Namespaces
Variants
Actions

deduction guides for std::map

From cppreference.com
< cpp‎ | container‎ | map
 
 
 
std::map
Member functions
             
Non-member functions
(until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
Deduction guides (C++17)
 
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 Alloc >

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

    -> map<Key, T, std::less <Key>, Alloc>;
(4) (since C++17)
template< ranges::input_range R, class Compare = std::less </*range-key-t*/<R>,

          class Alloc = std::allocator </*range-to-alloc-t*/<R>> >
map( std::from_range_t, R&&, Compare = Compare(), Alloc = Alloc() )

    -> map</*range-key-t*/<R>, range_mapped_t<R>, Compare, Alloc>;
(5) (since C++23)
template< ranges::input_range R, class Alloc >

map( std::from_range_t, R&&, Alloc )
    -> map</*range-key-t*/<R>, range_mapped_t<R>,

       map std::less </*range-key-t*/<R>>, Alloc>;
(6) (since C++23)
Exposition-only helper type aliases
template< class InputIt >

using /*iter-val-t*/ =

    typename std::iterator_traits <InputIt>::value_type;
(exposition only*)
template< class InputIt >

using /*iter-key-t*/ =

    std::remove_const_t < std::tuple_element_t <0, /*iter-val-t*/<InputIt>>>;
(exposition only*)
template< class InputIt >

using /*iter-mapped-t*/ =

    std::tuple_element_t <1, /*iter-val-t*/<InputIt>>;
(exposition only*)
template< class InputIt >

using /*iter-to-alloc-t*/ =
    std::pair <std::add_const_t <tuple_element_t<0, /*iter-val-t*/<InputIt>>>,

              std::tuple_element_t <1, /*iter-val-t*/<InputIt>>>;
(exposition only*)
template< ranges::input_range Range >

using /*range-key-t*/ =

    std::remove_const_t <typename ranges::range_value_t <Range>::first_type>;
(since C++23)
(exposition only*)
template< ranges::input_range Range >

using /*range-mapped-t*/ =

    typename ranges::range_value_t <Range>::second_type;
(since C++23)
(exposition only*)
template< ranges::input_range Range >

using /*range-to-alloc-t*/ =
    std::pair <std::add_const_t <typename ranges::range_value_t <Range>::first_type>,

              typename ranges::range_value_t <Range>::second_type>;
(since C++23)
(exposition only*)
1-4) These deduction guides are provided for map to allow deduction from an iterator range (overloads (1,3)) and std::initializer_list (overloads (2,4)).
5,6) These deduction guides are provided for map to allow deduction from a std::from_range_t tag and an input_range.

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.

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion; overload (5,6)

[edit] 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
}

[edit] 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 (2,4) take std::pair <const Key, T> use std::pair <Key, T>

AltStyle によって変換されたページ (->オリジナル) /