std::ranges::view_interface<D>::operator bool

explicit constexpr operator bool() requires /* see below */;

explicit constexpr operator bool() const requires /* see below */;

The default implementation of operator bool member function checks whether the view is non-empty. It makes the derived type contextually convertible to bool.

Parameters

(none).

Return value

false if the value of the derived type is empty (determined by std::ranges::empty), true otherwise.

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.

Example

#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';
}

Has odd numbers: true 
Has negative numbers: false

See also