deductionguidesforstd::map(3)

NAME
deductionguidesforstd::map - deductionguidesforstd::map

Synopsis
   Defined in header <map>
   template< class InputIt,
   class Comp = std::less<iter_key_t<InputIt>>,
   class Alloc = std::allocator<iter_to_alloc_t<InputIt>> > (1) (since C++17)
   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>> > (2) (since C++17)
   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 ) (3) (since C++17)
   -> 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 ) (4) (since C++17)
   -> 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 >
                        (exposition
   using iter_key_t = std::remove_const_t< only)
   typename std::iterator_traits<InputIt>::value_type::first_type>;
   template< class InputIt > (exposition
   using iter_val_t = typename only)
   std::iterator_traits<InputIt>::value_type::second_type;
   template< class InputIt >
   using iter_to_alloc_t = std::pair<
   std::add_const_t<typename (exposition
   std::iterator_traits<InputIt>::value_type::first_type>, only)
   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
   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.

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
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3025 C++17 initializer-list guides take use std::pair<Key, T>
                       std::pair<const Key, T>