How do I clear the std::queue efficiently?

A common idiom for clearing standard containers is swapping with an empty version of the container:

void clear( std::queue<int> &q )
{
   std::queue<int> empty;
   std::swap( q, empty );
}

It is also the only way of actually clearing the memory held inside some containers (std::vector)

Yes - a bit of a misfeature of the queue class, IMHO. This is what I do:

#include <queue>
using namespace std;;

int main() {
    queue <int> q1;
    // stuff
    q1 = queue<int>();  
}

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