deductionguidesforstd::map(3)

okay bestie, welcome to the deductionguidesforstd::map(3) page from the C++ Standard Libary manual 💅✨ we’re talking about the deduction guides for std::map, and yes, every single overload is getting its moment. no cap.

💖 NAME

deductionguidesforstd::map — deduction guides for std::map ✨

✨ Synopsis ✨

Defined in header <map> 🎀

🌈 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>> >
map( InputIt, InputIt, Comp = Comp(), Alloc = Alloc() )
    -> map<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>> >
map( std::initializer_list<std::pair<Key, T>>, Comp = Comp(), Alloc = Alloc() )
    -> map<Key, T, Comp, Alloc>;

🌈 Overload (3) — since C++17

template< class InputIt, class Alloc >
map( InputIt, InputIt, Alloc )
    -> map<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 >
map( std::initializer_list<std::pair<Key, T>>, Alloc )
    -> map<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>> >
map( std::from_range_t, R&&, Compare = Compare(), Alloc = Alloc() )
    -> map<range_key_t<R>, range_mapped_t<R>, Compare, Alloc>;

🌈 Overload (6) — since C++23

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

🧚 Exposition-only helper type aliases

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

(exposition only*)

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

(exposition only*)

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

(exposition only*)

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*)

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*)

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

(since C++23) · (exposition only*)

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*)

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

5,6) These deduction guides are provided for map 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, and Comp does not satisfy Allocator.

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.

📝 Notes

Feature-test macro table row, but make it cute:

• 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

#include <map>

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

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

🔍 Defect reports

okay history time: 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>

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okay that’s the whole page bestie 🫶 source/version: http://cppreference.com, date 2024.06.10, and the manual page is deductionguidesforstd::map(3) — go deduce something iconic 💅✨