Home - News ( United States | United Kingdom | Italy | Germany ) - Football scores

Showing content from https://en.cppreference.com/w/cpp/algorithm/../../cpp/language/../container/map/extract.html below:

std::map<Key,T,Compare,Allocator>::extract - cppreference.com

node_type extract( const_iterator pos );

node_type extract( const Key& k );

template< class K >
node_type extract( K&& x );

Unlinks the node that contains the element pointed to by

and returns a

that owns it.

If the container has an element with key equivalent to

, unlinks the node that contains that element from the container and returns a

that owns it. Otherwise, returns an empty node handle.

This overload participates in overload resolution only if

is

, and neither

nor

is implicitly convertible from

. It allows calling this function without constructing an instance of

.

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.

A node handle that owns the extracted element, or empty node handle in case the element is not found in (2,3).

1) Throws nothing.

2,3) Any exceptions thrown by the Compare object.

1) Amortized constant.

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"}}

#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);
}

Output:

Start: 1(a) 2(b) 3(c)
After extract and before insert: 2(b) 3(c)
End: 2(b) 3(c) 4(a)

RetroSearch is an open source project built by @garambo | Open a GitHub Issue

Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo

HTML: 3.2 | Encoding: UTF-8 | Version: 0.7.4