C++23)(C++23)
(C++26)(C++26)
(C++26)(C++26)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
(C++23)(C++23)
constexpr explicit operator bool() requires /* see below */;
(1)
(since C++20)
constexpr explicit operator bool() const requires /* see below */;
(2)
(since C++20)
The default implementation of operator bool member function checks whether the view is non-empty. It makes the derived type contextually convertible to bool.
1) Let
derived be static_cast<D&>(*this). The expression in the requires-clause is equal to requires { ranges::empty (derived); }, and the function body is equivalent to return !ranges::empty (derived);.2) Same as (1), except that
derived is static_cast<const D&>(*this).Contents
[edit] Return value
false if the value of the derived type is empty (determined by std::ranges::empty ), true otherwise.
[edit] Notes
In C++20, no type derived from std::ranges::view_interface in the standard library provides their own operator bool. Almost all of these types use the default implementation.
A notable exception is std::ranges::basic_istream_view . For its iterator type never satisfies forward_iterator, the view cannot use the inherited operator bool.
[edit] Example
Run this code
#include <array> #include <iostream> #include <ranges> int main() { const std::array ints {0, 1, 2, 3, 4}; auto odds = ints | std::views::filter ([](int i) { return 0 != i % 2; }); auto negs = ints | std::views::filter ([](int i) { return i < 0; }); std::cout << std::boolalpha << "Has odd numbers: " << (!!odds) << ' ' << '\n' << "Has negative numbers: " << (!!negs) << ' ' << '\n'; }
Output:
Has odd numbers: true Has negative numbers: false
[edit] See also
returns whether the derived view is empty, provided only if it satisfies
(public member function) [edit]
sized_range or forward_range (public member function) [edit]