Re: about casting
"Tony Johansson" <johansson.andersson@telia.com> ha scritto nel messaggio
news:MrXZk.4591$U5.34211@newsb.telia.net...
Thanks a lot for you answer.
You have been of great help!
You are welcome.
class MyTest
{
...
IHandle_DS* m_handle_ds;
Question numer 1.
Can I for example assign the passed handle_ds pointer of type IHandle_DS
in class MyTest shown above like this
in InitRules2
MyTest* myTest = new MyTest();
myTest->m_handle_ds = handle_ds
You can do that, but it is important to respect the COM reference counting
rules to avoid resource/object leaks.
Every COM interface derives from IUnknown (so, also you IHandle_DS derives
from IUknown).
IUknown exposes three methods: AddRef(), Release(), and QueryInterface().
QueryInterface() is the equivalent of the C/C++ "cast" (or maybe better:
dynamic cast).
AddRef() and Release() have the purpose of managing reference counting of
COM interfaces.
In fact, in contrast to automatic garbage collection of C#, COM interfaces
are managed using explicit reference counting.
When you pass a COM interface to someone, you call AddRef() to increase the
reference counter (meaning: "I'm going to use this interface").
When you don't need the interface anymore, you call Release() to decrease
the ref counter (meaning: "Thank you, I have finished using it").
When the reference counter reaches 0, it means that the interface is no
required by anyone, so it is deleted (ATL implements the body of AddRef,
Release, and other boilerplate code for you).
So, you can store the COM interface pointer using a data member in another
C++ class, like you asked. But you should pay attention to call
m_handle_ds->Release() when you don't need that interface anymore. You could
do that e.g. in MyTest destructor. Proper thing to do is to also set the
pointer to NULL, to avoid dangling references, et.c.
// Safely release COM interface e.g. in MyTest destructor
if ( m_handle_ds != NULL )
{
m_handle_ds->Release();
m_handle_ds = NULL;
}
There are also smart pointer classes from ATL to automatically manage COM
reference counting, one of these is CComPtr:
http://msdn.microsoft.com/en-us/library/ezzw7k98(vs.80).aspx
So, you could use CComPtr< IHandle_DS > instead of raw pointer IHandle_DS*.
When the CComPtr goes out of scope, Release() is automatically called, etc.
Question numer 2.
In this class Handle_DS I have two object defined like this
CDataSource DS;
CSession Session;
Is it any restiction when I want to use these for example passing this
Session object into another method named foo
Could I for example do
foo(handle_ds->Session);
This is not clear to me.
Handle_DS is a C++ class; IHandle_DS is the COM interface. As previously
written, COM interfaces don't have "data members", they just expose pure
virtual methods via v-tables. If you mean the C++ Handle_DS class (without
the "I" prefix), being it a C++ class (and not a COM interface), you can do
everything inside there, as you do for normal C++ classes.
Giovanni