std::ranges::empty

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

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)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_value_t	range_reference_trange_rvalue_reference_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Semiregular wrapper
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	views::counted
Adaptors
views::all_tviews::all join_viewviews::join take_viewviews::take basic_istream_viewistream_view	ref_view                                split_viewviews::split take_while_viewviews::take_while elements_viewviews::elements	common_viewviews::common transform_viewviews::transform drop_viewviews::drop keys_viewviews::keys	reverse_viewviews::reverse filter_viewviews::filter drop_while_viewviews::drop_while values_viewviews::values

 

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);

Determines whether or not t has any elements.

Let t be an object of type T. A call to ranges::empty is expression-equivalent to:

1. bool(std::forward<T>(t).empty()), if that expression is valid.
2. Otherwise, (ranges::size(std::forward<T>(t)) == 0), if that expression is valid.
3. Otherwise, bool(ranges::begin(t) == ranges::end(t))

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, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

Customization point objects

The name ranges::empty denotes a customization point object, which is a const function object of a literal semiregular class type (denoted, for exposition purposes, as empty_ftor). All instances of empty_ftor are equal. 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_ftor will satisfy std::invocable<const empty_ftor&, Args...>. Otherwise, no function call operator of empty_ftor participates in overload resolution.

Example

Run this code

#include <iostream>
#include <ranges>
#include <vector>
 
template <std::ranges::input_range R>
void print(R&& r)
{
    if (std::ranges::empty(r)) {
        std::cout << "\tEmpty\n";
        return;
    }
 
    std::cout << "\tElements:";
    for (const auto& element : r) {
        std::cout << ' ' << element;
    }
 
    std::cout << '\n';
}
 
int main()
{
    {
        auto v = std::vector<int>{1, 2, 3};
        std::cout << "1. calling ranges::empty on std::vector:\n";
        print(v);
 
        v.clear();
        print(v);
    }
    {
        std::cout << "2. calling ranges::empty on std::initializer_list:\n";
        auto il = {7, 8, 9};
        print(il);
 
        print(std::initializer_list<int>{});
    }
    {
        std::cout << "2. calling ranges::empty on a raw array:\n";
        int array[] = {4, 5, 6}; // array has a known bound
        print(array);
    }
    {
        struct NoEmptyNorSize : private std::vector<int> {
            auto begin() { return std::vector<int>::begin(); }
            auto end() { return std::vector<int>::end(); }
        };
 
        std::cout << "3. calling ranges::empty on an object that satisfies only case 3):\n";
        print(NoEmptyNorSize{});
    }
}

Output:

1. calling ranges::empty on std::vector:
	Elements: 1 2 3
	Empty
2. calling ranges::empty on std::initializer_list:
	Elements: 7 8 9
	Empty
2. calling ranges::empty on a raw array:
	Elements: 4 5 6
3. calling ranges::empty on an object that satisfies only case 3):
	Empty

See also

empty (C++17)

checks whether the container is empty (function template)