Re: dynamic_cast<> perfomans

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Wed, 14 Feb 2007 00:32:26 -0500

Message-ID:

<equ6ta$kbi$1@murdoch.acc.Virginia.EDU>

Grizlyk wrote:

Kai-Uwe Bux wrote:

//C++ do not support
#define interface

//******************
//ITF0
//******************
class Base interface
{
protected:
    static uint type;

public:
    virtual uint itype()const {return type;}

    enum { _type=0x0001 };
    enum { mask=0x0001 };
};

//******************
//interface_cast
//******************
template<class Tdst,class Tsrc>
Tdst* interface_cast(Tsrc* ptr)
{
fprintf(stderr,"src: %u, dst: %u, &=%u\n", ptr->itype(), Tdst::mask,
(ptr->itype()&Tdst::mask) );
    return ( ptr->itype() & Tdst::mask )?
        reinterpret_cast<Tdst*>(ptr):

You need a static_cast<> or a dynamic_cast<> here. Otherwise, you invite
undefined behavior of the _serious_ sort: in the case of multiple
inheritance, casting pointers up and down really does change bit-patterns
and reinterpret_cast<> is not required to do anything like that for you.
(Of course, the return value is formally unspecified even in the case of
single inheritance.)

The dynamic_cast<> can not be used because we are making replacement for
dynamic_cast<>.

Agreed.

Also we are assuming case of ordinary inheritance.

Also real class of object "*ptr" is completely defined by bit-pattern so
if condition (ptr->itype()&Tdst::mask) is true, then "*ptr" is object of
"Tdst" class, so reinterpret_cast<Tdst*> is what we need here - no
surprises - address change its type to correct and I am not sure that
static_cast<> will do the same here.

Your remarks in defense of reinterpret_cast<> are besides the technical
point. You are plain wrong about the behavior of reinterpret_cast<> and
static_cast<>. Please refer to the standard, [5.2.9/8-9] and [5.2.10].

Best

Kai-Uwe Bux