NAME

std::unordered_map::clear - std::unordered_map::clear

Synopsis

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.

Parameters

(none)

Return value

(none)

Complexity

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

Example

// Run this code

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