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.
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);.derived is static_cast<const D&>(*this).(none).
false if the value of the derived type is empty (determined by std::ranges::empty), true otherwise.
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.
#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
|
(C++20) | checks whether a range is empty (customization point object) |
|
(C++20) | returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range. (public member function) |
|
(C++17) | checks whether the container is empty (function template) |
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