WeakRef Class

Artikel
06/05/2017

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Represents a weak reference that can be used by only the Windows Runtime, not classic COM. A weak reference represents an object that might or might not be accessible.

Syntax

class WeakRef : public ComPtr<IWeakReference>

Remarks

A WeakRef object maintains a strong reference, which is associated with an object, and can be valid or invalid. Call the As() or AsIID() method to obtain a strong reference. When the strong reference is valid, it can access the associated object. When the strong reference is invalid (nullptr), the associated object is inaccessible.

A WeakRef object is typically used to represent an object whose existence is controlled by an external thread or application. For example, construct a WeakRef object from a reference to a file object. While the file is open, the strong reference is valid. But if the file is closed, the strong reference becomes invalid.

Note that there is a behavior change in the As, AsIID and CopyTo methods in the Windows 10 SDK. Previously, after calling any of these methods, you could check the WeakRef for nullptr to determine if a strong reference was successfully obtained, as in the following code:

WeakRef wr;  
strongComptrRef.AsWeak(&wr);  
  
// Now suppose that the object strongComPtrRef points to no longer exists  
// and the following code tries to get a strong ref from the weak ref:  
ComPtr<ISomeInterface> strongRef;  
HRESULT hr = wr.As(&strongRef);  
  
// This check won't work with the Windows 10 SDK version of the library.  
// Check the input pointer instead.  
if(wr == nullptr)  
{  
    wprintf(L"Couldn’t get strong ref!");  
}

The above code does not work when using the Windows 10 SDK (or later). Instead, check the pointer that was passed in for nullptr.

if (strongRef == nullptr)  
{  
    wprintf(L"Couldn't get strong ref!");  
 }

Members

Public Constructors

Name	Description
WeakRef::WeakRef Constructor	Initializes a new instance of the WeakRef class.
WeakRef::~WeakRef Destructor	Deinitializes the current instance of the WeakRef class.

Public Methods

Name	Description
WeakRef::As Method	Sets the specified ComPtr pointer parameter to represent the specified interface.
WeakRef::AsIID Method	Sets the specified ComPtr pointer parameter to represent the specified interface ID.
WeakRef::CopyTo Method	Assigns a pointer to an interface, if available, to the specified pointer variable.

Public Operators

Name	Description
WeakRef::operator& Operator	Returns a ComPtrRef object that represents the current WeakRef object.

Inheritance Hierarchy

ComPtr

WeakRef

Requirements

Header: client.h

Namespace: Microsoft::WRL

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