Question

For example:

abstract class goal
class priority
class childgoal

multimap<priority, goal> mm;
mm.insert(make_pair(priority(), childgoal());

I get the error:

cannot declare field 'std::pair<priority, goal>::second'
to be of abstract type 'goal'

If I remember correctly, Pair uses the default constructor to create second before assigning, which is the cause of the distress. I may be incorrect, but it would explain the error.

How do I get around this… how can i accomplish inserting into a multimap (or probably map for that matter) when one of the types is abstract?

Answer 1

You can make a map of pointers:

#include <memory>
#include <map>

class Base { };
class Derived1 : public Base { };
class Derived2 : public Base { };

typedef std::shared_ptr<Base> MyPtr;
typedef std::multimap<Key, MyPtr> MyMap;

int main()
{
  MyMap m { { Key(1), MyPtr(new Derived1) }, { Key(2), MyPtr(new Derived2) } };
}

Uniform initialization syntax requires C++11 support. In older versions, you can insert instead:

m.insert(MyMap::value_type(Key(1), MyPtr(new Derived1)));

If nobody else needs the mapped objects, you might get away with a std::unique_ptr<Base> instead. A simple matter of changing the typedef.