deduction guides for std::map

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C++
Containers library
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 > , std:: less < range_key_t < R >> , Alloc > ;
(6) (since C++23)
Exposition-only helper type aliases
template < class InputIter >

using iter_val_t =

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

using iter_key_t =

std:: remove_const_t < std:: tuple_element_t < 0 , iter_val_t < InputIter >>> ;
( exposition only* )
template < class InputIter >

using iter_mapped_t =

std:: tuple_element_t < 1 , iter_val_t < InputIter >> ;
( exposition only* )
template < class InputIter >

using iter_to_alloc_t =
std:: pair < std:: add_const_t < tuple_element_t < 0 , iter_val_t < InputIter >>> ,

std:: tuple_element_t < 1 , iter_val_t < InputIter >>> ;
( 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.

Notes

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

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