Question

I’d like to have a function to “remove one element” from unordered_set.

However, when it’s implemented using erase(begin()), it becomes extremely slow.
(This is in g++-4.5.3; maybe begin() has to traverse a larger number of empty hash buckets?)

See the example code below with surprising timings.

Is there some other way to implement “remove one element” that would have greater efficiency? (I do want to allow other intervening set operations which would invalidate iterators.)

#include <unordered_set>
#include <iostream>
#include <chrono>
using namespace std;

struct Timer {
    Timer(string s) : _s(s), _start(Clock::now()) { }
    ~Timer() {
        auto t=chrono::duration_cast<chrono::milliseconds>(Clock::now()-_start).count();
        cerr << "Timer(" << _s << ") = " << t << "ms\n";
    }
 private:
    typedef chrono::high_resolution_clock Clock;
    string _s;
    Clock::time_point _start;
};

int main()
{
    unordered_set<int> s;
    const int num=200000;
    { Timer t("insert"); for (int i=0;i<num;i++) { s.insert(i); } }
    { Timer t("remove half"); for (int i=0;i<num/2;i++) { s.erase(s.begin()); } }
    long long s1=0, s2=0;
    { Timer t("access begin()"); for (int i=0;i<num/2;i++) { s1+=*s.begin(); } }
    { Timer t("access all"); for (auto it=s.begin();it!=s.end();++it) { s2+=*it; } }
    cerr << s1 << " " << s2 << "\n";
    return 0;
}

// Timer(insert) = 36ms
// Timer(remove half) = 3039ms
// Timer(access begin()) = 5958ms
// Timer(access all) = 1ms

Answer 1

It looks like an issue with that version of the GNU library, fixed in more recent versions. Here are the results of my tests, using the two versions I happen to have installed:

mikes@seymour-desktop:~$ g++-4.4.5 -std=c++0x -O3 test.cpp
mikes@seymour-desktop:~$ ./a.out 
Timer(insert) = 15ms
Timer(remove half) = 3815ms
Timer(access begin()) = 7722ms
Timer(access all) = 0ms
10000000000 14999950000

mikes@seymour-desktop:~$ g++-4.6.1 -std=c++0x -O3 test.cpp
mikes@seymour-desktop:~$ ./a.out 
Timer(insert) = 16ms
Timer(remove half) = 2ms
Timer(access begin()) = 0ms
Timer(access all) = 1ms
10000000000 14999950000

I also got similarly fast results by using boost::unordered_set, so that’s an option if you can’t update your compiler.