Re: Alternative pattern for virtual template

On 5 mai, 17:24, Noah Roberts <n...@nowhere.com> wrote:


On of this approach is to use dynamic polymorphism, that is if you can
define a same contract on your type. As an example if your foo is only
displaying a message with the data in parameter, your contract if that
the type should support (ostream<<T), you define the interface:

struct foo_ostreamable
{
  virtual ostream& output(ostream&)const=0;
};

Then you define a templated foo_streambable implementation:

template<typename T>
struct foo_ostreamable_imp: foo_ostreamable
{
  T value;
  foo_ostreamable_imp(const T& t):value(t){}

  virtual ostream& output(ostream& os)const{return os<<value;}
};

And finally the parameter of SuperClass::foo with templated
constructor:

struct foo_param
{
  template<typename T>
  foo_param(const T& t){data.reset(new foo_ostreamable_imp<T>(t));

  scoped_ptr<foo_ostreamable> data;
};

// foo_param can be written into ostream
ostream& operator<<(ostream& os,const foo_param& p)
{
 return p.data->output(os);
}

And finally, you define your classes:
class SuperClass
{
public:
        virtual void foo(const foo_param&) const = 0;
};

class SubClass: public SuperClass
{
public:
        virtual void foo(const foo_param& p) const { cout << p <<
endl;}
} ;

The compiler will automatically resolve with correct subtype:
SubClass s;
s.foo(1);
s.foo(0.1);
s.foo("bar");
...

If you want to keep/compare foo_param values, there is some management
to do such as deep copy and others to put it into canonical form. If a
type doesn't react as you want, you overload foo_param constructor.

There is an overhead but that depends on what you want.

--
Michael