aggregatable

Article
01/03/2018

The latest version of this topic can be found at aggregatable.

Indicates that the class supports aggregation.

Syntax

  
      [ aggregatable(   
   value  
) ]

Parameters

value (optional)
A parameter to indicate when the COM object can be aggregated:

never The COM object cannot be aggregated.
allowed The COM object can be created directly or it can be aggregated. This is the default.
always The COM object cannot be created directly and can only be aggregated. When you call CoCreateInstance for this object, you must specify the aggregating object's IUnknown interface (the controlling IUnknown).

Remarks

The aggregatable C++ attribute has the same functionality as the aggregatable MIDL attribute. This means that the compiler will pass the aggregatable attribute through to the generated .idl file.

This attribute requires that the coclass, progid, or vi_progid attribute (or another attribute that implies one of these) also be applied to the same element. If any single attribute is used, the other two are automatically applied. For example, if progid is applied, vi_progid and coclass are also applied.

ATL Projects

If this attribute is used within a project that uses ATL, the behavior of the attribute changes. In addition to the previously described behavior, the attribute also adds one of the following macros to the target class:

Parameter value	Inserted macro
Never	DECLARE_NOT_AGGREGATABLE
Allowed	DECLARE_POLY_AGGREGATABLE
Always	DECLARE_ONLY_AGGREGATABLE

Example

// cpp_attr_ref_aggregatable.cpp  
// compile with: /LD  
#define _ATL_ATTRIBUTES  
#include "atlbase.h"  
#include "atlcom.h"  
  
[module(name="MyModule")];  
  
[ coclass, aggregatable(allowed),  
  uuid("1a8369cc-1c91-42c4-befa-5a5d8c9d2529")]  
class CMyClass {};

Requirements

Attribute Context

Applies to	class, `struct`
Repeatable	No
Required attributes	One or more of the following: coclass, progid, or vi_progid.
Invalid attributes	None

For more information about the attribute contexts, see Attribute Contexts.

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