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deduction guides for std::multimap

Defined in header <map> 
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>;
 (1) (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<const Key, T>>, Comp = Comp(), Alloc = Alloc())
  -> multimap<Key, T, Comp, Alloc>;
 (2) (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>;
 (3) (since C++17) 
template<class Key, class T, class Allocator>
multimap(std::initializer_list<std::pair<const Key, T>>, Allocator)
  -> multimap<Key, T, std::less<Key>, Allocator>;
 (4) (since C++17)

where the type aliases iter_key_t, iter_val_t, iter_to_alloc_t are defined as if as follows.

template<class InputIt>
using iter_key_t = std::remove_const_t<
                     typename std::iterator_traits<InputIt>::value_type::first_type>;
 (exposition only) 
template<class InputIt>
using iter_val_t = typename std::iterator_traits<InputIt>::value_type::second_type;
 (exposition only) 
template<class InputIt>
using iter_to_alloc_t = std::pair<
  std::add_const_t<typename std::iterator_traits<InputIt>::value_type::first_type>,
  typename std::iterator_traits<InputIt>::value_type::second_type
>
 (exposition only)

This deduction guide is provided for multimap to allow deduction from an iterator range (overloads (1,3)) and std::initializer_list (overloads (2,4)). These overloads only participate in overload resolution if InputIt satisfies InputIterator, Alloc satisfies Allocator, and Comp does not satisfy Allocator.

Note: the extent to which the library determines that a type does not satisfy InputIterator 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.

Example

#include <map>
int main() {
// std::multimap m1 = {{"foo", 1}, {"bar", 2}}; // Error: braced-init-list has no type;
                                                // cannot deduce pair<const Key, T> from
                                                // {"foo", 1} or {"bar", 2}
 
   std::multimap m1 = std::initializer_list<
                        std::pair<char const* const, int>>({{"foo", 2}, {"bar", 3}}); // guide #2
   std::multimap m2(m1.begin(), m1.end()); // guide #1
}

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