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

From cppreference.com
 
 
Containers library
Sequence
(C++11)
Associative
Unordered associative
Adaptors
Views
(C++20)
 
 
void clear() noexcept;
(since C++11)

Erases all elements from the container. After this call, size() returns zero.

Invalidates any references, pointers, or iterators referring to contained elements. May also invalidate past-the-end iterators.

Contents

[edit] Parameters

(none)

[edit] Return value

(none)

[edit] Complexity

Linear in the size of the container, i.e., the number of elements.

[edit] Example

#include <algorithm>
#include <iostream>
#include <unordered_map>
 
int main()
{
    std::unordered_map<int, char> container{{1, 'x'}, {2, 'y'}, {3, 'z'}};
 
    auto print = [](std::pair<const int, char>& n) { 
        std::cout << " " << n.first << '(' << n.second << ')'; 
    };
 
    std::cout << "Before clear:";
    std::for_each(container.begin(), container.end(), print);
    std::cout << "\nSize=" << container.size() << '\n';
 
    std::cout << "Clear\n";
    container.clear();
 
    std::cout << "After clear:";
    std::for_each(container.begin(), container.end(), print);
    std::cout << "\nSize=" << container.size() << '\n';
}

Possible output:

Before clear: 1(x) 2(y) 3(z)
Size=3
Clear
After clear:
Size=0

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 2550 C++11 for unordered associative containers, unclear if complexity
is linear in the number of elements or buckets
clarified that it's linear in the number of elements

[edit] See also

(C++11)
erases elements
(public member function) [edit]