deductiongu...ordered_set(3)

okay bestie, buckle up because this is deductiongu...ordered_set(3) from the C++ Standard Libary (yes, “Libary” — we respect the typo 💅). It’s all about deduction guides for std::unordered_set, and honestly it’s giving “template girlie knows exactly what type you meant” energy. ✨

🌸 NAME 🌸

deductionguidesforstd::unordered_set — literally the deduction guides for std::unordered_set. That’s the whole name, no cap.

✨ Synopsis ✨

First thing: it’s defined in header <unordered_set> 🫶

template<
    class InputIt,
    class Hash = std::hash<typename std::iterator_traits<InputIt>::value_type>,
    class Pred = std::equal_to<typename std::iterator_traits<InputIt>::value_type>,
    class Alloc = std::allocator<typename std::iterator_traits<InputIt>::value_type> >                      (1)
(since C++17)
   unordered_set( InputIt, InputIt,
                  typename /* see below */::size_type = /* see below */,
                  Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
       -> unordered_set<typename std::iterator_traits<InputIt>::value_type,
                        Hash, Pred, Alloc>;
template< class T,
          class Hash = std::hash<T>,
          class Pred = std::equal_to<T>,
          class Alloc = std::allocator<T> >
   unordered_set( std::initializer_list<T>,                          (2)
(since C++17)
                  typename /* see below */::size_type = /* see below */,
                  Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
       -> unordered_set<T, Hash, Pred, Alloc>;
template< class InputIt, class Alloc >
   unordered_set( InputIt, InputIt, typename /* see below */::size_type, Alloc )
       -> unordered_set<typename std::iterator_traits<InputIt>::value_type,                        (3)
(since C++17)
                        std::hash<typename std::iterator_traits<InputIt>::value_type>,
                        std::equal_to<typename std::iterator_traits<InputIt>::value_type>,
                        Alloc>;
template< class InputIt, class Hash, class Alloc >
   unordered_set( InputIt, InputIt, typename /* see below */::size_type, Hash, Alloc )
       -> unordered_set<typename                                     (4)
(since C++17)
          std::iterator_traits<InputIt>::value_type, Hash,
                        std::equal_to<typename std::iterator_traits<InputIt>::value_type>,
                        Alloc>;
template< class T, class Alloc >
   unordered_set( std::initializer_list<T>, typename /* see below    (5)
(since C++17)
 */::size_type, Alloc )
       -> unordered_set<T, std::hash<T>, std::equal_to<T>, Alloc>;
template< class T, class Hash, class Alloc >
   unordered_set( std::initializer_list<T>, typename /* see below
 */::size_type,                                                    (6)
(since C++17)
                  Hash, Alloc )
       -> unordered_set<T, Hash, std::equal_to<T>, Alloc>;
template< ranges::input_range R,
          class Hash = std::hash<ranges::range_value_t<R>>,
          class Pred = std::equal_to<ranges::range_value_t<R>>,
          class Alloc = std::allocator<ranges::range_value_t<R>>
>
   unordered_set( std::from_range_t, R&&,                            (7)
(since C++23)
                  typename /* see below */::size_type = /* see below */,
                  Hash = Hash(), Pred = Pred(), Alloc = Alloc() )
       -> unordered_set<ranges::range_value_t<R>, Hash, Pred, Alloc>;
template< ranges::input_range R, class Alloc >
   unordered_set( std::from_range_t, R&&,
                  typename /* see below */::size_type, Alloc )
       -> unordered_set<ranges::range_value_t<R>,                    (8)
(since C++23)
          hash<ranges::range_value_t<R>>,
                        std::equal_to<ranges::range_value_t<R>>,
                        Alloc>;
template< ranges::input_range R, class Alloc >
   unordered_set( std::from_range_t, R&&, Alloc )
       -> unordered_set<ranges::range_value_t<R>,                    (9)
(since C++23)
          hash<ranges::range_value_t<R>>,
                        std::equal_to<ranges::range_value_t<R>>,
                        Alloc>;
template< ranges::input_range R, class Hash, class Alloc >
   unordered_set( std::from_range_t, R&&,
                  typename /* see below */::size_type, Hash, Alloc )
(10) (since C++23)
       -> unordered_set<ranges::range_value_t<R>, Hash,
                        std::equal_to<ranges::range_value_t<R>>,
                        Alloc>;

okay so here’s the tea, bestie ☕:

1-6) These deduction guides are provided for unordered_set to allow deduction from an iterator range (overloads (1,3,4)) and std::initializer_list (overloads (2,5,6)). This overload participates in overload resolution only if InputIt satisfies LegacyInputIterator, Alloc satisfies Allocator, neither Hash nor Pred satisfy Allocator, Hash is not an integral type. 🫶

7-10) These deduction guides are provided for unordered_set to allow deduction from a std::from_range_t tag and an input_range.

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.

And a crucial little size_type moment 👑: the size_type parameter type in these guides refers to the size_type member type of the type deduced by the deduction guide. iykyk 💅

🦋 Notes 🦋

Feature-test macro table? It’s giving one-row table realness, so here it is in bullet form because tables are not the vibe in this terminal:

• __cpp_lib_containers_ranges — value 202202L, Std (C++23), feature: Ranges-aware construction and insertion; overloads (7-10).

🎀 Example 🎀

Run this code, bestie:

// Run this code

#include <unordered_set>

int main()
{
    // guide #2 deduces std::unordered_set<int>
    std::unordered_set s = {1, 2, 3, 4};

    // guide #1 deduces std::unordered_set<int>
    std::unordered_set s2(s.begin(), s.end());
}

and that’s the whole template flex 💖

Source/version is http://cppreference.com, dated 2024.06.10, and the footer tag is deductiongu...ordered_set(3). Mwah, bye bestie 💋🌈