deduction guides for std::map

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>;

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>;

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>;

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

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>;

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

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
>

This deduction guide is provided for map 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 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.

Example

#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

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