event-source.md

title	description	ms.date	f1_keywords	helpviewer_keywords
event_source (C++ COM Attribute)	Learn how to use the Microsoft C++ extension `event_source` COM attribute.	11/20/2020	vc-attr.event_source	event handling, attributes event logs, event source event sources, creating event_source attribute event sources event handling, creating event source

event_source attribute

Creates an event source.

Note

Event attributes in native C++ are incompatible with Standard C++. They don't compile when you specify /permissive- conformance mode.

Syntax

[ event_source(type, optimize=[speed | size], decorate=[true | false]) ]

Parameters

type
An enumeration of one of the following values:

• native for unmanaged C/C++ code (default for unmanaged classes).

• com for COM code. Use coclass when type=com. This value requires that you include the following header files:

  #define _ATL_ATTRIBUTES
  #include <atlbase.h>
  #include <atlcom.h>

optimize
When type is native, you can specify optimize=size, to indicate that there are 4 bytes of storage (minimum) for all events in a class or optimize=speed (the default) to indicate that there are 4 * (# of events) bytes of storage.

decorate
When type is native, you can specify decorate=false, to indicate that the expanded name in the merged (.mrg) file shouldn't include the enclosing class name. /Fx lets you generate .mrg files. decorate=false, which is the default, results in fully qualified type names in the merged file.

Remarks

The event_source C++ attribute specifies that the class or structure to which it's applied will be an event source.

event_source is used in conjunction with the event_receiver attribute and the __event keyword. Use event_receiver to create event receivers. Use __event on methods within the event source to specify those methods as events.

Note

A templated class or struct cannot contain events.

Requirements

Attribute context	Value
Applies to	class, struct
Repeatable	No
Required attributes	coclass when type=com
Invalid attributes	None

For more information, see Attribute Contexts.

See also

Compiler attributes
event_receiver
__event
__hook
__unhook
Class attributes