Question

I have a multiset and I’m getting a range from it. I want to add this range in a vector to use it later, this is what I’m doing:

class foo
{
    public:
        int a;
        foo(int a) : a(a){}
};
class FooPointerCompare
{
    public:
        bool operator()(const foo* a, const foo* b)
        {
            return a->a < b->a;
        }
};

std::multiset<foo*, FooPointerCompare> m;
std::vector<std::multiset<foo*, FooPointerCompare>::iterator> v;
auto pair = m.equal_range(new foo(5)); //leak here, I know
v.insert(v.end(), pair.first, pair.second);

but I get these errors:

No matching constructor for initialization of 'std::__1::__tree_const_iterator<foo *, const std::__1::__tree_node<foo *, void *> *, int>'

No viable overloaded '='

when I use a simple for(auto it = pair.first; it != pair.second; it++) v.push_back(it); it works perfectly. What is wrong with my vector::insert call?

Answer 1

What’s wrong is that

v.insert(v.end(), pair.first, pair.second);

is equivalent to

for (auto it = pair.first; it != pair.second; ++ it)
    v.push_back(*it);
//              ^

which is not the same as your intention. I don’t think there’s any standard algorithms to do what you want. It’s better you just write out the for loop.