# std::map<Key,T,Compare,Allocator>::clear

From cppreference.com
< cpp‎ | container‎ | map

void clear(); |  | (until C++11)
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. Any past-the-end iterator remains valid.

### 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 <map>

    int main()
    {
        std::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';
    }

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 224 | C++98 | the complexity was log(size()) + N, but N was not defined | corrected to 'linear in size()'

### See also

erase | erases elements (public member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/map/clear&oldid=135930"