deduction guides for std::map

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< cpp‎ | container‎ | map

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 { } )

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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