Re: Maintaining stack semantics but using polymorphic code

From:

nroberts <roberts.noah@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 25 Apr 2012 12:13:22 -0700 (PDT)

Message-ID:

<95779a5c-8ee4-49fa-a3f4-9a9b806a6b15@v2g2000vbv.googlegroups.com>

On Apr 18, 4:57 am, "asif.lse" <asif.l...@gmail.com> wrote:

I want to specify only ONE return type in this function. I suspect I
could auto here and use decltype to declare a pointer to the returned
type so that I could assign that pointer/ref to a pointer/ref to an
abstract class for polymorphic access. I haven't read up much on these
new C++ features but could somebody help me here? I want to use stack
variables and yet want to execute polymorphic code. The problem is
that I have to specify the concrete class name and return type. So,
apparently, there is no polymorphism. I am just thinking off the top
of my head but I feel that this is very doable. Can I use templates
here?

I'll be thankful for your help.

You can use the handle/body idiom together to get there...you'll need
a prototype as well:

struct actual_class
{
     virtual ~actual_class() {}
     virtual void function_a() = 0;
     ....
     void function_b() { ... } // non-virtual stuff too!!!
};

struct my_wrapper
{
     void function_a() { pimpl->function_a(); }
     void function_b() { pimpl->function_b(); }
     ....

     my_wrapper(actual_class * vf) : pimpl(vf) {}
     ~my_wrapper() { delete pimpl; }

     my_wrapper(my_wrapper const& other)
         : pimpl(other.vf->clone())
     {}
     // assignment...

private:
     actual_class * pimpl;
};

struct my_concrete_thingy : actual_class
{
     // you get the idea...
};

The handle/body or pimpl idiom can actually be used to hide all kinds
of things. Inheritance, singletons...etc...

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