deduction guides for std::map

 

C++

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Technical Specifications

 

Containers library

array (C++11)

vector

deque

forward_list (C++11)

list

set

multiset

map

multimap

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

stack

queue

priority_queue

span (C++20)

 

std::map

Member functions

map::map

map::~map

map::operator=

map::get_allocator

Element access

map::at

map::operator[]

Iterators

map::beginmap::cbegin (C++11)

map::endmap::cend (C++11)

map::rbeginmap::crbegin (C++11)

map::rendmap::crend (C++11)

Capacity

map::empty

map::size

map::max_size

Modifiers

map::clear

map::insert

map::insert_or_assign (C++17)

map::emplace (C++11)

map::emplace_hint (C++11)

map::try_emplace (C++17)

map::erase

map::swap

map::extract (C++17)

map::merge (C++17)

Lookup

map::count

map::find

map::contains (C++20)

map::equal_range

map::lower_bound

map::upper_bound

Observers

map::key_comp

map::value_comp

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 Allocator> map(std::initializer_list<std::pair<Key, T>>, Allocator)   -> map<Key, T, std::less<Key>, Allocator>;	(4)	(since C++17)

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<                      typename std::iterator_traits<InputIt>::value_type::first_type>;		(exposition only)
template<class InputIt> using iter_val_t = typename std::iterator_traits<InputIt>::value_type::second_type;		(exposition only)
template<class InputIt> using iter_to_alloc_t = std::pair<   std::add_const_t<typename std::iterator_traits<InputIt>::value_type::first_type>,   typename std::iterator_traits<InputIt>::value_type::second_type >		(exposition only)

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.

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>