std::ranges::empty

 

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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

 

Defined in header <ranges>
inline namespace /*unspecified*/ {     inline constexpr auto empty = /*unspecified*/; }		(since C++20) (customization point object)
Call signature
template< class T >     requires /* see below */ constexpr bool 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.

Expression-equivalent

Expression e is expression-equivalent to expression f, if

• e and f have the same effects, and
• either both are constant subexpressions or else neither is a constant subexpression, and
• either both are potentially-throwing or else neither is potentially-throwing (i.e. noexcept(e) == noexcept(f)).

Customization point objects

The name ranges::empty denotes a customization point object, which is a const function object of a literal semiregular class type. For exposition purposes, the cv-unqualified version of its type is denoted as __empty_fn.

All instances of __empty_fn are equal. The effects of invoking different instances of type __empty_fn on the same arguments are equivalent, regardless of whether the expression denoting the instance is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified instance is not required to be invocable). Thus, ranges::empty can be copied freely and its copies can be used interchangeably.

Given a set of types Args..., if std::declval<Args>()... meet the requirements for arguments to ranges::empty above, __empty_fn models

• std::invocable<__empty_fn, Args...>,
• std::invocable<const __empty_fn, Args...>,
• std::invocable<__empty_fn&, Args...>, and
• std::invocable<const __empty_fn&, Args...>.
  

Otherwise, no function call operator of __empty_fn participates in overload resolution.

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 (const auto& 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 : private std::vector<int> {
            using std::vector<int>::begin;
            using std::vector<int>::end;
            using std::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)