std::ranges::view_interface<D>::empty

 

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begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
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to (C++23)
范围原语
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)
悬垂迭代器处理
dangling	borrowed_iterator_t	borrowed_subrange_t
范围概念
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
视图
view_interface	subrange
范围工厂
empty_viewviews::empty single_viewviews::single	iota_viewviews::iota repeat_viewviews::repeat (C++23)(C++23)	basic_istream_viewviews::istream cartesian_product_viewviews::cartesian_product_view (C++23)(C++23)
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views::all_tviews::all ref_view owning_view as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while	drop_viewviews::drop drop_while_viewviews::drop_while join_viewviews::join join_with_viewviews::join_with (C++23)(C++23) lazy_split_viewviews::lazy_split split_viewviews::split views::counted common_viewviews::common	reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   views::pairwise_transform (C++23)   chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)
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std::generator (C++23)
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range_adaptor_closure (C++23)
辅助项
copyable-box	simple-view

 

std::ranges::view_interface

成员函数

empty (C++20)

cbegin (C++23)

cend (C++23)

operator bool (C++20)

data (C++20)

size (C++20)

front (C++20)

back (C++20)

operator[] (C++20)

 

constexpr bool empty() requires ranges::forward_range<D>;	(1)	(C++20 起)
constexpr bool empty() const requires ranges::forward_range<const D>;	(2)	(C++20 起)

empty 成员函数的默认实现检查起始迭代器与哨位是否比较相等。

1) 令 derived 为绑定到 static_cast<D&>(*this) 的引用。等价于 return ranges::begin(derived) == ranges::end(derived); 。

2) 同 (1) ，除了 derived 为 static_cast<const D&>(*this) 。

参数

（无）

返回值

若派生类型对象的起始迭代器与哨位比较相等则为 true ，否则为 false 。

注解

下列派生类型可使用 empty 的默认实现：

• std::ranges::common_view
• std::ranges::drop_view
• std::ranges::drop_while_view
• std::ranges::elements_view
• std::ranges::filter_view
• std::ranges::iota_view
• std::ranges::join_view
• std::ranges::reverse_view
• std::ranges::single_view
• std::ranges::split_view
• std::ranges::take_view
• std::ranges::take_while_view
• std::ranges::transform_view

尽管 std::ranges::basic_istream_view 继承自 std::ranges::view_interface 并且不声明 empty 成员函数，它仍无法使用默认实现，因为它决不满足 std::ranges::forward_range 。

示例

运行此代码

#include <array>
#include <ranges>
 
int main() {
    constexpr std::array a {0, 1, 2, 3, 4};
    static_assert( ! std::ranges::single_view(a).empty() );
    static_assert(   (a | std::views::take(0)).empty() );
    static_assert( ! (a | std::views::take(2)).empty() );
    static_assert(   (a | std::views::drop(9)).empty() );
    static_assert( ! (a | std::views::drop(2)).empty() );
    static_assert(   std::views::iota(0,0).empty() );
    static_assert( ! std::views::iota(0).empty() );
}

参阅

empty (C++17)	检查容器是否为空 (函数模板)
ranges::empty (C++20)	检查范围是否为空 (定制点对象)