Re: Destructor not called with forward declaration instead of include

From:
Triple-DES <DenPlettfrie@gmail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 22 Apr 2008 10:52:59 CST
Message-ID:
<41792d18-a785-46cd-9c7a-8df04bc3ed6f@s50g2000hsb.googlegroups.com>
On 22 Apr, 04:43, internetpet <internet...@hotmail.com> wrote:

Say you have these files:

BigClazz.h

     class Clazz; // Note the forward declaration only , no include
"Clazz.h"
     class BigClazz
     {
         public:
             BigClazz();
             ~BigClazz();
             Clazz* pclazz;
     };

BigClazz.cpp

     #include "BigClazz.h"
     BigClazz::BigClazz(){}
     BigClazz::~BigClazz()
     {
         delete pclazz; // Here the destructor of of the Clazz object
will not be called
     }

Clazz.h

     class Clazz
     {
         public:
             Clazz();
             ~Clazz();
     };

Clazz.cpp

     #include "Clazz.h"
     Clazz::Clazz(){};
     Clazz::~Clazz(){};

main.cpp

     int main(int argc, char *argv[])
     {
         BigClazz* pVM = new BigClazz();
         pVM->pclazz = new Clazz();
         delete pVM;
     }

If you run this you'll see that the Clazz destructor will not be
called when BigClazz does "delete pclazz;" in it's own destructor. But
if you replace the forward declaration in BigClazz (class Clazz;) with
an include
(#include "Clazz.h") then it works.

Any idea why?


Yes. According to [expr.delete]/5 in the standard (with my comments in
parens):

If the object being deleted has incomplete class type (in this case,
forward declared) at the point of deletion and the complete class has
a non-trivial destructor (you declared your own dtor, so it's non-
trivial) or a deallocation function, the behavior is undefined (which
usually means not calling the destructor in this case).

To fix this, simply #include "Clazz.h" in BigClazz.cpp, so that the
type is complete at the point of deletion.

--
DP

      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
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