deductionguidesforstd::map (3) - Linux Manuals

deductionguidesforstd::map: deductionguidesforstd::map


deductionguidesforstd::map - deductionguidesforstd::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>>> (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>, (2) (since C++17)
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) (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>
using iter_key_t = std::remove_const_t< (exposition only)
typename std::iterator_traits<InputIt>::value_type::first_type>;
template<class InputIt> (exposition only)
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>, (exposition 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 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.


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