deductiongu...red_multimap(3)

💖 NAME 💖

deductionguidesforstd::unordered_multimap — deductionguidesforstd::unordered_multimap

okay bestie, welcome to the deductiongu...red_multimap(3) page from the C++ Standard Libary. yes, the manual name is literally that, and the whole vibe is “deduction guides for std::unordered_multimap, but make it fashion.” 💅

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✨ Synopsis ✨

Defined in header <unordered_map> — write that down, it's important.

template< class InputIt,

          class Hash = std::hash<iter_key_t<InputIt>>,
          class Pred = std::equal_to<iter_key_t<InputIt>>,
          class Alloc =
std::allocator<iter_to_alloc_t<InputIt>> >
unordered_multimap( InputIt, InputIt,
                    typename /*see below*/::size_type =      (1)
(since C++17)
/*see below*/,
                    Hash = Hash(), Pred = Pred(), Alloc =
Alloc() )
    -> unordered_multimap<iter_key_t<InputIt>,
iter_val_t<InputIt>,

                          Hash, Pred, Alloc>;
template< class Key, class T, class Hash = std::hash<Key>,

          class Pred = std::equal_to<Key>,
          class Alloc = std::allocator<std::pair<const Key,
T>> >
unordered_multimap( std::initializer_list<std::pair<Key,
T>>,                                                         (2)
(since C++17)
                    typename /*see below*/::size_type =
/*see below*/,
                    Hash = Hash(), Pred = Pred(), Alloc =
Alloc() )

    -> unordered_multimap<Key, T, Hash, Pred, Alloc>;
template< class InputIt, class Alloc >

unordered_multimap( InputIt, InputIt, typename /*see
below*/::size_type, Alloc )
    -> unordered_multimap<iter_key_t<InputIt>,               (3)
(since C++17)
iter_val_t<InputIt>,
                          std::hash<iter_key_t<InputIt>>,

std::equal_to<iter_key_t<InputIt>>, Alloc>;
template< class InputIt, class Alloc >

unordered_multimap( InputIt, InputIt, Alloc )
    -> unordered_multimap<iter_key_t<InputIt>,
iter_val_t<InputIt>,                                         (4)
(since C++17)
                          std::hash<iter_key_t<InputIt>>,

std::equal_to<iter_key_t<InputIt>>, Alloc>;
template< class InputIt, class Hash, class Alloc >

unordered_multimap( InputIt, InputIt, typename /*see
below*/::size_type, Hash, Alloc )
    -> unordered_multimap<iter_key_t<InputIt>,               (5)
(since C++17)
iter_val_t<InputIt>, Hash,

std::equal_to<iter_key_t<InputIt>>, Alloc>;
template< class Key, class T, typename Alloc >

unordered_multimap( std::initializer_list<std::pair<Key,
T>>,
                    typename /*see below*/::size_type, Alloc (6)
(since C++17)
)

    -> unordered_multimap<Key, T, std::hash<Key>,
std::equal_to<Key>, Alloc>;
template< class Key, class T, typename Alloc >

unordered_multimap( std::initializer_list<std::pair<Key,
T>>, Alloc )                                                 (7)
(since C++17)

    -> unordered_multimap<Key, T, std::hash<Key>,
std::equal_to<Key>, Alloc>;
template< class Key, class T, class Hash, class Alloc >

unordered_multimap( std::initializer_list<std::pair<Key,
T>>,
                    typename /*see below*/::size_type, Hash, (8)
(since C++17)
Alloc )

    -> unordered_multimap<Key, T, Hash, std::equal_to<Key>,
Alloc>;
template< ranges::input_range R,

          class Hash = std::hash<range_key_t<R>>,
          class Pred = std::equal_to<range_key_t<R>>,
          class Alloc = std::allocator<range_to_alloc_t<R>>
>
unordered_multimap( std::from_range_t, R&&,                  (9)
(since C++23)
                    typename /* see description
*/::size_type = /* see description */,
                    Hash = Hash(), Pred = Pred(), Alloc =
Alloc() )
    -> unordered_multimap<range_key_t<R>, range_mapped_t<R>,

                          Hash, Pred, Alloc>;
template< ranges::input_range R, class Alloc >

unordered_multimap( std::from_range_t, R&&,
                    typename /* see description
*/::size_type, Alloc )                                       (10)
(since C++23)
    -> unordered_multimap<range_key_t<R>, range_mapped_t<R>,
                          std::hash<range_key_t<R>>,

                          std::equal_to<range_key_t<R>>,
Alloc>;
template< ranges::input_range R, class Alloc >

unordered_multimap( std::from_range_t, R&&, Alloc )
    -> unordered_multimap<range_key_t<R>, range_mapped_t<R>, (11)
(since C++23)
                          std::hash<range_key_t<R>>,

                          std::equal_to<range_key_t<R>>,
Alloc>;
template< ranges::input_range R, class Hash, class Alloc >

unordered_multimap( std::from_range_t, R&&, typename /* see
description */::size_type,
                    Hash, Alloc )                            (12)
(since C++23)
    -> unordered_multimap<range_key_t<R>, range_mapped_t<R>,
Hash,

                          std::equal_to<range_key_t<R>>,
Alloc>;

🧚 Exposition-only helper type aliases

these little helpers are exposition-only, which is fancy-speak for “bestie, these are here to explain the vibes, not for you to directly use.”

template< class InputIter >

using iter_val_t =
(exposition only*)

    typename std::iterator_traits<InputIter>::value_type;
template< class InputIter >

using iter_key_t =
(exposition only*)

    std::remove_const_t< std::tuple_element_t<0,
iter_val_t<InputIter>>>;
template< class InputIter >

using iter_mapped_t =
(exposition only*)

    std::tuple_element_t<1, iter_val_t<InputIter>>;
template< class InputIter >

using iter_to_alloc_t =
    std::pair<std::add_const_t<tuple_element_t<0,
(exposition only*)
iter_val_t<InputIter>>>,

              std::tuple_element_t<1,
iter_val_t<InputIter>>>;
template< ranges::input_range Range >

using range_key_t =
(since C++23)
                                                                  (exposition
only*)
    std::remove_const_t<typename
ranges::range_value_t<Range>::first_type>;
template< ranges::input_range Range >
                                                                  (since
C++23)
using range_mapped_t =
(exposition only*)

    typename ranges::range_value_t<Range>::second_type;
template< ranges::input_range Range >

using range_to_alloc_t =
    std::pair<std::add_const_t<typename
(since C++23)
ranges::range_value_t<Range>::first_type>,
(exposition only*)

              typename
ranges::range_value_t<Range>::second_type>;

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• 1-8) These deduction guide are provided for unordered_multimap to allow deduction from an iterator range (overloads (1,3-5)) and std::initializer_list (overloads (2,6-8)). periodt.
• 9-12) These deduction guides are provided for unordered_multimap to allow deduction from a std::from_range_t tag and an input_range.

These overloads participate in overload resolution only if InputIt satisfies LegacyInputIterator, Alloc satisfies Allocator, neither Hash nor Pred satisfy Allocator, and Hash is not an integral type.

Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type Alloc::value_type must exist and the expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.

The size_type parameter type in these guides refers to the size_type member type of the type deduced by the deduction guide.

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🌈 Notes 🌈

feature-test macro check, bestie:

• __cpp_lib_containers_ranges — value 202202L — Std (C++23) — feature: Ranges-aware construction and insertion; overloads (9-12).

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🍓 Example 🍓

// Run this code

#include <unordered_map>

int main()
{
    // std::unordered_multimap m1 = {{"foo", 1}, {"bar", 2}};
        // Error: braced-init-list has no type cannot
        // deduce pair<Key, T> from {"foo", 1} or {"bar", 2}

    std::unordered_multimap m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide #2
    std::unordered_multimap m2(m1.begin(), m1.end()); // guide #1
}

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💀 Defect reports 💀

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

• LWG 3025 — applied to C++17 — behavior as published: initializer-list guides take std::pair<const Key, T> — correct behavior: use std::pair<Key, T>.

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and that's the whole deductiongu...red_multimap(3) moment, bestie. source/version: http://cppreference.com; date: 2024.06.10. bye and slay 🫶💖