std::map extract() method
- since C++23
- since C++17
// (1) Non const version only
node_type extract( const_iterator position );
// (2) Non const version only
node_type extract( const Key& k );
// (3) Non const version only
template< class K >
node_type extract( K&& x );
// (1) Non const version only
node_type extract( const_iterator position );
// (2) Non const version only
node_type extract( const Key& k );
- (1) Unlinks the node that contains the element pointed to by position and returns a node handle that owns it.
- (2) If the container has an element with key equivalent to
k
, unlinks the node that contains that element from the container and returns a node handle that owns it. Otherwise, returns an empty node handle. - (3) Same as (2). This overload participates in overload resolution only if the qualified-id
Compare::is_transparent
is valid and denotes a type, and neitheriterator
norconst_iterator
is implicitly convertible fromK
. It allows calling this function without constructing an instance ofKey
.
In either case, no elements are copied or moved, only the internal pointers of the container nodes are repointed (rebalancing may occur, as with erase()
).
Extracting a node invalidates only the iterators to the extracted element.
Pointers and references to the extracted element remain valid, but cannot be used while element is owned by a node handle: they become usable if the element is inserted into a container.
Parameters
position
- a valid iterator into this containerk
- a key to identify the node to be extractedx
- a value of any type that can be transparently compared with a key identifying the node to be extracted
Return value
A node handle
that owns the extracted element, or empty node handle
in case the element is not found (oveerloads (2) and (3)).
Exceptions
- (1) Throws nothing.
- (2-3) Any exceptions thrown by the
Compare
object.
Complexity
- (1) Amortized constant.
- (2,3) Logarithmic in the size of the container - O(log size()).
Notes
Using extract()
is the only way to change a key of a map element without reallocation:
std::map<int, std::string> m {{1, "mango"}, {2, "papaya"}, {3, "guava"}};
auto nh = m.extract(2);
nh.key() = 4;
m.insert(std::move(nh));
// m == {{1, "mango"}, {3, "guava"}, {4, "papaya"}}
Feature testing macro: __cpp_lib_associative_heterogeneous_erasure
(for overload (3)).
Example
#include <algorithm>
#include <iostream>
#include <string_view>
#include <map>
void print(std::string_view comment, const auto& data)
{
std::cout << comment;
for (auto [k, v] : data)
std::cout << ' ' << k << '(' << v << ')';
std::cout << '\n';
}
int main()
{
std::map<int, char> cont{{1, 'a'}, {2, 'b'}, {3, 'c'}};
print("Start:", cont);
// Extract node handle and change key
auto nh = cont.extract(1);
nh.key() = 4;
print("After extract and before insert:", cont);
// Insert node handle back
cont.insert(std::move(nh));
print("End:", cont);
}
Start: 1(a) 2(b) 3(c)
After extract and before insert: 2(b) 3(c)
End: 2(b) 3(c) 4(a)
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