std::unorde...red_multiset(3)

🌸 NAME 🌸

std::unordered_multiset::unordered_multiset — bestie, this is the constructor page for std::unordered_multiset, aka the std::unorde...red_multiset(3) page from the C++ Standard Libary manual. It’s the “how do I even build one of these” page, and yes it has a lot of overloads. Iconic behavior, honestly 💅

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✨ Synopsis ✨

Okay so here’s the entire overload lineup, and I am not touching the tokens because code is sacred, periodt. 🫶

          unordered_multiset()
       (since C++11)
              : unordered_multiset(size_type(/* implementation-defined
       (until C++20)
          */)) {}
          unordered_multiset();
       (since C++20)
          explicit unordered_multiset( size_type bucket_count,

                                       const Hash& hash = Hash(),
                                       const key_equal& equal =             (2)
       (since C++11)
          key_equal(),

                                       const Allocator& alloc =
          Allocator() );
          unordered_multiset( size_type bucket_count,

                              const Allocator& alloc )                      (3)
       (since C++14)

              : unordered_multiset(bucket_count, Hash(), key_equal(),
          alloc) {}
          unordered_multiset( size_type bucket_count,

                              const Hash& hash,
                              const Allocator& alloc )                      (4)
       (since C++14)

              : unordered_multiset(bucket_count, hash, key_equal(),
          alloc) {}
          explicit unordered_multiset( const Allocator& alloc );            (5)
       (since C++11)
          template< class InputIt >

          unordered_multiset( InputIt first, InputIt last,
                              size_type bucket_count = /*
          implementation-defined */,                                        (6)
       (since C++11)
                              const Hash& hash = Hash(),
                              const key_equal& equal = key_equal(),

                              const Allocator& alloc = Allocator() );
          template< class InputIt >

          unordered_multiset( InputIt first, InputIt last,
                              size_type bucket_count,
                              const Allocator& alloc )                      (7)
       (since C++14)
              : unordered_multiset(first, last,

                                   bucket_count, Hash(), key_equal(),
          alloc) {}
          template< class InputIt >

          unordered_multiset( InputIt first, InputIt last,
                              size_type bucket_count,
                              const Hash& hash,                             (8)
       (since C++14)
                              const Allocator& alloc )
              : unordered_multiset(first, last,

                                   bucket_count, hash, key_equal(),
          alloc) {}
          unordered_multiset( const unordered_multiset& other );            (9)
       (since C++11)
          unordered_multiset( const unordered_multiset& other, const
       (10) (since C++11)
          Allocator& alloc );                                           (1)
          unordered_multiset( unordered_multiset&& other );
       (11) (since C++11)
          unordered_multiset( unordered_multiset&& other, const
       (12) (since C++11)
          Allocator& alloc );
          unordered_multiset( std::initializer_list<value_type> init,

                              size_type bucket_count = /*
          implementation-defined */,
       (13) (since C++11)
                              const Hash& hash = Hash(),
                              const key_equal& equal = key_equal(),

                              const Allocator& alloc = Allocator() );
          unordered_multiset( std::initializer_list<value_type> init,

                              size_type bucket_count,
                              const Allocator& alloc )
       (14) (since C++14)
              : unordered_multiset(init, bucket_count,

                                   Hash(), key_equal(), alloc) {}
          unordered_multiset( std::initializer_list<value_type> init,

                              size_type bucket_count,
                              const Hash& hash,
       (15) (since C++14)
                              const Allocator& alloc )
              : unordered_multiset(init, bucket_count,

                                   hash, key_equal(), alloc) {}
          template< container-compatible-range<value_type> R >

          unordered_multiset( std::from_range_t, R&& rg,
                              size_type bucket_count = /* see
          description */,
       (16) (since C++23)
                              const Hash& hash = Hash(),
                              const key_equal& equal = key_equal(),

                              const Allocator& alloc = Allocator() );
          template< container-compatible-range<value_type> R >

          unordered_multiset( std::from_range_t, R&& rg,
                              size_type bucket_count,
                              const Allocator& alloc )
       (17) (since C++23)
              : unordered_multiset(std::from_range,
          std::forward<R>(rg),

                                   bucket_count, Hash(), key_equal(),
          alloc) {}
          template< container-compatible-range<value_type> R >

          unordered_multiset( std::from_range_t, R&& rg,
                              size_type bucket_count,
                              const Hash& hash,
                              const Alloc& alloc )
       (18) (since C++23)
              : unordered_multiset(std::from_range,
          std::forward<R>(rg),

                                   bucket_count, hash, key_equal(),
          alloc) {}

So what’s the vibe? It constructs a new container from a variety of data sources. Optionally it uses your user supplied bucket_count as a minimal number of buckets to create, hash as the hash function, equal as the function to compare keys, and alloc as the allocator. Slay.

1. Overloads 1-5 — Constructs empty container. Sets max_load_factor() to 1.0. For the default constructor, the number of buckets is implementation-defined.
2. Overloads 6-8 — Constructs the container with the contents of the range [first, last). Sets max_load_factor() to 1.0.
3. Overloads 9,10 — Copy constructor. Constructs the container with the copy of the contents of other, copies the load factor, the predicate, and the hash function as well. If alloc is not provided, allocator is obtained by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction(other.get_allocator()). The template parameter Allocator is only deduced from the first argument while used in class template argument deduction. (since C++23)
4. Overloads 11,12 — Move constructor. Constructs the container with the contents of other using move semantics. If alloc is not provided, allocator is obtained by move-construction from the allocator belonging to other. The template parameter Allocator is only deduced from the first argument while used in class template argument deduction. (since C++23)
5. Overloads 13-15 — Initializer-list constructor. Constructs the container with the contents of the initializer list init, same as unordered_multiset(init.begin(), init.end()).
6. Overloads 16-18 — Constructs the container with the contents of rg.

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💖 Parameters 💖

• alloc — allocator to use for all memory allocations of this container.
• bucket_count — minimal number of buckets to use on initialization. If it is not specified, implementation-defined default value is used.
• hash — hash function to use.
• equal — comparison function to use for all key comparisons of this container.
• first, last — the range [first, last) to copy the elements from.
• rg — a container compatible range, that is, an input_range whose elements are convertible to value_type.
• other — another container to be used as source to initialize the elements of the container with.
• init — initializer list to initialize the elements of the container with.

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🦋 Type requirements 🦋

• InputIt must meet the requirements of LegacyInputIterator.

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⭐ Complexity ⭐

• 1-5 — Constant.
• 6-8 — Average case linear (i.e. O(N), where N is std::distance(first, last)), worst case quadratic, i.e. O(N^2).
• 9,10 — Linear in size of other.
• 11,12 — Constant. If alloc is given and alloc != other.get_allocator(), then linear.
• 13-15 — Average case O(N) (N is std::size(init)), worst case O(N^2).
• 16-18 — Average case O(N) (N is ranges::distance(rg)), worst case O(N^2).

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🔥 Exceptions 🔥

Calls to Allocator::allocate may throw. Not the vibe, but forewarned is forearmed 😭

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🌈 Notes 🌈

After container move construction (overload (4)), references, pointers, and iterators (other than the end iterator) to other remain valid, but refer to elements that are now in *this. The current standard makes this guarantee via the blanket statement in [container.reqmts]/67, and a more direct guarantee is under consideration via LWG issue 2321.

Although not formally required until C++23, some implementations have already put the template parameter Allocator into non-deduced contexts in earlier modes.

Feature-test macro cuteness:

• Feature-test macro: __cpp_lib_containers_ranges — Value: 202202L — Std: (C++23) — Feature: Ranges-aware construction and insertion; overloads (16-18).

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🎀 Example 🎀

This section is incomplete. Reason: no example. Bestie, not the missing example 😭

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👑 Defect reports 👑

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

• DR: LWG 2193 — Applied to: C++11 — Behavior as published: the default constructor (1) was explicit — Correct behavior: made non-explicit.

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🌸 See also 🌸

• operator= assigns values to the container (public member function).

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💜 Category 💜

• * Todo no example

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Signing off from http://cppreference.com, dated 2024.06.10, under the std::unorde...red_multiset(3) banner — stay hydrated and may your buckets always be implementation-defined in exactly the way you need 💖✨