std::ranges::empty

 

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Ranges library(C++20)

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Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata reserve_hint (C++26)   empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    common_range	sized_range viewable_range    view	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	approximately_sized_range (C++26) constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while common_viewviews::common views::counted to_input_viewviews::to_input (C++26)(C++26)	ref_view drop_viewviews::drop drop_while_viewviews::drop_while lazy_split_viewviews::lazy_split split_viewviews::split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   concat_viewviews::concat (C++26)(C++26) cache_latest_viewviews::cache_latest (C++26)(C++26)	owning_view 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)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

Defined in header <ranges>
Defined in header <iterator>
inlinenamespace/*unspecified*/{     inlineconstexprauto empty =/*unspecified*/; }		(since C++20) (customization point object)
Call signature
template<class T >     requires /* see below */ constexprbool empty( T&& t );		(since C++20)

Determines whether or not t has any elements.

A call to ranges::empty is expression-equivalent to:

1. bool(t.empty()), if that expression is valid.
2. Otherwise, (ranges::size(t)==0), if that expression is valid.
3. Otherwise, bool(ranges::begin(t)==ranges::end(t)), if that expression is valid and decltype(ranges::begin(t)) models std::forward_iterator.

In all other cases, a call to ranges::empty is ill-formed, which can result in substitution failure when ranges::empty(t) appears in the immediate context of a template instantiation.

Customization point objects

The name ranges::empty denotes a customization point object, which is a const function object of a literalsemiregular class type. See CustomizationPointObject for details.

Example

Run this code

#include <iostream>#include <ranges>#include <vector>   template<std::ranges::input_range R>void print(char id, R&& r){if(std::ranges::empty(r)){std::cout<<'\t'<< id <<") Empty\n";return;}   std::cout<<'\t'<< id <<") Elements:";for(constauto& element : r)std::cout<<' '<< element;std::cout<<'\n';}   int main(){{auto v =std::vector<int>{1, 2, 3};std::cout<<"(1) ranges::empty uses std::vector::empty:\n"; print('a', v);   v.clear(); print('b', v);}{std::cout<<"(2) ranges::empty uses ranges::size(initializer_list):\n";auto il ={7, 8, 9}; print('a', il);   print('b', std::initializer_list<int>{});}{std::cout<<"(2) ranges::empty on a raw array uses ranges::size:\n";int array[]={4, 5, 6};// array has a known bound print('a', array);}{struct Scanty :privatestd::vector<int>{usingstd::vector<int>::begin;usingstd::vector<int>::end;usingstd::vector<int>::push_back;// Note: both empty() and size() are hidden};   std::cout<<"(3) calling ranges::empty on an object w/o empty() or size():\n"; Scanty y; print('a', y); y.push_back(42); print('b', y);}}

Output:

(1) ranges::empty uses std::vector::empty: a) Elements: 1 2 3 b) Empty (2) ranges::empty uses ranges::size(initializer_list): a) Elements: 7 8 9 b) Empty (2) ranges::empty on a raw array uses ranges::size: a) Elements: 4 5 6 (3) calling ranges::empty on an object w/o empty() or size(): a) Empty b) Elements: 42

See also

empty (C++17)

checks whether the container is empty (function template)