std::ranges::view_interface<D>::operator bool
From cppreference.com
< cpp | ranges | view interface
|
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)
Parameters
(none)
Return value
false if the value of the derived type is empty (determined by std::ranges::empty), true
Notes
In C++20, no type derived from std::ranges::view_interface in the standard library provides their own operator bool
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
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
See also
|
(C++20)
|
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) |
|
|
(C++17)
|
checks whether the container is empty (function template) |