Posts Tagged ‘ATL’

Set resource handle in MFC and ATL

March 23, 2009 5 comments

When working in MFC/ATL and then using a resource dll can at times cause bugs because resource handle is not properly set. For e.g. if you have a dialog resource in a dll and you instantiate a CDialog which results in an error because resource cannot be located in this application or dll since it’s in the resource dll. So solution for above problem will be to set correct resource handle.

MFC provides two functions…

  1. AfxGetResourceHandle
  2. AfxSetResourceHandle

pretty obvious names. 🙂

So we should first save our current resource handle and then set new resource handle before loading such a resource. Also don’t forget to set our old handle since its good practice.

Sample code snippet.

extern HINSTANCE hResDll = NULL;
HINSTANCE hDefInstance = AfxGetResourceHandle();
CBitmap Bmp;
Bmp.LoadBitmap( IDB_NIBUS_FACE );


Internally MFC calls FindResource and LoadResource using this handle, so if it’s not properly set this will cause failure in loading resources.

In ATL it’s quite similar except that we’ve got a new function called AddResourceInstance which adds our new resource handle to existing list of handles. So when a look up is done given resource handle is also used. Following functions are provided in ATL to work with resource handles…

  1. AddResourceInstance
  2. GetResourceInstance – Similar to AfxGetResourceHandle
  3. SetResourceInstance – Similar to AfxGetSetResourceHandle

For newbies this is always a painful bug to resolve as they don’t know what went wrong since they expect this to be automagically done.

How to convert ANSI string to UNICODE string and vice versa?

July 2, 2008 6 comments

Well quite simple,  but still quite frequently asked in forums… 🙂

There are two macros that does this for us. They are as follows.

Note: You must include atlconv.h


Before using these two macros you have to use this macro too…


Here is a code snippet for you… 😉

#include <atlconv .h>
//An example for converting from ANSI to UNICODE

//use this first

//An ANSI string
LPSTR lpsz_ANSI_String = "An ANSI String";

//ANSI string being converted to a UNICODE string
LPWSTR lpUnicodeStr = A2W( lpsz_ANSI_String )

//Another example for converting from UNICODE to ANSI

//Use this first

//A UNICODE string
LPWSTR lp_UNICODE_STR = L"A Unicode String";

//UNICODE string being converted to a ANSI string

Another option is to call WideCharToMultiByte and MultiByteToWideChar directly, it’s quite easy to use, at least easier than above macros. If you have doubts on usage of above functions then take a look at AfxW2AHelper and AfxA2WHelper functions. Since these are the functions that A2W and W2A internally calls.

Also as a homework take a look at CW2AEX and CA2WEX classes. Will be similar to above macros, looks to me like a  secure version overload.

Dynamically determining the base class of a class using templates

November 27, 2007 Leave a comment

Well nearly dynamically! When working on the ATL COM framework I came across a piece of framework code which makes a template parameter as the base class of a class which looked quite useful to me.

Here is a demo…

template<class BaseClass = CWnd> class SomeWnd : public BaseClass
      LRESULT WindowProc(UINT uMessage, WPARAM wParam, LPARAM lParam );

template</class><class BaseClass>
LRESULT SomeWnd<baseclass>::WindowProc( UINT uMessage, WPARAM wParam, LPARAM lParam )
    //... Do some additional processing here ...
    return BaseClass::WindowProc( uMessage, wParam, lParam );

class MyDialog : public SomeWnd<cdialog>

class MyFrame : public SomeWnd<cframewnd>

class MyCustomWindow : public SomeWnd<> // Use default template param

Now you may ask why I needed this, well I had to write some code in the WindowProc of a window class, but the base differed from implementation to implementation, so I adapted this idea from the ATL COM framework.

Since WindowProc is found in all CWnd derived window classes and at the same time I also needed to call the correct the default base implementation too, so this helped.

The ATL team

October 18, 2007 2 comments

The Microsoft(r) ATL team…

It’s meant to be an easter egg -> See here


Got this BMP from the resource section of AtlObj.dll using my ResourceDigger.

rename_namespace ( #import )

When importing a type library it could be at times cumbersome to use the default namespace name.

#import provides a nice option called rename_namespace.


#import "SomeTypelib.tlb" rename_namespace( "NibuNamespace" )

Using no_namespace is lazy programming 😐 . Namespaces reduces name collision hence usage should be encouraged.

MIDL “string” attribute

June 4, 2007 7 comments

Sometime back I was wondering about whether there was a way to transfer our good ol’ NULL terminated strings using COM apart from using BSTR’s.

That’s how I bumped into “string” attribute provided by MIDL.

Here is how we use it…

typedef [string] char NormalString[1024];

[id(1), helpstring( "Displays null terminated UNICODE strings" )]
HRESULT ShowMsgUnicodeString( [in,string] wchar_t* wCharString_i );

[id(2), helpstring( "Displays null terminated ANSI strings" )]
HRESULT ShowMsgAnsiString( [in,string] char* cString_i );

Well if we don’t specify “string” then MIDL treats the parameter as an address of a wchar/char.

Callers can invoke the call as they would normally do with a function.

Using CComCreator

Easiest way to construct a COM interface from it’s co class.

ISomeInterfacePtr sipPtr;
CComCreator< CComObject< CSomeCoClass > >::CreateInstance( 0,
                                                           reinterpret_cast( void**, &sipPtr ));


Must be an ATL/COM Project.

Categories: COM Tags: , , ,
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