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    std::empty

    From cppreference.com
     
    C++
     
    Iterator library
    Iterator concepts
    Iterator primitives
    Algorithm concepts and utilities
    Indirect callable concepts
    Common algorithm requirements
    (C++20)
    (C++20)
    (C++20)
    Utilities
    (C++20)
    Iterator adaptors
    Range access
    (C++11)(C++14)
    (C++14)(C++14)  
    (C++11)(C++14)
    (C++14)(C++14)  
    (C++17)(C++20)
    (C++17)
    (C++17)
     
    Defined in header <array>
    Defined in header <deque>
    Defined in header <flat_map>
     (since C++23)
    Defined in header <flat_set>
     (since C++23)
    Defined in header <forward_list>
    Defined in header <inplace_vector>
     (since C++26)
    Defined in header <iterator>
    Defined in header <list>
    Defined in header <map>
    Defined in header <optional>
     (since C++26)
    Defined in header <regex>
    Defined in header <set>
    Defined in header <span>
     (since C++20)
    Defined in header <stacktrace>
     (since C++23)
    Defined in header <string>
    Defined in header <string_view>
    Defined in header <unordered_map>
    Defined in header <unordered_set>
    Defined in header <valarray>
    Defined in header <vector> 
    template< class C >
constexpr auto empty( const C& c ) noexcept(noexcept(c.empty()))
    -> decltype(c.empty());
    		(1) (since C++17) 
    template< class T, std::size_t N >
constexpr bool empty( const T (&array)[N] ) noexcept;
    		(2) (since C++17)

    Returns whether the given range is empty.

    1) Returns c.empty().
    2) Returns false.

    Parameters

    c - a container or view with an empty member function
    array - an array of arbitrary type

    Return value

    1) c.empty()
    2) false

    Exceptions

    1) What and when the underlying c.empty() call throws.

    Notes

    Feature-test macro Value Std Feature
    __cpp_lib_nonmember_container_access 201411L (C++17) std::size(), std::data(), and std::empty()
    __cpp_lib_initializer_list 202511L (C++26)
    (DR11)    		 data and empty member functions of std::initializer_list;
    removing unnecessary free functions for std::initializer_list[1]
    1. ↑ As of 2026-05-27, libstdc++ has not treated the addition and removal of functions from P3016R6 as a defect report against C++11, and only applies the changes since C++26.
      The overload for std::initializer_list was necessary before P3016R6 because it did not have a member function empty.

    Possible implementation

    First version 
    template<class C>
[[nodiscard]] constexpr auto empty(const C& c) noexcept(noexcept(c.empty()))
    -> decltype(c.empty())
{
    return c.empty();
}
    Second version 
    template<class T, std::size_t N>
[[nodiscard]] constexpr bool empty(const T (&array)[N]) noexcept
{
    return false;
}

    Example

    Run this code
    #include <iostream>
#include <vector>

template<class T>
void print(const T& container)
{
    if (std::empty(container))
        std::cout << "Empty\n";
    else
    {
        std::cout << "Elements:";
        for (const auto& element : container)
            std::cout << ' ' << element;
        std::cout << '\n';
    }
}

int main()
{
    std::vector<int> c = {1, 2, 3};
    print(c);
    c.clear();
    print(c);
    
    int array[] = {4, 5, 6};
    print(array);
    
    auto il = {7, 8, 9};
    print(il);
}

    Output: 

    Elements: 1 2 3
Empty
Elements: 4 5 6
Elements: 7 8 9

    Defect report

    The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

    DR Applied to Behavior as published Correct behavior
    P3016R6 C++17 1) non-member std::empty overload was provided for std::initializer_list
    2) std::empty was not required to propagate exception specification     		1) removed
    2) required

    See also

    (C++20)
    		 checks whether a range is empty
    (customization point object)[edit]