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Showing posts from March, 2006

Do not delete [] a scalar pointer !!!

Recently I got tangled into this problem in my code - Calling a vector dtor for a scalar pointer. We all know that it is perfectly illegal to do that. For example, if we allocate something like this:-

OurClass *p = new OurClass();

and try to delete like this:-
delete []p;

then we are going to end up in trouble. Ofcourse we know that we will end up in trouble. But I have really not given a thought HOW ?

When we allocate an array of items eg. OurClass pa[] = new[5] pa(), the compiler actually allocates the necessary amount of memory, calls the ctors for each allocated class and also prefixes the block of memory of the 'n' items allocated with the number of items allocated.

NumItems | OurClassObject1 | OurClassObject2 | ...... | OurClassObjectn

But pa always points to the first item in the allocation, thereby the item count prefix remains hidden. When we call delete[] pa, the compiler uses the item count prefix to delete the allocated objects and call the dtors.

Now i thin…

Where do you QueryInterface ???

For an ATL class, the QueryInterface is implemented in CComObject. The figure below is the inheritance hierarchy for a class generated by the wizard representing an ATL-COM object.

CComObjectRootBase has an InternalQueryInterface method, which uses the interface map built by the BEGIN_COM_MAP macro to resolve IID -> interface pointer. The BEGIN_COM_MAP macro also defines a method _InternalQueryInterface, which passes the map on to InternalQueryInterface. CComObject implements QueryInterface, and calls _InternalQueryInterface.

NOTE:
CComObjectRootEx: Provides methods to handle object reference count management for both nonaggregated and aggregated objects.


CComObject: Implements IUnknown for a nonaggregated object. It is a template class that takes a class like CSomeClass derived from CComObjectRootEx.


CComObjectNoLock: Implements IUnknown for a nonaggregated object, but does not increment the module lock count in the constructor. ATL uses CComObjectNoLock internally for class factorie…

Use Of Class Factories !!!

To understand quickly and to explain in the simplest way, Class Factories are the factory classes that create a COM object. A class factory may be responsible for creating one or more COM objects. In the case of COM OutOfProc servers, the server registers the class factories for objects that it can create in a system-global table using CoRegisterClassObject. Whenever a client does CoGetClassObject for a CLSID, the COM run-time can look it up in the system global table, and return the factory instance. The case with InProc servers is also similar but through the DLLGetClassObject.

The point here is that class factories are required [irrespective of how they exist physically in the servers, either as seperate instances or the COM object itself behaving as a factory for the objects of that type], they abstract the creation of the COM object through IClassfactory::CreateInstance.

Unsafe Operations with STL !!!

It is UNSAFE to do any operation on an STL container that will modify its size while holding a reference to one of its exisiting element. What could happen is, when you do an operation, say push_back on a vector, it determines if there is enough space available to add a new element. If there is not sufficient space available, it allocates new space for whole of the data structure and deletes the old buffer. At this point, any reference to one of its elements created prior to push_back would have gotten corrupted.

For example, the following usage of code is dangerous when used within a single scope.

SomeClass &sc = m_Vector.back();
m_Vector.push_back(someotherobject);
.
.
.
sc.SomeMethodCall(); // Code might crash here.

Consoles for Mr.GUI !!!

Learnt something new, a small one but very useful.
Many times I have seen GUI applications accompanied by console windows that show logs or trace information of the application. How do we do that for our application ?

Any GUI application can create its own console window just by calling AllocConsole Win32 API. Actually any process can use that API to allocate a new console. And the application must also learn to be disciplined enough to FreeConsole. Ok, fine. I used that in my small MFC application and was happy to see the console. But I did not see anything displayed on the console. As we know, each process has its own stdin, stdout and stderr. So redirect the console output of your parent application to the console. How do you do that ?

Use the FILE *freopen(const char *path, const char *mode, FILE *stream); API. The freopen function closes the file currently associated with stream and reassigns stream to the file specified by path. By that way, call freopen as follows:-

FILE *fpStdOut …

Setting Environment Variables !!!

Need to change or set the value of an environment variable programmatically and without the need to restart/log off the machine. I need the change to reflect for all processes, ie, I need to change the global environment value and not the one in the PEB [Process Environment Block] of a process. Frustated with setting the value of an environment variable !!!

For getting the set of environment variables or to get the value of an environment vaible from your C# program, there is the GetEnvironmentVariables/GetEnvironmentVariable API in the System.Environment class. But there is no API for setting the value of an environment variable.

The system environment variables are stored in the registry under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment.

The [current] user environment variables are stored in the registry under HKEY_CURRENT_USERE\Environment.

When the system boots up, the environment is built from this list in the registry. If we change the value direc…

CoMarshal.... working in NT, Not working in XP !!!

Problem:-

I have created a multi-threaded application which works without any problems on a NT-4.0 Workstation/Server. When I try to run the same application in Windows XP, I get an error in a call to CoMarshalInterThreadInterfaceInStream which returns -2147418113.
I have provided a snippet of the code below where the call fails in Windows XP.
Environment - Windows-XP,SP-2,Visual Studio 6.0,SP-4,ATL-3.0
Should I be doing anything different in Windows XP?

HRESULT hr = S_OK;
IUnknown** pp = p->m_vec.begin();
while (pp <>m_vec.end() && hr == S_OK)
{
if (*pp != NULL)
{
IEvent* pEvent = (IEvent*)*pp;
IStream* pIStream;
HRESULT hr = CoMarshalInterThreadInterfaceInStream(IID_IEvent, pEvent, &pIStream);
if(SUCCEEDED(hr))
{
CComPtr pMarshalEvent;
hr = CoGetInterfaceAndReleaseStream(pIStream, IID_IEvent, (void**)&pMarshalEvent);

if(SUCCEEDED(hr))
hr = pMarshalEvent->NewCurrentCassette(m_pCurrentCassette, m_se…

Consts in .NET !!!

I was doing some programming with C# and I had to use some 'const's as everybody does generally in programming. I had a class that simply had const string variables for my DB table names and stuff like that. My program was not working well and I started debugging and in the debugger, I was shocked to see that the const variables did not show the string values I had assigned. I did rebuilt and other non-sensical stuff like that until I learnt this about the consts in .NET:- 'const' variables in .NET do not exist as variables out of the assembly they exist in. Instead, during compilation, they get embedded [hard-coded] where ever you use them, and so when you debug, you do not see the proper value that you had assigned. For debugging purposes you have to output diagnostic trace messages and verify.