Re: [boost] Why no non-null smart pointers?

Subject: Re: [boost] Why no non-null smart pointers?
From: Jeff Hill (johill_at_[hidden])
Date: 2014-04-29 10:32:54

> You've overlooked the use case of reclaiming ownership from smart_ptr.

Sorry, apparently a bit more explanation is needed.

In the code above I have this assignment.

 *this = NullObjectTypes < T > :: factory (); 

My idea is that shared_ptr <T> is returned from this factory function so
that it can be assigned to *this, and so ownership /is/ properly maintained.

The default templated version of NullObjectTypes < T > :: factory () would
return a default constructed shared_ptr <T>. This is necessary of course for
backwards compatibility.

template < class T >
struct NullObjectTypes 
    shared_ptr < ChessPiece  >   factory () { return shared_ptr < T > (); }
};

The benefit comes when T is an interface class. In this situation we can
specialize NullObjectTypes < T > as follows.

Lets say that T is the ChessPiece class, and we will implement the null
object pattern for it.

struct ChessPiece {
    virtual void move() =  0;
};

struct ChessPieceNull : public ChessPiece {
    void move() {}
};

template <>
NullObjectTypes < ChessPiece   >
    shared_ptr < ChessPiece  >   factory ();
};

template <>
shared_ptr < ChessPiece  > NullObjectTypes < ChessPiece   > ::  factory ()
{
    static ChessPieceNull chessPieceNull;
    return shared_ptr < ChessPiece > ( &chessPieceNull, nullDeleter () );
}

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