deduction guides for std::map

< cpp‎ | container‎ | map

Defined in header <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>;
(1) (since C++17)
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>;
(2) (since C++17)
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>;
(3) (since C++17)
template<class Key, class T, class Allocator>

map(std::initializer_list<std::pair<Key, T>>, Allocator)

  -> map<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 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.


#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 std::pair<const Key, T> use std::pair<Key, T>