NAME¶

deductionguidesforstd::map - deductionguidesforstd::map

Synopsis¶

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>,
class Alloc = std::allocator<std::pair<const Key, (2) (since C++17)
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 Alloc >

map( std::initializer_list<std::pair<Key, T>>, Alloc ) (4) (since C++17)

-> map<Key, T, std::less<Key>, Alloc>;
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 = (5) (since C++23)
Alloc() )

-> map<range_key_t<R>, range_mapped_t<R>, Compare,
Alloc>;
template< ranges::input_range R, class Alloc >

map( std::from_range_t, R&&, Alloc ) (6) (since C++23)

-> map<range_key_t<R>, range_mapped_t<R>,
std::less<range_key_t<R>>, Alloc>;
Exposition-only helper type aliases
template< class InputIter >

using iter_val_t = (exposition only*)

typename std::iterator_traits<InputIter>::value_type;
template< class InputIter >

using iter_key_t = (exposition only*)

std::remove_const_t< std::tuple_element_t<0,
iter_val_t<InputIter>>>;
template< class InputIter >

using iter_mapped_t = (exposition only*)

std::tuple_element_t<1, iter_val_t<InputIter>>;
template< class InputIter >

using iter_to_alloc_t =
std::pair<std::add_const_t<tuple_element_t<0, (exposition only*)
iter_val_t<InputIter>>>,

std::tuple_element_t<1,
iter_val_t<InputIter>>>;
template< ranges::input_range Range >

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

typename ranges::range_value_t<Range>::second_type;
template< ranges::input_range Range >

using range_to_alloc_t =
std::pair<std::add_const_t<typename (since C++23)
ranges::range_value_t<Range>::first_type>, (exposition only*)

typename
ranges::range_value_t<Range>::second_type>;

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