Re: A subtle access issue (may be advanced :-) )

"Alf P. Steinbach" <>
Sun, 30 Aug 2009 04:42:11 +0200
* ld:

On 29 ao?t, 09:11, "Alf P. Steinbach" <> wrote:

The following code compiles as-is with g++ and Comeau Online, but not when then
the commented lines are uncommented:

#include <stddef.h>

template< class T > T* create();

class Base
template< class T > friend T* create();
     static void* operator new( size_t size )
         return ::operator new( size );

// static void operator delete( void* p )
// {
// ::operator delete( p );
// }

     virtual ~Base() {}


class Derived
     : public Base
     Derived() {}
     virtual ~Derived() {}


template< class T >
T* create() { return new T; }

int main()


With uncommenting the commented code both compilers complain that the Derived
destructor can't access Base::operator delete.

They don't complain that the Derived constructor can't access Base::operator new.

I can understand the lack of complaint for the constructor: the constructor
doesn't need to access the allocation function, which is invoked by the new
expression which is in the context of the create function which has access.

I don't understand the complaint for the destructor. The new expression has to
potentially deallocate, if an expression is thrown. But it manages well to use
the allocation function, so why can't it also use the deallocation function?

In short, why this different treatment?

It almost seems as if the standard supports an implementation technique where
the call to the deallocation function is made directly from *within* the most
derived class' destructor?

[snip explanation of machine code level -- I don't think it should influence

BTW, why don't you declare Base::operator delete as protected instead
of private since its role is more or less the same as the destructor
~Base() ?

That's what I had to do, but the reason for the "private:" is to express in the
language that a derived class' code can't do 'delete this'.

With protected access it can. :-(



Thanks :-) Although I disagree (very strongly) that access rules should be
affected by possible implementation techniques for the compiler.


- Alf

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