Re: Abstract base class as template parameter (for std::set comparator)

"=?iso-8859-1?q?Daniel_Kr=FCgler?=" <>
Fri, 23 Feb 2007 03:36:46 CST
On 22 Feb., 22:16, "Aaron J. M." <> wrote:

I'm creating std::set objects with my own comparators. My comparators
are in following hierarchy, rooted at the abstract base class

class BaseCompare
     virtual bool operator()(const SomeOtherClass &lhs, const
SomeOtherClass &rhs) const = 0;


Don't forget the virtual destructor!


I want to dynamically set what comparator is used when I create a
set. I want to be able to do something like this:

void something(BaseCompare &compareRef)
     std::set<SomeOtherClass, BaseCompare> aSet(compareRef);



Here I'd pass either an ACompare or a BCompare. Unfortunately, my
compiler complains about BaseCompare's virtual function. Is there a
way to fix this?

The compiler warn's you rightly because of the slicing effect.
You comparison object must be at least copy constructible (if not
assignable). The solution is a simple non-polymorphic wrapper class
(i.e. a variant of the bridge pattern):

class BaseCompareWrapper
     explicit BaseCompareWrapper(const BaseCompare& cmp) : pcmp(&cmp)

      bool operator()(const SomeOtherClass &lhs, const
        SomeOtherClass &rhs) const
         return (*pcmp)(lhs, rhs);

      const BaseCompare* pcmp;

Now do this:

void something(const BaseCompare &compareRef)
      std::set<SomeOtherClass, BaseCompareWrapper>

Greetings from Bremen,

Daniel Kr?gler

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