deductiongu...iority_queue(3)

🎀 NAME 🎀

deductiongu...iority_queue(3) — deduction guides for std::priority_queue, from the C++ Standard Libary (yes, the header spells it like that, and we are keeping it 💅). This is the whole page for the std::priority_queue deduction guides, bestie. Defined in header <queue>. Overloads (1)–(6) are since C++17; overloads (7)–(9) are since C++23. No cap.

🦋 Synopsis 🦋

Okay so here's the overload lineup. The arrows are literally “deduces to”, iykyk 👑

template< class Comp, class Container >
priority_queue( Comp, Container )
(1) (since C++17)
    -> priority_queue<typename Container::value_type, Container, Comp>;

template< class InputIt,
          class Comp = std::less</*iter-value-type*/<InputIt>>,
          class Container = std::vector</*iter-value-type*/<InputIt> > >
priority_queue( InputIt, InputIt, Comp = Comp(), Container = Container() )
(2) (since C++17)
    -> priority_queue</*iter-value-type*/<InputIt>, Container, Comp>;

template< class Comp, class Container, class Alloc >
priority_queue( Comp, Container, Alloc )
(3) (since C++17)
    -> priority_queue<typename Container::value_type, Container, Comp>;

template< class InputIt, class Alloc >
priority_queue( InputIt, InputIt, Alloc )
(4) (since C++17)
    -> priority_queue</*iter-value-type*/<InputIt>,
                      std::vector</*iter-value-type*/<InputIt>, Alloc>,
                      std::less</*iter-value-type*/<InputIt>>>;

template< class InputIt, class Comp, class Alloc >
priority_queue( InputIt, InputIt, Comp, Alloc )
(5) (since C++17)
    -> priority_queue</*iter-value-type*/<InputIt>,
                      std::vector</*iter-value-type*/<InputIt>, Alloc>,
                      Comp>;

template< class InputIt, class Comp, class Container, class Alloc >
priority_queue( InputIt, InputIt, Comp, Container, Alloc )
(6) (since C++17)
    -> priority_queue<typename Container::value_type, Container, Comp>;

template< ranges::input_range R,
          class Comp = std::less<ranges::range_value_t<R>> >
priority_queue( std::from_range_t, R&&, Comp = Comp() )
(7) (since C++23)
    -> priority_queue<ranges::range_value_t<R>,
                      std::vector<ranges::range_value_t<R>>,
                      Comp>;

template< ranges::input_range R, class Comp, class Alloc >
priority_queue( std::from_range_t, R&&, Comp, Alloc )
(8) (since C++23)
    -> priority_queue<ranges::range_value_t<R>,
                      std::vector<ranges::range_value_t<R>, Alloc>,
                      Comp>;

template< ranges::input_range R, class Alloc >
priority_queue( std::from_range_t, R&&, Alloc )
(9) (since C++23)
    -> priority_queue<ranges::range_value_t<R>,
                      std::vector<ranges::range_value_t<R>, Alloc>>;

These are the deduction guides that std::priority_queue gets, bestie:

• (1-6) — allow deduction from the underlying container type and from an iterator range. For any type It, /*iter-value-type*/<It> denotes typename std::iterator_traits<It>::value_type.
• (7-9) — allow deduction from a std::from_range_t tag and an input_range.

These overloads participate in overload resolution only if:

• InputIt satisfies LegacyInputIterator,
• Comp does not satisfy Allocator,
• Container does not satisfy Allocator,
• for overloads (4,5), (since C++23) Alloc satisfies Allocator, and
• for overloads (3,6), std::uses_allocator_v<Container, Alloc> is true.

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 check, iykyk:

• __cpp_lib_containers_ranges — value 202202L, std (C++23), feature: Ranges-aware construction and insertion; overloads (7-9) ✨

✨ Example ✨

Okay bestie, run this code. Periodt 💅

       // Run this code

        #include <functional>
        #include <iostream>
        #include <queue>
        #include <vector>

        int main()
        {
            const std::vector<int> v = {1, 2, 3, 4};
            std::priority_queue pq1{std::greater<int>{}, v}; // deduces
       std::priority_queue<
                                                             //     int,
       std::vector<int>,
                                                             //
       std::greater<int>>
            for (; !pq1.empty(); pq1.pop())
                std::cout << pq1.top() << ' ';
            std::cout << '\n';

            std::priority_queue pq2{v.begin(), v.end()}; // deduces
       std::priority_queue<int>

            for (; !pq2.empty(); pq2.pop())
                std::cout << pq2.top() << ' ';
            std::cout << '\n';
        }

🎀 Output 🎀

        1 2 3 4
        4 3 2 1

👑 Defect reports 👑

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. Not me reading DRs for fun, but here we are 🙈

• LWG 3506 — applied to C++17; behavior as published: deduction guides from iterator and allocator were missing; correct behavior: added.

That's the whole page, bestie 🫶 — from http://cppreference.com, version/date 2024.06.10, manual page deductiongu...iority_queue(3). Slay 💖✨