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

From:
"=?iso-8859-1?q?Daniel_Kr=FCgler?=" <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Fri, 23 Feb 2007 03:36:46 CST
Message-ID:
<1172181144.679638.311470@q2g2000cwa.googlegroups.com>
On 22 Feb., 22:16, "Aaron J. M." <ajm...@ns.sympatico.ca> wrote:

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

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
{
public:
     explicit BaseCompareWrapper(const BaseCompare& cmp) : pcmp(&cmp)
{}

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

private:
      const BaseCompare* pcmp;
};

Now do this:

void something(const BaseCompare &compareRef)
{
      std::set<SomeOtherClass, BaseCompareWrapper>
aSet(BaseCompareWrapper(compareRef));
      ...
}

Greetings from Bremen,

Daniel Kr?gler

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