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Showing content from https://cplusplus.com/reference/unordered_map/unordered_multimap/clear/ below:

public member function

<unordered_map>

std::unordered_multimap::clear

Clear content

All the elements in the unordered_multimap container are dropped: their destructors are called, and they are removed from the container, leaving it with a size of 0.

Parameters none

Return value none

Example 
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// clearing unordered_multimap
#include <iostream>
#include <string>
#include <unordered_map>

int main ()
{
  std::unordered_multimap<std::string,std::string> myumm =
         { {"Tom","Produce"}, {"Bob","Toys"}, {"Bob","Garden"} };

  std::cout << "myumm contains:";
  for (auto& x: myumm) std::cout << " " << x.first << ":" << x.second;
  std::cout << std::endl;

  myumm.clear();
  myumm.insert(std::make_pair("Bob","Jail"));

  std::cout << "myumm contains:";
  for (auto& x: myumm) std::cout << " " << x.first << ":" << x.second;
  std::cout << std::endl;

  return 0;
}

Possible output: 
myumm contains: Tom:Produce Bob:Toys Bob:Garden
myumm contains: Bob:Jail


Complexity Linear on size (destructors).

Iterator validity All iterators, pointers and references are invalidated.

See also
unordered_multimap::erase
Erase elements (public member function)
unordered_multimap::empty
Test whether container is empty (public member function)

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