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, and 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 <iostream>
#include <string_view>
#include <unordered_map>

void print_info(std::string_view rem, const std::unordered_map<int, char>& v)
{
std::cout << rem << "{ ";
for (const auto& [key, value] : v)
std::cout << '[' << key << "]:" << value << ' ';
std::cout << "}\n";
std::cout << "Size=" << v.size() << '\n';
}

int main()
{
std::unordered_map<int, char> container{{1, 'x'}, {2, 'y'}, {3, 'z'}};
print_info("Before clear: ", container);
container.clear();
print_info("After clear: ", container);
}

Possible output:¶

Before clear: { [1]:x [2]:y [3]:z }
Size=3
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

See also¶

erase erases elements
(public member function)