How do I clear the std::queue efficiently?

Question

I am using std::queue for implementing JobQueue class. ( Basically this class process each job in FIFO manner). In one scenario, I want to clear the queue in one shot( delet

Answer 1

Apparently, there are two most obvious ways to clear std::queue: swapping with empty object and assignment to empty object.

I would suggest using assignment because it simply faster, more readable, and unambiguous.

I measured performance using following simple code and I found that swapping in C++03 version works 70-80% slower than assignment to an empty object. In C++11 there is no difference in performance, however. Anyway, I would go with assignment.

#include 
#include 
#include 
#include 
#include 

int main()
{
    std::cout << "Started" << std::endl;

    std::queue q;

    for (int i = 0; i < 10000; ++i)
    {
        q.push(i);
    }

    std::vector > queues(10000, q);

    const std::clock_t begin = std::clock();

    for (std::vector::size_type i = 0; i < queues.size(); ++i)
    {
        // OK in all versions
        queues[i] = std::queue();

        // OK since C++11
        // std::queue().swap(queues[i]);

        // OK before C++11 but slow
        // std::queue empty;
        // std::swap(empty, queues[i]);
    }

    const double elapsed = double(clock() - begin) / CLOCKS_PER_SEC;

    std::cout << elapsed << std::endl;

    return 0;
}