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

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).



Return value

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';


Has odd numbers: true
Has negative numbers: false

See also

checks whether a range is empty
(customization point object)
returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range.
(public member function)
checks whether the container is empty
(function template)

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