std::unorde...nordered_set(3)

💅 std::unordered_set::unordered_set 💅

from the C++ Standard Libary, manual section 3. okay so bestie, this constructor page is giving main character energy and there are eighteen overloads here. that is a lot of ways to build one unordered_set, and every single one is important. 🫶

✨ NAME ✨

std::unordered_set::unordered_set — std::unordered_set::unordered_set

🌸 SYNOPSIS 🌸

the signatures are copied exactly, because we respect code in this house 💅

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

okay so what is this constructor actually doing? it constructs a new container from a variety of data sources. optionally it uses the 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. 🌟

• overloads 1–5 — constructs an empty container. sets max_load_factor() to 1.0. for the default constructor, the number of buckets is implementation-defined.
• overloads 6–8 — constructs the container with the contents of the range [first, last). sets max_load_factor() to 1.0. if multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).
• overloads 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, 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.
• 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.
  • (since C++23) the template parameter Allocator is only deduced from the first argument while used in class template argument deduction.
• overloads 13–15 — initializer-list constructor. constructs the container with the contents of the initializer list init, same as unordered_set(init.begin(), init.end()).
• overloads 16–18 — constructs the container with the contents of rg. if multiple elements in the range have keys that compare equivalent, it is unspecified which element is inserted (pending LWG2844).

🎀 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 is the whole warning, bestie. 💀

💅 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:

• __cpp_lib_containers_ranges — value 202202L; std (C++23); feature: ranges-aware construction and insertion; overloads (16-18).

🍓 EXAMPLE 🍓

This section is incomplete

Reason: no example

not me staring at “no example” like it’s going to manifest one 😭

🌈 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 💀

category vibes: * Todo no example

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xoxo, your cpp bestie 💖 — source/version is http://cppreference.com, date is 2024.06.10, and this cutie came from the std::unorde...nordered_set(3) page. periodt. 🎀