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

 

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std::unordered_map

Member types
Member functions
unordered_map::unordered_map unordered_map::~unordered_map unordered_map::operator= unordered_map::get_allocator Iterators unordered_map::beginunordered_map::cbegin unordered_map::endunordered_map::cend Capacity unordered_map::size unordered_map::max_size unordered_map::empty Modifiers unordered_map::clear unordered_map::erase unordered_map::swap unordered_map::extract (C++17) unordered_map::merge (C++17) unordered_map::insert unordered_map::insert_range (C++23) unordered_map::insert_or_assign (C++17)   unordered_map::emplace unordered_map::emplace_hint unordered_map::try_emplace (C++17)	Lookup unordered_map::at unordered_map::operator[] unordered_map::count unordered_map::find unordered_map::contains (C++20) unordered_map::equal_range Bucket interface unordered_map::begin(size_type)unordered_map::cbegin(size_type) unordered_map::end(size_type)unordered_map::cend(size_type) unordered_map::bucket_count unordered_map::max_bucket_count unordered_map::bucket_size unordered_map::bucket Hash policy unordered_map::load_factor unordered_map::max_load_factor unordered_map::rehash unordered_map::reserve Observers unordered_map::hash_function unordered_map::key_eq
Non-member functions
operator==operator!= (until C++20)	std::swap(std::unordered_map) erase_if(std::unordered_map) (C++20)
Deduction guides(C++17)

 

void clear() noexcept;		(since C++11) (constexpr since C++26)

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.

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
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

See also

erase

erases elements (public member function)

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