Re: [boost] Adding polymorphic_value to boost

Subject: Re: [boost] Adding polymorphic_value to boost
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2017-11-22 23:31:21

Peter Dimov wrote:
> You can have
>
>     vector<polymorphic_value<Shape>> v;
>
> and then
>
>     v.push_back( Circle(10) );
>     v.push_back( Ellipse(10, 15) );
>     v.push_back( Square(11) );

Good example.  Yes please.

There's another way of implementing this, though; rather than
pointing to the object in the heap you can store it in-place
by allocating enough space for the largest possible derived
subclass:

class Base { ... };
class Derived: public Base { ... };

template <typename B>
class polymorhpic_value_2
{
   B base;
   char enough_space_for_dervided_classes_members[1024];

public:
   template <typename D> // requires D is same as or derived from B
   void operator=(const D& d)
   {
     B::~B(this);
     new (this) D(d);
   }

};

polymorphic_value_2<Base> p;
Derived d;
p = d;

That's just a sketch; many details omitted!  There are some obvious
advantages and disadvantages of each approach and I'm definitely not
saying this is better than the pointer-to-heap design.  But I mention it
for this reason:

  * If your proposed interface for polymorphic_value could also work
    for this method of implementation, then it is probably a better
    interface.

> It's unfortunate we have to use -> instead of . but that's what's available. 

There have been proposals for operator. ; has this been evaluated to
see how it would work with them?  Similarly the unified a.b() vs. b(a)
syntax proposal.

Regards, Phil.