std::unorde...red_multimap(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💖 std::unordered_multimap::unordered_multimap 💖

okay bestie, welcome to the C++ Standard Libary manual page for the std::unordered_multimap constructor. yes, the title is a whole mouthful, and yes, we are literally building the container from scratch: empty, from ranges, from other containers, from init lists, all of it. no cap, it's giving “choose your own constructor adventure” ✨

🌸 NAME 🌸

std::unordered_multimap::unordered_multimap — that's the constructor, bestie. the name on the page is literally std::unordered_multimap::unordered_multimap, so we're not gonna pretend it's deeper than that 💅

✨ Synopsis ✨

okay copy-paste goblin mode: here are the overloads exactly as the page gives them. do not come for me about the spacing, I'm just the messenger 🫶

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

so what's the tea? constructs a new container from a variety of data sources. optionally it uses your supplied bucket_count as the minimal number of buckets to create, hash as the hash function, equal as the function to compare keys, and alloc as the allocator. it's giving “I can be empty, I can be copied, I can be moved, I can be built from a range” 💅

  • 1-5) — constructs an empty container. sets max_load_factor() to 1.0. for the default constructor, the number of buckets is implementation-defined.
  • 6-8) — constructs the container with the contents of the range [first, last). sets max_load_factor() to 1.0.
  • 9,10) — copy constructor. constructs the container with the copy of the contents of other, and copies the load factor, the predicate, and the hash function as well. if alloc is not provided, the allocator is obtained by calling std::allocator_traits<allocator_type>::select_on_container_copy_construction(other.get_allocator()).

(since C++23) the template parameter Allocator is only deduced from the first argument while used in class template argument deduction.

  • 11,12) — move constructor. constructs the container with the contents of other using move semantics. if alloc is not provided, the allocator is obtained by move-construction from the allocator belonging to other.

(since C++23) the template parameter Allocator is only deduced from the first argument while used in class template argument deduction.

  • 13-15) — initializer-list constructor. constructs the container with the contents of the initializer list init, same as unordered_multimap(init.begin(), init.end()).
  • 16-18) — constructs the container with the contents of rg.

🦋 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.

🌈 Type requirements 🌈

  • InputIt must meet the requirements of LegacyInputIterator.

🔥 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).

😭 Exceptions 😭

Calls to Allocator::allocate may throw. that's it, that's the whole warning. stay safe 🫶

💜 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 tea ☕:

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

🎀 Example 🎀

This section is incomplete Reason: no example

💀 Defect reports 💀

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

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

🌟 See also 🌟

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

🏷️ Category 🏷️

  • * Todo no example

okay bye bestie 🫶 this page came from http://cppreference.com, version/date 2024.06.10, and it's the std::unorde...red_multimap(3) page. slay ✨

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