设置/映射迭代器不可递增

为什么在迭代时不能编辑集合/映射？例：

bool flag;
set <int> :: iterator aIterator;
set <int> :: iterator bIterator;
bIterator = B.begin();
for (bIterator=B.begin(); bIterator!=B.end(); ++bIterator)
{
    flag = true;
    aIterator = A.begin(); 
    for (aIterator=A.begin(); aIterator!=A.end(); ++aIterator)
    {
        if (*bIterator == *aIterator)// if an elemnet in B is found in A, delete that element from A and check the next element of B
        {           
            A.erase(aIterator);
            flag = false;
            break;
        }
    }
    if (flag == true)
        A.insert(*bIterator); //if not found then add that element to A(which now represents the set of symmetric difference)
}

我该如何处理？我的意思是，如何在迭代时编辑集合？