Re: Destructor not called with forward declaration instead of include

internetpet 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?
> 
> Thanks
> Eric
> 
> 

C++ standard -- 5.3.5.5:
"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."

A forward declaration gives you an incomplete type.  Thus what you are
trying to do is undefined behavior.

Think about it from a compiler implementor's viewpoint.  If you are
emitting code for "delete pclazz;", but you don't know whether or not
pclazz even has a destructor, how are you going to emit code that calls
it?

-- 
Alan Johnson

      [ See http://www.gotw.ca/resources/clcm.htm
for info about ]
      [ comp.lang.c++.moderated.    First time posters: Do this! ]