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std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::erase

From cppreference.com
 
 
Containers library
Sequence
(C++11)
Associative
Unordered associative
Adaptors
Views
(C++20)
 
 
(1)
iterator erase( iterator pos );
(since C++11)
iterator erase( const_iterator pos );
(since C++11)
(2)
iterator erase( const_iterator first, const_iterator last );
(since C++11)
size_type erase( const key_type& key );
(3) (since C++11)
template< class K >
size_type erase( K&& x );
(4) (since C++23)

Removes specified elements from the container.

1) Removes the element at pos.
2) Removes the elements in the range [first; last), which must be a valid range in *this.
3) Removes the element (if one exists) with the key equivalent to key.
4) Removes the element (if one exists) with key that compares equivalent to the value x. This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type, and neither iterator nor const_iterator is implicitly convertible from K. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.

References and iterators to the erased elements are invalidated. Other iterators and references are not invalidated.

The iterator pos must be valid and dereferenceable. Thus the end() iterator (which is valid, but is not dereferenceable) cannot be used as a value for pos.

The order of the elements that are not erased is preserved. (This makes it possible to erase individual elements while iterating through the container.)

Contents

[edit] Parameters

pos - iterator to the element to remove
first, last - range of elements to remove
key - key value of the elements to remove
x - a value of any type that can be transparently compared with a key denoting the elements to remove

[edit] Return value

1-2) Iterator following the last removed element.
3,4) Number of elements removed (0 or 1).

[edit] Exceptions

1,2) Throws nothing.
3,4) Any exceptions thrown by the Hash and KeyEqual object.

[edit] Complexity

Given an instance c of unordered_map:

1) Average case: constant, worst case: c.size()
2) Average case: std::distance(first, last), worst case: c.size()
3) Average case: c.count(key), worst case: c.size()
4) Average case: c.count(x), worst case: c.size()

[edit] Example

#include <unordered_map>
#include <iostream>
int main()
{
    std::unordered_map<int, std::string> c = {
        {1, "one" }, {2, "two" }, {3, "three"},
        {4, "four"}, {5, "five"}, {6, "six"  }
    };
 
    // erase all odd numbers from c
    for(auto it = c.begin(); it != c.end(); ) {
        if(it->first % 2 != 0)
            it = c.erase(it);
        else
            ++it;
    }
 
    for(auto& p : c) {
        std::cout << p.second << ' ';
    }
}

Possible output:

two four six

[edit] 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 2059 C++11 overload for one const_iterator introduced new ambiguity overload for iterator added
LWG 2356 C++11 the order of element that are not erased was unspecified required to be preserved

[edit] See also

(C++11)
clears the contents
(public member function) [edit]