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, T>> > (2) (since C++17)
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 Allocator >

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

-> map<Key, T, std::less<Key>, Allocator>;

where the type aliases iter_key_t, iter_val_t, iter_to_alloc_t are defined as if as
follows

template< class InputIt >
(exposition
using iter_key_t = std::remove_const_t< only)

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

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

>

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
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.

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>