Re: sup****ting header-less blocks in memory allocators...

On Oct 3, 3:56=A0pm, Michel Decima <michel.dec...@[EMAIL PROTECTED]
> wrote:

> > derived2* d2 =3D new derived2 (); // CRASH! Allocate memory only for
> > derived1, derived2 will not fit into that memory
>
> But if you use the 'sz' parameter of operator new to call malloc(),
> then you will allocate enough memory for derived2.
> And you can overload the class-local delete operator with a second
> size_t parameter, you will known the size of the object to deallocate:


Cool! I completely miss this moment.
The point is that if user calls delete for object and object doesn't
have virtual destructor, then static and dynamic types of object MUST
be identical. So compiler can always determine correct size.

ISO C++ 12.5/4
If a delete-expression begins with a unary :: operator, the
deallocation function=92s name is looked up in
global scope. Otherwise, if the delete-expression is used to
deallocate a class object whose static type has a
virtual destructor, the deallocation function is the one found by the
lookup in the definition of the dynamic
type=92s virtual destructor (12.4).104) Otherwise, if the delete-
expression is used to deallocate an object of
class T or array thereof, the static and dynamic types of the object
shall be identical and the deallocation
function=92s name is looked up in the scope of T.


The only thing I don't like is dynamic dispatch on size.
Allocation/deallocation functions in fixed size allocator can be as
small as 5 machine instructions. So dynamic dispatch on size can
increase cost of allocation several times!

The only thing we can do is:

    __forceinline static void* operator new ( size_t sz )
    {
        // resolution at runtime
        if ( sz <=3D 4 && sizeof(derived_t) <=3D 4 )
            return alloc_4() ;
        else if ( sz <=3D 8 && sizeof(derived_t) <=3D 8 )
            return alloc_8() ;
        else if ( sz <=3D 16 && sizeof(derived_t) <=3D 16 )
            return alloc_16() ;
        else
            return ::malloc( sz ) ;
    }

and pray for inlining :)

Dmitriy V'jukov