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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

std::ranges::view_interface

Member functions

empty (C++20)

operator bool (C++20)

data (C++20)

size (C++20)

front (C++20)

back (C++20)

operator[] (C++20)

 

explicit constexpr operator bool() requires /* see below */;	(1)	(since C++20)
explicit constexpr 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 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

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

ranges::empty (C++20)	checks whether a range is empty (customization point object)
empty (C++20)	Returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range. (public member function)
empty (C++17)	checks whether the container is empty (function template)