[reSIProcate] Another question on KeepAlive

[Alan Hawrylyshen]

On May 3, 2006, at 02.18, John Draper wrote:

>
> I stopped at the "if keepAlive" - then checked the following variables
>
> message is 0x887ae00
> keepAlive = 0x0
>
> Shouldn't the keepAlive be the same pointer as "message", only it's  
> just dynamic casted?
>
> Is there something I overlooked?  Could the compiler be messed up?

Interfaces are sometimes constrained to a set of object that are more  
general than required.
This is a C++ understanding issue, not a resiprocate issue.



From:
"The C++ Programming Language" - Bjarne Stroustrup [you already have  
this book, I'm sure]

Section 15.4 Run-Time Type Information

[...]
Recovering the "lost" type information [from the general base-class  
interface] of an object requires us to somehow ask the object to  
reveal its type. Any operation on an object requires us to have a  
pointer or reference of a suitable type for the object. Consequently,  
the most obvious and useful operation for inspecting the type of of  
an object at run time is a type conversion operation that returns a  
valid pointer if the object is of the expected type and a null  
pointer if it isn't. The dynamic_cast operator does exactly that. [...]

15.4.1 dynamic_cast
[...]
Consider the following:
	dynamic_cast<T*>(p)

If 'p' is of type T*, or an accessible base class of T, the result is  
exactly as if we had simply assigned p to a T*. [..]

The purpose of dynamic_cast is to deal with the case in which the  
correctness of the conversion cannot be determined by the compiler.  
Im that case,

	dynamic_cast<T*>(p)

looks at the obbject pointed to by p (if any). If that object is of  
class T or has a unique base class of type T, then dynamic_cast  
returns a pointer of type T* to that object; otherwise 0 is returned.  
If the value of p is 0, dynamic_cast<T*>(p) returns 0.

A