Inserting iterator range in std::vector

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?

Btw, it's easier to write foo f(5); auto pair = m.equal_range(&f); than to write //leak here, I know ;-p — Steve Jessop
– Steve Jessop, Commented Nov 21, 2012 at 18:24
@SteveJessop true enough :P, in the real code I was using a smart pointer, but the reference to the local object sounds like a better alternative. — Luke B.
– Luke B., Commented Nov 21, 2012 at 18:38

kennytm · Accepted Answer · 2012-11-21 18:16:29Z

3

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.

answered Nov 21, 2012 at 18:16

kennytm

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Luke B. Over a year ago

Ah, that makes perfect sense! Thanks for the help.

Steve Jessop · Accepted Answer · 2012-11-21 18:22:21Z

3

vector::insert expects iterators that point to the values to be inserted into the vector.

Since you have a vector of iterators, you would have to pass it something that iterates over the iterators. But you're passing it something that iterates over the elements in your multiset.

So, this fails for the same reason that you can't do:

std::vector<char*> v;
char foo[10];
v.insert(v.end(), foo, foo+10);

or:

std::vector<int> v;
v.insert(v.end(), 0, 10);

One solution to all three problems is boost::counting_iterator. In your case:

v.insert(v.end(),
    boost::make_counting_iterator(pair.first), 
    boost::make_counting_iterator(pair.second)
);

edited Nov 21, 2012 at 18:22

answered Nov 21, 2012 at 18:16

Steve Jessop

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