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CComCoClass Class

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The latest version of this topic can be found at CComCoClass Class.

This class provides methods for creating instances of a class, and obtaining its properties.

Syntax

template <class T, const CLSID* pclsid = &CLSID_NULL>  
class CComCoClass

Parameters

T
Your class, derived from CComCoClass.

pclsid
A pointer to the CLSID of the object.

Members

Public Methods

Name Description
CComCoClass::CreateInstance (Static) Creates an instance of the class and queries for an interface.
CComCoClass::Error (Static) Returns rich error information to the client.
CComCoClass::GetObjectCLSID (Static) Returns the object's class identifier.
CComCoClass::GetObjectDescription (Static) Override to return the object's description.

Remarks

CComCoClass provides methods for retrieving an object's CLSID, setting error information, and creating instances of the class. Any class registered in the object map should be derived from CComCoClass.

CComCoClass also defines the default class factory and aggregation model for your object. CComCoClass uses the following two macros:

You can override either of these defaults by specifying another macro in your class definition. For example, to use CComClassFactory2 instead of CComClassFactory, specify the DECLARE_CLASSFACTORY2 macro:

class ATL_NO_VTABLE CMyClass2 :
   public CComObjectRootEx<CComSingleThreadModel>,
   public CComCoClass<CMyClass2, &CLSID_MyClass>,
   public IDispatchImpl<IMyClass, &IID_IMyClass, &LIBID_NVC_ATL_COMLib, /*wMajor =*/ 1, /*wMinor =*/ 0>,
   public IDispatchImpl<IMyDualInterface, &__uuidof(IMyDualInterface), &LIBID_NVC_ATL_COMLib, /* wMajor = */ 1, /* wMinor = */ 0>
{
public:
   DECLARE_CLASSFACTORY2(CMyLicense)

   // Remainder of class declaration omitted

Requirements

Header: atlcom.h

CComCoClass::CreateInstance

Use these CreateInstance functions to create an instance of a COM object and retrieve an interface pointer without using the COM API.

template <class  Q>
static HRESULT CreateInstance( Q** pp);

template <class  Q>
static HRESULT CreateInstance(IUnknown* punkOuter, Q** pp);

Parameters

Q
The COM interface that should be returned via pp.

punkOuter
[in] The outer unknown or controlling unknown of the aggregate.

pp
[out] The address of a pointer variable that receives the requested interface pointer if creation succeeds.

Return Value

A standard HRESULT value. See CoCreateInstance in the Windows SDK for a description of possible return values.

Remarks

Use the first overload of this function for typical object creation; use the second overload when you need to aggregate the object being created.

The ATL class implementing the required COM object (that is, the class used as the first template parameter to CComCoClass) must be in the same project as the calling code. The creation of the COM object is carried out by the class factory registered for this ATL class.

These functions are useful for creating objects that you have prevented from being externally creatable by using the OBJECT_ENTRY_NON_CREATEABLE_EX_AUTO macro. They are also useful in situations where you want to avoid the COM API for reasons of efficiency.

Note that the interface Q must have an IID associated with it that can be retrieved using the __uuidof operator.

Example

In the following example, CDocument is a wizard-generated ATL class derived from CComCoClass that implements the IDocument interface. The class is registered in the object map with the OBJECT_ENTRY_NON_CREATEABLE_EX_AUTO macro so clients can't create instances of the document using CoCreateInstance. CApplication is a CoClass that provides a method on one of its own COM interfaces to create instances of the document class. The code below shows how easy it to create instances of the document class using the CreateInstance member inherited from the CComCoClass base class.

STDMETHODIMP CMyApp::CreateDocument( /* [out, retval] */ IDocument** ppDoc)
{
   *ppDoc = NULL;
   return CMyDoc::CreateInstance(ppDoc);
}

CComCoClass::Error

This static function sets up the IErrorInfo interface to provide error information to the client.

static HRESULT WINAPI Error(
    LPCOLESTR lpszDesc,
    const IID& iid = GUID_NULL,
    HRESULT hRes = 0);

static HRESULT WINAPI Error(
    LPCOLESTR lpszDesc,
    DWORD dwHelpID,
    LPCOLESTR lpszHelpFile,
    const IID& iid = GUID_NULL,
    HRESULT hRes = 0);

static HRESULT WINAPI Error(
    LPCSTR lpszDesc,
    const IID& iid = GUID_NULL,
    HRESULT hRes = 0);

static HRESULT WINAPI Error(
    LPCSTR lpszDesc,
    DWORD dwHelpID,
    LPCSTR lpszHelpFile,
    const IID& iid = GUID_NULL,
    HRESULT hRes = 0);

static HRESULT WINAPI Error(
    UINT nID,
    const IID& iid = GUID_NULL,
    HRESULT hRes = 0,
    HINSTANCE hInst = _AtlBaseModule.GetResourceInstance ());

static HRESULT Error(
    UINT nID,
    DWORD dwHelpID,
    LPCOLESTR lpszHelpFile,
    const IID& iid = GUID_NULL,
    HRESULT hRes = 0,
    HINSTANCE hInst = _AtlBaseModule.GetResourceInstance());

Parameters

lpszDesc
[in] The string describing the error. The Unicode version of Error specifies that lpszDesc is of type LPCOLESTR; the ANSI version specifies a type of LPCSTR.

iid
[in] The IID of the interface defining the error or GUID_NULL (the default value) if the error is defined by the operating system.

hRes
[in] The HRESULT you want returned to the caller. The default value is 0. For more details about hRes, see Remarks.

nID
[in] The resource identifier where the error description string is stored. This value should lie between 0x0200 and 0xFFFF, inclusively. In debug builds, an ASSERT will result if nID does not index a valid string. In release builds, the error description string will be set to "Unknown Error."

dwHelpID
[in] The help context identifier for the error.

lpszHelpFile
[in] The path and name of the help file describing the error.

hInst
[in] The handle to the resource. By default, this parameter is _AtlModule::GetResourceInstance, where _AtlModule is the global instance of CAtlModule.

Return Value

A standard HRESULT value. For details, see Remarks.

Remarks

To call Error, your object must implement the ISupportErrorInfo Interface interface.

If the hRes parameter is nonzero, then Error returns the value of hRes. If hRes is zero, then the first four versions of Error return DISP_E_EXCEPTION. The last two versions return the result of the macro MAKE_HRESULT( 1, FACILITY_ITF, nID ).

CComCoClass::GetObjectCLSID

Provides a consistent way of retrieving the object's CLSID.

static const CLSID& WINAPI GetObjectCLSID();

Return Value

The object's class identifier.

CComCoClass::GetObjectDescription

This static function retrieves the text description for your class object.

static LPCTSTR WINAPI GetObjectDescription();

Return Value

The class object's description.

Remarks

The default implementation returns NULL. You can override this method with the DECLARE_OBJECT_DESCRIPTION macro. For example:

class ATL_NO_VTABLE CMyDoc :
   public CComObjectRootEx<CComSingleThreadModel>,
   public CComCoClass<CMyDoc, &CLSID_MyDoc>,
   public IDocument
{
public:
   DECLARE_OBJECT_DESCRIPTION("My Document Object 1.0")

   // Remainder of class declaration omitted.

GetObjectDescription is called by IComponentRegistrar::GetComponents. IComponentRegistrar is an Automation interface that allows you to register and unregister individual components in a DLL. When you create a Component Registrar object with the ATL Project Wizard, the wizard will automatically implement the IComponentRegistrar interface. IComponentRegistrar is typically used by Microsoft Transaction Server.

For more information about the ATL Project Wizard, see the article Creating an ATL Project.

See Also

Class Overview