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

public member function

<unordered_set>

std::unordered_set::clear

Clear content

All the elements in the unordered_set 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_set
#include <iostream>
#include <string>
#include <unordered_set>

int main ()
{
  std::unordered_set<std::string> myset =
    { "chair", "table", "lamp", "sofa" };

  std::cout << "myset contains:";
  for (const std::string& x: myset) std::cout << " " << x;
  std::cout << std::endl;

  myset.clear();
  myset.insert("bed");
  myset.insert("wardrobe");
  myset.insert("nightstand");

  std::cout << "myset contains:";
  for (const std::string& x: myset) std::cout << " " << x;
  std::cout << std::endl;

  return 0;
}

Possible output: 
myset contains: sofa lamp table chair
myset contains: nightstand wardrobe bed


Complexity Linear on size (destructors).

Iterator validity All iterators, pointers and references are invalidated.

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

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