Re: Making a smart pointer which works with incomplete types

"Alf P. Steinbach" <>
Sun, 07 Sep 2008 19:46:10 +0200
* Kai-Uwe Bux:

Alf P. Steinbach wrote:

* James Kanze:

On Sep 7, 4:11 pm, "Alf P. Steinbach" <> wrote:

* Kai-Uwe Bux:

Alf P. Steinbach wrote:

* Kai-Uwe Bux:

Alf P. Steinbach wrote:

* Juha Nieminen:

  Then it occurred to me: Is there any reason this pointer cannot
static? Like this:
template<typename Data_t>
class SmartPointer
    static void(*deleterFunc)(Data_t*);
    SmartPointer(Data_t* d): data(d)
        deleterFunc = &deleter;
template<typename Data_t>
void(*SmartPointer<Data_t>::deleterFunc)(Data_t*) = 0;
  This way the pointer to the deleter will be stored in the program
once, and most importantly it will not increment the size of the
smart pointer.
  This feels so glaringly obvious to me now that I really wonder
this wasn't suggested to me to begin with. Is there some error here
I'm missing?

Yeah. Successive smart pointer instantiations can change the common
deleter func pointer.


Sorry, I reacted to the static pointer. As I wrote use a template
parameter instead. The only valid reason for having a pointer to that
function is to change the pointer, and that's apparently done in the
constructor body,

Really? Then a smart pointer working with incomplete types
(in the sense above) either does not qualify as a valid
reason or there must be a way without that (static) pointer.
Which is it? and if it is the second, which way of making a
smart pointer work with incomplete types do you have in

Consider the following:
   template< typename T >
   void destroy( T* );
   template< typename T >
   class SmartPointer
       ~SmartPointer() { if( ... ) { destroy( myReferent ); } }
It achieves the same as the original code without any static pointer.

No. With the original code, you can delete the pointer in a
context where the type is incomplete. with your code, you

Well, when you say something like that then you start me thinking if I may
have overlooked something, e.g something really basic. You force me to
actually code up a test case and check it. With at least two different

Could you post the code? I have a little trouble deciding when and where to
put the definition for the template

    template< typename T >
    void destroy( T* );


Following is the code for James' comments.

Although this uses template parameter (which is what I recommended for ease of
use) it is trivial to remove that feature, requiring a litte more client code.

<code file="sp.h">
#ifndef SP_H
#define SP_H

template< typename T >
void destroy( T* );

template< typename T, void (*doDestroy)(T*) = &destroy<T> >
class SmartPtr
     T* myReferent;

     SmartPtr( SmartPtr const& );
     SmartPtr& operator=( SmartPtr const& );

     SmartPtr( T* p ): myReferent( p ) {}
     ~SmartPtr() { doDestroy( myReferent ); }


<code file="x.cpp">
#include "sp.h"

class X;
X* newX();
void deleteX( X* );

int main()
     SmartPtr<X, deleteX> p( newX() );

<code file="y.cpp">
#include <iostream>
using namespace std;

class X
     X( X const& );
     X& operator=( X& );
     X() { cout << "X::<init>" << endl; }
     ~X() { cout << "X::<destroy>" << endl; }

X* newX() { return new X; }
void deleteX( X* p ) { delete p; }

Cheers & hth.,

- Alf

A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

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