deductiongu...td::multimap(3)

🌸 NAME 🌸

Okay so this is the C++ Standard Library manual page for deduction guides for std::multimap, section 3, and the name is literally deduction guides for std::multimap — deduction guides for std::multimap. No cap, that's the whole NAME situation. 🫶

✨ Synopsis ✨

Defined in header <map> — yes bestie, <map> is the header, write it down. 💅

🌟 Overload (1) — since C++17 🌟

template< class InputIt,
          class Comp = std::less<iter_key_t<InputIt>>,
          class Alloc = std::allocator<iter_to_alloc_t<InputIt>> >
multimap( InputIt, InputIt, Comp = Comp(), Alloc = Alloc() )
    -> multimap<iter_key_t<InputIt>, iter_val_t<InputIt>,
                Comp, Alloc>;

🌟 Overload (2) — since C++17 🌟

template< class Key,
          class T,
          class Comp = std::less<Key>,
          class Alloc = std::allocator<std::pair<const Key, T>> >
multimap( std::initializer_list<std::pair<Key, T>>, Comp = Comp(), Alloc = Alloc() )
    -> multimap<Key, T, Comp, Alloc>;

🌟 Overload (3) — since C++17 🌟

template< class InputIt, class Alloc >
multimap( InputIt, InputIt, Alloc )
    -> multimap<iter_key_t<InputIt>, iter_val_t<InputIt>,
                std::less<iter_key_t<InputIt>>, Alloc>;

🌟 Overload (4) — since C++17 🌟

template< class Key, class T, class Alloc >
multimap( std::initializer_list<std::pair<Key, T>>, Alloc )
    -> multimap<Key, T, std::less<Key>, Alloc>;

🌟 Overload (5) — since C++23 🌟

template< ranges::input_range R, class Compare = std::less<range_key_t<R>>,
          class Alloc = std::allocator<range_to_alloc_t<R>> >
multimap( std::from_range_t, R&&, Compare = Compare(), Alloc = Alloc() )
    -> multimap<range_key_t<R>, range_mapped_t<R>, Compare, Alloc>;

🌟 Overload (6) — since C++23 🌟

template< ranges::input_range R, class Alloc >
multimap( std::from_range_t, R&&, Alloc )
    -> multimap<range_key_t<R>, range_mapped_t<R>,
                std::less<range_key_t<R>>, Alloc>;

🌈 Exposition-only helper type aliases 🌈

iter_val_t

template< class InputIter >
using iter_val_t =
    typename std::iterator_traits<InputIter>::value_type;

(exposition only*)

iter_key_t

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

(exposition only*)

iter_mapped_t

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

(exposition only*)

iter_to_alloc_t

template< class InputIter >
using iter_to_alloc_t =
    std::pair<std::add_const_t<tuple_element_t<0,
        iter_val_t<InputIter>>>,
        std::tuple_element_t<1,
        iter_val_t<InputIter>>>;

(exposition only*)

range_key_t

template< ranges::input_range Range >
using range_key_t =
    std::remove_const_t<typename
        ranges::range_value_t<Range>::first_type>;

(since C++23) (exposition only*)

range_mapped_t

template< ranges::input_range Range >
using range_mapped_t =
    typename ranges::range_value_t<Range>::second_type;

(since C++23) (exposition only*)

range_to_alloc_t

template< ranges::input_range Range >
using range_to_alloc_t =
    std::pair<std::add_const_t<typename
        ranges::range_value_t<Range>::first_type>,
        typename ranges::range_value_t<Range>::second_type>;

(since C++23) (exposition only*)

1-4) These deduction guides are provided for multimap to allow deduction from an iterator range (overloads (1,3)) and std::initializer_list (overloads (2,4)). Slay.

5,6) These deduction guides are provided for multimap to allow deduction from a std::from_range_t tag and an input_range. Adorable.

These overloads participate in overload resolution only if InputIt satisfies LegacyInputIterator, Alloc satisfies Allocator, and Comp does not satisfy Allocator.

Note, bestie: 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.

🦋 Notes 🦋

Feature-test macro

• Feature-test macro: __cpp_lib_containers_ranges
• Value: 202202L
• Std: (C++23)
• Feature: Ranges-aware construction and insertion; overload (5,6)

💅 Example 💅

Run this code, bestie — the comments are literally doing the teaching here. ✨

// Run this code

#include <map>

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

🎀 Defect reports 🎀

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

• DR 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> periodt.

---

That's the whole page, bestie 💖 — source/version and date: cppreference.com, 2024.06.10. You survived the template soup. 🫶