Re: Deleting a pointer to an incomplete class

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Wed, 11 Jul 2007 19:44:12 +0200
Message-ID:
<139a5nelc6gp306@corp.supernews.com>
* Kai-Uwe Bux:

Alf P. Steinbach wrote:

You can "delete" a pointer to incomplete type. But IIRC you have
Undefined Behavior if that type then defines a non-trivial destructor.
And that was the problem with Herb Sutter's original GOTW on the PIMPL
idiom, where he employed std::auto_ptr for the implementation object.


Thanks! You are refering to [5.3.4/5], right?

  If the object being deleted has incomplete class type at the point of
  deletion and the complete class has a non-trivial destructor or a
  deallocation function, the behavior is undefined.


If I had looketh in the Holy Standard I guess that would be it, yes.

Now, the question becomes: which line in the program is the point of
deletion? I would think, in the snippet I posted, it's the line in the
destructor of Y, i.e., the following has UB:

#include <iostream>
#include <ostream>

struct X;

struct Y {

  X * x_ptr;

  Y ( X * ptr = 0 )
    : x_ptr ( ptr )
  {}

  ~Y ( void ) {


Uh, C-ism, please don't do that Kai! Hurts me... ;-)

    delete x_ptr;
  }

};

struct X {

  ~X ( void ) {
    std::cout << "crash me\n";
  }
  
};

int main ( void ) {
  Y y ( new X );
}

Thus, it makes perfect sense that it does not print anything with g++ :-)


Yes.

However, I was more interested in templates anyway (posted an oversimplified
piece of code). So what about these:

A (typedef)
===========

#include <iostream>
#include <ostream>

template < typename X>
struct Y {

  X * x_ptr;

  Y ( X * ptr = 0 )
    : x_ptr ( ptr )
  {}

  ~Y ( void ) {
    delete x_ptr;
  }

};

struct X;

typedef Y<X> YX;

struct X {

  ~X ( void ) {
    std::cout << "crash me\n";
  }
  
};

int main ( void ) {
  YX yx ( new X );
}


Hm. Hm. I /think/ the template isn't instantiated until 'main' (i.e.,
the typedef doesn't do anything other than introduce a name), where the
definition of X is known, and so it should be OK.

However, there's also the practical matter of compilers that don't
implement two-phase template instantiation.

B (inheritance)
===============

#include <iostream>
#include <ostream>

template < typename X>
struct Y {

  X * x_ptr;

  Y ( X * ptr = 0 )
    : x_ptr ( ptr )
  {}

  ~Y ( void ) {
    delete x_ptr;
  }

};

struct X;

struct YX : public Y<X> {


I think that right here Y<X> is instantiated, with still incomplete type
X, and thus Undefined Behavior.

  YX ( X * ptr = 0 )
    : Y<X>( ptr )
  {}
  
};

 >

struct X {

  ~X ( void ) {
    std::cout << "crash me\n";
  }
  
};

int main ( void ) {
  YX yx ( new X );
}

C (position of function definition)
===================================

#include <iostream>
#include <ostream>

template < typename X>
struct Y {

  X * x_ptr;

  Y ( X * ptr = 0 )
    : x_ptr ( ptr )
  {}

  ~Y ( void ) {
    delete x_ptr;
  }

};

struct X;

void f ( X * ptr = 0 ) {
  Y<X> y ( ptr );
}


Instantiation with incomplete type X, Undefined Behavior.

struct X {

  ~X ( void ) {
    std::cout << "crash me\n";
  }
  
};

int main ( void ) {
  f ( new X );
}

In my experiments with g++, all these programs print "crash me".


Hm, B and C "should", as far as I understand it, exhibit UB, which of
course means they could by chance print "crash me", but in practical
terms a compiler-generated empty X destructor should be invoked.

Checking...

I get the same result as your g++-testing by using MSVC 7.1. However,
using MingW g++ 3.4.4 B and C do not print anything (and warnings are
issued for the destructor calls), while A does print "crash me" (and no
warnings). This is consistent with g++ generally being much more
standard-conforming wrt. templates than MSVC.

Where is
the "point of deletion" in these programs when template instantiation
issues enter the picture? Which of the above have UB?


See above. I think I got it right; at least, testing above indicates
that. So, A is OK (A OK ;-)), while B and C are UB.

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