Re: polymorphism on template parameters

Joe Greer <>
Tue, 5 Aug 2008 13:36:44 +0000 (UTC)
Renato Golin <> wrote in

Hi all,

I'm fiddling with policies but I'm having some problems...

I get the error:

In function ???int main()??":
error: cannot convert ???Stone*??" to ???Thing<MovePolicy, EatPolicy,
ReproducePolicy>*??" in initialization

Any pointers (that doesn't start with 0x...) on the subject are


Sadly, that is both the curse and blessing of policy based design.
Different template parameters create completely new types. The template
name itself doesn't imply interaction capability between the instances.
That is one of the reasons that shared_ptr<> in TR1 isn't a policy based
smart pointer.

I have two suggestions as to how to organize things to get around this
feature of templates.

First, Instead of policies, you can use strategies that get
configured/set by the constructor. In other words, don't use templates
in this way if you want the resulting types to interact as if they were
the same type.

Or, you can define an interface for the basic 'Thing'-ness and make all
your templated classes inherit from that and only use the template for
construction. i.e.

class IThing
   virtual void DoThing() = 0;
   virtual ~IThing() {}

class MovePolicy { };
class EatPolicy { };
class ReproducePolicy { };

template <
     class Move = MovePolicy,
     class Eat = EatPolicy,
     class Reproduce = ReproducePolicy

class Thing : public IThing, Move, Eat, Reproduce
     Thing() { }
     virtual ~Thing() { }
     virtual DoThing() {}

class NoMove : public MovePolicy { };
class NoEat : public EatPolicy { };
class NoReproduce : public ReproducePolicy { };

class Stone : public Thing <NoMove, NoEat, NoReproduce>
     Stone() { }
     ~Stone() { }

main () {
     IThing * s = static_cast<IThing *>(new Stone());
     s ->DoThing();
     delete s; // virtual destructor lets this work
     return 0;


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