std::unordered_map - Man Page

using insert_return_type
using node_type

typedef _Hashtable::key_type key_type
Public typedefs.
typedef _Hashtable::value_type value_type
Public typedefs.
typedef _Hashtable::mapped_type mapped_type
Public typedefs.
typedef _Hashtable::hasher hasher
Public typedefs.
typedef _Hashtable::key_equal key_equal
Public typedefs.
typedef _Hashtable::allocator_type allocator_type
Public typedefs.

typedef _Hashtable::pointer pointer
Iterator-related typedefs.
typedef _Hashtable::const_pointer const_pointer
Iterator-related typedefs.
typedef _Hashtable::reference reference
Iterator-related typedefs.
typedef _Hashtable::const_reference const_reference
Iterator-related typedefs.
typedef _Hashtable::iterator iterator
Iterator-related typedefs.
typedef _Hashtable::const_iterator const_iterator
Iterator-related typedefs.
typedef _Hashtable::local_iterator local_iterator
Iterator-related typedefs.
typedef _Hashtable::const_local_iterator const_local_iterator
Iterator-related typedefs.
typedef _Hashtable::size_type size_type
Iterator-related typedefs.
typedef _Hashtable::difference_type difference_type
Iterator-related typedefs.

unordered_map ()=default
Default constructor.
template<typename _InputIterator > unordered_map (_InputIterator __first, _InputIterator __last, size_type __n, const allocator_type &__a)
template<typename _InputIterator > unordered_map (_InputIterator __first, _InputIterator __last, size_type __n, const hasher &__hf, const allocator_type &__a)
template<typename _InputIterator > unordered_map (_InputIterator __first, _InputIterator __last, size_type __n=0, const hasher &__hf=hasher(), const key_equal &__eql=key_equal(), const allocator_type &__a=allocator_type())
Builds an unordered_map from a range.
unordered_map (const allocator_type &__a)
Creates an unordered_map with no elements.
unordered_map (const unordered_map &)=default
Copy constructor.
unordered_map (const unordered_map &__umap, const allocator_type &__a)
unordered_map (initializer_list< value_type > __l, size_type __n, const allocator_type &__a)
unordered_map (initializer_list< value_type > __l, size_type __n, const hasher &__hf, const allocator_type &__a)
unordered_map (initializer_list< value_type > __l, size_type __n=0, const hasher &__hf=hasher(), const key_equal &__eql=key_equal(), const allocator_type &__a=allocator_type())
Builds an unordered_map from an initializer_list.
unordered_map (size_type __n, const allocator_type &__a)
unordered_map (size_type __n, const hasher &__hf, const allocator_type &__a)
unordered_map (size_type __n, const hasher &__hf=hasher(), const key_equal &__eql=key_equal(), const allocator_type &__a=allocator_type())
Default constructor creates no elements.
unordered_map (unordered_map &&)=default
Move constructor.
unordered_map (unordered_map &&__umap, const allocator_type &__a) noexcept(noexcept(_Hashtable(std::move(__umap._M_h), __a)))
iterator begin () noexcept
local_iterator begin (size_type __n)
Returns a read/write iterator pointing to the first bucket element.
size_type bucket (const key_type &__key) const
size_type bucket_count () const noexcept
Returns the number of buckets of the unordered_map.
size_type bucket_size (size_type __n) const
void clear () noexcept
template<typename... _Args> std::pair< iterator, bool > emplace (_Args &&... __args)
Attempts to build and insert a std::pair into the unordered_map.
template<typename... _Args> iterator emplace_hint (const_iterator __pos, _Args &&... __args)
Attempts to build and insert a std::pair into the unordered_map.
bool empty () const noexcept
Returns true if the unordered_map is empty.
iterator end () noexcept
local_iterator end (size_type __n)
Returns a read/write iterator pointing to one past the last bucket elements.
size_type erase (const key_type &__x)
Erases elements according to the provided key.
iterator erase (const_iterator __first, const_iterator __last)
Erases a [__first,__last) range of elements from an unordered_map.
node_type extract (const key_type &__key)
Extract a node.
node_type extract (const_iterator __pos)
Extract a node.
allocator_type get_allocator () const noexcept
Returns the allocator object used by the unordered_map.
hasher hash_function () const
Returns the hash functor object with which the unordered_map was constructed.
template<typename _InputIterator > void insert (_InputIterator __first, _InputIterator __last)
A template function that attempts to insert a range of elements.
iterator insert (const_iterator, node_type &&__nh)
Re-insert an extracted node.
void insert (initializer_list< value_type > __l)
Attempts to insert a list of elements into the unordered_map.
insert_return_type insert (node_type &&__nh)
Re-insert an extracted node.
template<typename _Obj > pair< iterator, bool > insert_or_assign (const key_type &__k, _Obj &&__obj)
Attempts to insert a std::pair into the unordered_map.
template<typename _Obj > iterator insert_or_assign (const_iterator __hint, const key_type &__k, _Obj &&__obj)
Attempts to insert a std::pair into the unordered_map.
template<typename _Obj > iterator insert_or_assign (const_iterator __hint, key_type &&__k, _Obj &&__obj)
template<typename _Obj > pair< iterator, bool > insert_or_assign (key_type &&__k, _Obj &&__obj)
key_equal key_eq () const
Returns the key comparison object with which the unordered_map was constructed.
float load_factor () const noexcept
Returns the average number of elements per bucket.
size_type max_bucket_count () const noexcept
Returns the maximum number of buckets of the unordered_map.
float max_load_factor () const noexcept
Returns a positive number that the unordered_map tries to keep the load factor less than or equal to.
void max_load_factor (float __z)
Change the unordered_map maximum load factor.
size_type max_size () const noexcept
Returns the maximum size of the unordered_map.
template<typename _H2 , typename _P2 > void merge (unordered_map< _Key, _Tp, _H2, _P2, _Alloc > &&__source)
template<typename _H2 , typename _P2 > void merge (unordered_map< _Key, _Tp, _H2, _P2, _Alloc > &__source)
template<typename _H2 , typename _P2 > void merge (unordered_multimap< _Key, _Tp, _H2, _P2, _Alloc > &&__source)
template<typename _H2 , typename _P2 > void merge (unordered_multimap< _Key, _Tp, _H2, _P2, _Alloc > &__source)
unordered_map & operator= (const unordered_map &)=default
Copy assignment operator.
unordered_map & operator= (initializer_list< value_type > __l)
Unordered_map list assignment operator.
unordered_map & operator= (unordered_map &&)=default
Move assignment operator.
void rehash (size_type __n)
May rehash the unordered_map.
void reserve (size_type __n)
Prepare the unordered_map for a specified number of elements.
size_type size () const noexcept
Returns the size of the unordered_map.
void swap (unordered_map &__x) noexcept(noexcept(_M_h.swap(__x._M_h)))
Swaps data with another unordered_map.
template<typename... _Args> pair< iterator, bool > try_emplace (const key_type &__k, _Args &&... __args)
Attempts to build and insert a std::pair into the unordered_map.
template<typename... _Args> iterator try_emplace (const_iterator __hint, const key_type &__k, _Args &&... __args)
Attempts to build and insert a std::pair into the unordered_map.
template<typename... _Args> iterator try_emplace (const_iterator __hint, key_type &&__k, _Args &&... __args)
template<typename... _Args> pair< iterator, bool > try_emplace (key_type &&__k, _Args &&... __args)

const_iterator begin () const noexcept
const_iterator cbegin () const noexcept

const_iterator end () const noexcept
const_iterator cend () const noexcept

std::pair< iterator, bool > insert (const value_type &__x)
Attempts to insert a std::pair into the unordered_map.
std::pair< iterator, bool > insert (value_type &&__x)
Attempts to insert a std::pair into the unordered_map.
template<typename _Pair > __enable_if_t< is_constructible< value_type, _Pair && >::value, pair< iterator, bool > > insert (_Pair &&__x)
Attempts to insert a std::pair into the unordered_map.

iterator insert (const_iterator __hint, const value_type &__x)
Attempts to insert a std::pair into the unordered_map.
iterator insert (const_iterator __hint, value_type &&__x)
Attempts to insert a std::pair into the unordered_map.
template<typename _Pair > __enable_if_t< is_constructible< value_type, _Pair && >::value, iterator > insert (const_iterator __hint, _Pair &&__x)
Attempts to insert a std::pair into the unordered_map.

iterator erase (const_iterator __position)
Erases an element from an unordered_map.
iterator erase (iterator __position)
Erases an element from an unordered_map.

iterator find (const key_type &__x)
Tries to locate an element in an unordered_map.
template<typename _Kt > auto find (const _Kt &__x) -> decltype(_M_h._M_find_tr(__x))
Tries to locate an element in an unordered_map.
const_iterator find (const key_type &__x) const
Tries to locate an element in an unordered_map.
template<typename _Kt > auto find (const _Kt &__x) const -> decltype(_M_h._M_find_tr(__x))
Tries to locate an element in an unordered_map.

size_type count (const key_type &__x) const
Finds the number of elements.
template<typename _Kt > auto count (const _Kt &__x) const -> decltype(_M_h._M_count_tr(__x))
Finds the number of elements.

bool contains (const key_type &__x) const
Finds whether an element with the given key exists.
template<typename _Kt > auto contains (const _Kt &__x) const -> decltype(_M_h._M_find_tr(__x), void(), true)
Finds whether an element with the given key exists.

std::pair< iterator, iterator > equal_range (const key_type &__x)
Finds a subsequence matching given key.
template<typename _Kt > auto equal_range (const _Kt &__x) -> decltype(_M_h._M_equal_range_tr(__x))
Finds a subsequence matching given key.
std::pair< const_iterator, const_iterator > equal_range (const key_type &__x) const
Finds a subsequence matching given key.
template<typename _Kt > auto equal_range (const _Kt &__x) const -> decltype(_M_h._M_equal_range_tr(__x))
Finds a subsequence matching given key.

mapped_type & operator[] (const key_type &__k)
Subscript ( [] ) access to unordered_map data.
mapped_type & operator[] (key_type &&__k)
Subscript ( [] ) access to unordered_map data.

mapped_type & at (const key_type &__k)
Access to unordered_map data.
const mapped_type & at (const key_type &__k) const
Access to unordered_map data.

const_local_iterator begin (size_type __n) const
Returns a read-only (constant) iterator pointing to the first bucket element.
const_local_iterator cbegin (size_type __n) const
Returns a read-only (constant) iterator pointing to the first bucket element.

const_local_iterator end (size_type __n) const
Returns a read-only (constant) iterator pointing to one past the last bucket elements.
const_local_iterator cend (size_type __n) const
Returns a read-only (constant) iterator pointing to one past the last bucket elements.

template<typename _Key1 , typename _Tp1 , typename _Hash1 , typename _Pred1 , typename _Alloc1 > bool operator== (const unordered_map< _Key1, _Tp1, _Hash1, _Pred1, _Alloc1 > &, const unordered_map< _Key1, _Tp1, _Hash1, _Pred1, _Alloc1 > &)
template<typename , typename , typename > class std::_Hash_merge_helper

class std::unordered_map< _Key, _Tp, _Hash, _Pred, _Alloc >"A standard container composed of unique keys (containing at most one of each key value) that associates values of another type with the keys.

Since

C++11

Template Parameters

Meets the requirements of a container, and unordered associative container

The resulting value type of the container is std::pair<const _Key, _Tp>.

Base is _Hashtable, dispatched at compile time via template alias __umap_hashtable.

Public typedefs.

Iterator-related typedefs.

Iterator-related typedefs.

Iterator-related typedefs.

Iterator-related typedefs.

Iterator-related typedefs.

Public typedefs.

Iterator-related typedefs.

Public typedefs.

Public typedefs.

Iterator-related typedefs.

Public typedefs.

Iterator-related typedefs.

Iterator-related typedefs.

Iterator-related typedefs.

Public typedefs.

Default constructor.

Default constructor creates no elements.

Parameters

Builds an unordered_map from a range.

Parameters

Create an unordered_map consisting of copies of the elements from [__first,__last). This is linear in N (where N is distance(__first,__last)).

Copy constructor.

Move constructor.

Creates an unordered_map with no elements.

Parameters

Builds an unordered_map from an initializer_list.

Parameters

Create an unordered_map consisting of copies of the elements in the list. This is linear in N (where N is __l.size()).

Access to unordered_map data.

Parameters

Returns

A reference to the data whose key is equal to __k, if such a data is present in the unordered_map.

Exceptions

std::out_of_range If no such data is present.

Access to unordered_map data.

Parameters

Returns

A reference to the data whose key is equal to __k, if such a data is present in the unordered_map.

Exceptions

std::out_of_range If no such data is present.

Returns a read-only (constant) iterator that points to the first element in the unordered_map.

Returns a read/write iterator that points to the first element in the unordered_map.

Returns a read/write iterator pointing to the first bucket element.

Parameters

Returns

A read/write local iterator.

Returns a read-only (constant) iterator pointing to the first bucket element.

Parameters

Returns

A read-only local iterator.

Returns the number of buckets of the unordered_map.

Returns a read-only (constant) iterator that points to the first element in the unordered_map.

Returns a read-only (constant) iterator pointing to the first bucket element.

Parameters

Returns

A read-only local iterator.

Returns a read-only (constant) iterator that points one past the last element in the unordered_map.

Referenced by std::unordered_map< _Key, _Tp, _Hash, _Pred, _Alloc >::insert_or_assign(), and std::unordered_map< _Key, _Tp, _Hash, _Pred, _Alloc >::try_emplace().

Returns a read-only (constant) iterator pointing to one past the last bucket elements.

Parameters

Returns

A read-only local iterator.

Erases all elements in an unordered_map. Note that this function only erases the elements, and that if the elements themselves are pointers, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.

[inline]"

Finds whether an element with the given key exists.

Parameters

Returns

True if there is any element with the specified key.

Finds whether an element with the given key exists.

Parameters

Returns

True if there is any element with the specified key.

[inline]"

Finds the number of elements.

Parameters

Returns

Number of elements with specified key.

This function only makes sense for unordered_multimap; for unordered_map the result will either be 0 (not present) or 1 (present).

Finds the number of elements.

Parameters

Returns

Number of elements with specified key.

This function only makes sense for unordered_multimap; for unordered_map the result will either be 0 (not present) or 1 (present).

Attempts to build and insert a std::pair into the unordered_map.

Parameters

Returns

A pair, of which the first element is an iterator that points to the possibly inserted pair, and the second is a bool that is true if the pair was actually inserted.

This function attempts to build and insert a (key, value) pair into the unordered_map. An unordered_map relies on unique keys and thus a pair is only inserted if its first element (the key) is not already present in the unordered_map.

Insertion requires amortized constant time.

References std::forward().

Attempts to build and insert a std::pair into the unordered_map.

Parameters

Returns

An iterator that points to the element with key of the std::pair built from __args (may or may not be that std::pair).

This function is not concerned about whether the insertion took place, and thus does not return a boolean like the single-argument emplace() does. Note that the first parameter is only a hint and can potentially improve the performance of the insertion process. A bad hint would cause no gains in efficiency.

See https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints for more on hinting.

Insertion requires amortized constant time.

References std::forward().

Returns true if the unordered_map is empty.

Returns a read-only (constant) iterator that points one past the last element in the unordered_map.

Returns a read/write iterator that points one past the last element in the unordered_map.

Referenced by std::unordered_map< _Key, _Tp, _Hash, _Pred, _Alloc >::extract().

Returns a read/write iterator pointing to one past the last bucket elements.

Parameters

Returns

A read/write local iterator.

Returns a read-only (constant) iterator pointing to one past the last bucket elements.

Parameters

Returns

A read-only local iterator.

[inline]"

Finds a subsequence matching given key.

Parameters

Returns

Pair of iterators that possibly points to the subsequence matching given key.

This function probably only makes sense for unordered_multimap.

[inline]"

Finds a subsequence matching given key.

Parameters

Returns

Pair of iterators that possibly points to the subsequence matching given key.

This function probably only makes sense for unordered_multimap.

Finds a subsequence matching given key.

Parameters

Returns

Pair of iterators that possibly points to the subsequence matching given key.

This function probably only makes sense for unordered_multimap.

Finds a subsequence matching given key.

Parameters

Returns

Pair of iterators that possibly points to the subsequence matching given key.

This function probably only makes sense for unordered_multimap.

Erases elements according to the provided key.

Parameters

Returns

The number of elements erased.

This function erases all the elements located by the given key from an unordered_map. For an unordered_map the result of this function can only be 0 (not present) or 1 (present). Note that this function only erases the element, and that if the element is itself a pointer, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.

Erases a [__first,__last) range of elements from an unordered_map.

Parameters

Returns

The iterator __last.

This function erases a sequence of elements from an unordered_map. Note that this function only erases the elements, and that if the element is itself a pointer, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.

Erases an element from an unordered_map.

Parameters

Returns

An iterator pointing to the element immediately following __position prior to the element being erased. If no such element exists, end() is returned.

This function erases an element, pointed to by the given iterator, from an unordered_map. Note that this function only erases the element, and that if the element is itself a pointer, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.

Erases an element from an unordered_map.

Parameters

Returns

An iterator pointing to the element immediately following __position prior to the element being erased. If no such element exists, end() is returned.

This function erases an element, pointed to by the given iterator, from an unordered_map. Note that this function only erases the element, and that if the element is itself a pointer, the pointed-to memory is not touched in any way. Managing the pointer is the user's responsibility.

Extract a node.

Extract a node.

References std::unordered_map< _Key, _Tp, _Hash, _Pred, _Alloc >::end().

[inline]"

Tries to locate an element in an unordered_map.

Parameters

Returns

Iterator pointing to sought-after element, or end() if not found.

This function takes a key and tries to locate the element with which the key matches. If successful the function returns an iterator pointing to the sought after element. If unsuccessful it returns the past-the-end ( end() ) iterator.

[inline]"

Tries to locate an element in an unordered_map.

Parameters

Returns

Iterator pointing to sought-after element, or end() if not found.

This function takes a key and tries to locate the element with which the key matches. If successful the function returns an iterator pointing to the sought after element. If unsuccessful it returns the past-the-end ( end() ) iterator.

Tries to locate an element in an unordered_map.

Parameters

Returns

Iterator pointing to sought-after element, or end() if not found.

This function takes a key and tries to locate the element with which the key matches. If successful the function returns an iterator pointing to the sought after element. If unsuccessful it returns the past-the-end ( end() ) iterator.

Tries to locate an element in an unordered_map.

Parameters

Returns

Iterator pointing to sought-after element, or end() if not found.

This function takes a key and tries to locate the element with which the key matches. If successful the function returns an iterator pointing to the sought after element. If unsuccessful it returns the past-the-end ( end() ) iterator.

Returns the allocator object used by the unordered_map.

Returns the hash functor object with which the unordered_map was constructed.

A template function that attempts to insert a range of elements.

Parameters

Complexity similar to that of the range constructor.

Attempts to insert a std::pair into the unordered_map.

Parameters

Returns

A pair, of which the first element is an iterator that points to the possibly inserted pair, and the second is a bool that is true if the pair was actually inserted.

This function attempts to insert a (key, value) pair into the unordered_map. An unordered_map relies on unique keys and thus a pair is only inserted if its first element (the key) is not already present in the unordered_map.

Insertion requires amortized constant time.

References std::forward().

Attempts to insert a std::pair into the unordered_map.

Parameters

Returns

A pair, of which the first element is an iterator that points to the possibly inserted pair, and the second is a bool that is true if the pair was actually inserted.

This function attempts to insert a (key, value) pair into the unordered_map. An unordered_map relies on unique keys and thus a pair is only inserted if its first element (the key) is not already present in the unordered_map.

Insertion requires amortized constant time.

Attempts to insert a std::pair into the unordered_map.

Parameters

Returns

An iterator that points to the element with key of __x (may or may not be the pair passed in).

This function is not concerned about whether the insertion took place, and thus does not return a boolean like the single-argument insert() does. Note that the first parameter is only a hint and can potentially improve the performance of the insertion process. A bad hint would cause no gains in efficiency.

See https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints for more on hinting.

Insertion requires amortized constant time.

References std::forward().

Attempts to insert a std::pair into the unordered_map.

Parameters

Returns

An iterator that points to the element with key of __x (may or may not be the pair passed in).

This function is not concerned about whether the insertion took place, and thus does not return a boolean like the single-argument insert() does. Note that the first parameter is only a hint and can potentially improve the performance of the insertion process. A bad hint would cause no gains in efficiency.

See https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints for more on hinting.

Insertion requires amortized constant time.

Attempts to insert a std::pair into the unordered_map.

Parameters

Returns

An iterator that points to the element with key of __x (may or may not be the pair passed in).

This function is not concerned about whether the insertion took place, and thus does not return a boolean like the single-argument insert() does. Note that the first parameter is only a hint and can potentially improve the performance of the insertion process. A bad hint would cause no gains in efficiency.

See https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints for more on hinting.

Insertion requires amortized constant time.

References std::move().

Re-insert an extracted node.

References std::move().

Attempts to insert a list of elements into the unordered_map.

Parameters

Complexity similar to that of the range constructor.

Re-insert an extracted node.

References std::move().

Attempts to insert a std::pair into the unordered_map.

Parameters

Returns

A pair, of which the first element is an iterator that points to the possibly inserted pair, and the second is a bool that is true if the pair was actually inserted.

This function attempts to insert a (key, value) pair into the unordered_map. An unordered_map relies on unique keys and thus a pair is only inserted if its first element (the key) is not already present in the unordered_map.

Insertion requires amortized constant time.

References std::move().

Attempts to insert a std::pair into the unordered_map.

Parameters

Returns

A pair, of which the first element is an iterator that points to the possibly inserted pair, and the second is a bool that is true if the pair was actually inserted.

This function attempts to insert a (key, value) pair into the unordered_map. An unordered_map relies on unique keys and thus a pair is only inserted if its first element (the key) is not already present in the unordered_map. If the pair was already in the unordered_map, the .second of the pair is assigned from __obj.

Insertion requires amortized constant time.

References std::unordered_map< _Key, _Tp, _Hash, _Pred, _Alloc >::cend(), and std::forward().

Attempts to insert a std::pair into the unordered_map.

Parameters

Returns

An iterator that points to the element with key of __x (may or may not be the pair passed in).

This function is not concerned about whether the insertion took place, and thus does not return a boolean like the single-argument insert() does.
If the pair was already in the unordered map, the .second of the pair is assigned from __obj. Note that the first parameter is only a hint and can potentially improve the performance of the insertion process. A bad hint would cause no gains in efficiency.

See https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints for more on hinting.

Insertion requires amortized constant time.

References std::forward().

Returns the key comparison object with which the unordered_map was constructed.

Returns the average number of elements per bucket.

Returns the maximum number of buckets of the unordered_map.

Returns a positive number that the unordered_map tries to keep the load factor less than or equal to.

Change the unordered_map maximum load factor.

Parameters

Returns the maximum size of the unordered_map.

Copy assignment operator.

Unordered_map list assignment operator.

Parameters

This function fills an unordered_map with copies of the elements in the initializer list __l.

Note that the assignment completely changes the unordered_map and that the resulting unordered_map's size is the same as the number of elements assigned.

Move assignment operator.

Subscript ( [] ) access to unordered_map data.

Parameters

Returns

A reference to the data of the (key,data) pair.

Allows for easy lookup with the subscript ( [] )operator. Returns data associated with the key specified in subscript. If the key does not exist, a pair with that key is created using default values, which is then returned.

Lookup requires constant time.

Subscript ( [] ) access to unordered_map data.

Parameters

Returns

A reference to the data of the (key,data) pair.

Allows for easy lookup with the subscript ( [] )operator. Returns data associated with the key specified in subscript. If the key does not exist, a pair with that key is created using default values, which is then returned.

Lookup requires constant time.

References std::move().

May rehash the unordered_map.

Parameters

Rehash will occur only if the new number of buckets respect the unordered_map maximum load factor.

Prepare the unordered_map for a specified number of elements.

Parameters

Same as rehash(ceil(n / max_load_factor())).

Returns the size of the unordered_map.

Swaps data with another unordered_map.

Parameters

This exchanges the elements between two unordered_map in constant time. Note that the global std::swap() function is specialized such that std::swap(m1,m2) will feed to this function.

Attempts to build and insert a std::pair into the unordered_map.

Parameters

Returns

A pair, of which the first element is an iterator that points to the possibly inserted pair, and the second is a bool that is true if the pair was actually inserted.

This function attempts to build and insert a (key, value) pair into the unordered_map. An unordered_map relies on unique keys and thus a pair is only inserted if its first element (the key) is not already present in the unordered_map. If a pair is not inserted, this function has no effect.

Insertion requires amortized constant time.

References std::unordered_map< _Key, _Tp, _Hash, _Pred, _Alloc >::cend(), and std::forward().

Attempts to build and insert a std::pair into the unordered_map.

Parameters

Returns

An iterator that points to the element with key of the std::pair built from __args (may or may not be that std::pair).

This function is not concerned about whether the insertion took place, and thus does not return a boolean like the single-argument emplace() does. However, if insertion did not take place, this function has no effect. Note that the first parameter is only a hint and can potentially improve the performance of the insertion process. A bad hint would cause no gains in efficiency.

See https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints for more on hinting.

Insertion requires amortized constant time.

References std::forward().