Question

Consider the following program. It creates a set of pointer-to-ints, and uses a custom indrect_less comparator that sorts the set by the value of the pointed-to integer. Once this is done, I then change the value of one of the pointed-to integers. Then, it can be seen the order of the set is no longer sorted (I suppose because the set doesn’t know something got changed).

(Don’t mind the C++0x loops, I’m running on VS2010)

#include <iostream>
#include <set>
using namespace std;

struct indirect_less {
    bool operator()(int* l, int* r) const
    {
        return *l < *r;
    }
};

int main()
{
    set<int*, indirect_less> myset;

    int* a = new int(5);
    int* b = new int(6);
    int* c = new int(7);

    myset.insert(a);
    myset.insert(b);
    myset.insert(c);

    cout << "Set contains: ";
    // (outputs: 5 6 7)

    for (auto i = myset.begin(), end = myset.end(); i != end; ++i)
    {
        cout << **i << " ";
    }

    cout << endl << "Modifying *a" << endl;
    *a = 9;         // point of interest
    cout << "Set contains: ";
    // (outputs: 9 6 7 - unsorted order)

    for (auto i = myset.begin(), end = myset.end(); i != end; ++i)
    {
        cout << **i << " ";
    }

    cout << endl;

    cin.get();

    return 0;
}

1) Am I right that I am invoking undefined behavior? Is the entire state of myset invalid after the line *a = 9;?

2) Is the only correct way to do this to erase then re-insert a?

3) Is there any way, once *a = 9; has been run, to re-balance the set in to sorted order, with well-defined behavior?

Answer 1

Yes, std::set assumes elements are immutable. It’s possible, if dangerous, to reorder it yourself after each change. I wouldn’t recommend it, though: use another collection type.