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WeakRef class

  • Artikkeli

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>;

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

Public operators

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

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.

There's a behavior change in the As, AsIID, and CopyTo methods in the Windows 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 doesn't 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!");
}

Inheritance hierarchy

ComPtr
 └ WeakRef

Requirements

Header: client.h

Namespace: Microsoft::WRL

WeakRef::WeakRef constructor

Initializes a new instance of the WeakRef class.

WeakRef();
WeakRef(
   decltype(__nullptr)
);

WeakRef(
   _In_opt_ IWeakReference* ptr
);

WeakRef(
   const ComPtr<IWeakReference>& ptr
);

WeakRef(
   const WeakRef& ptr
);

WeakRef(
   _Inout_ WeakRef&& ptr
);

Parameters

ptr
A pointer, reference, or rvalue-reference to an existing object that initializes the current WeakRef object.

Remarks

The first constructor initializes an empty WeakRef object. The second constructor initializes a WeakRef object from a pointer to the IWeakReference interface. The third constructor initializes a WeakRef object from a reference to a ComPtr<IWeakReference> object. The fourth and fifth constructors initialize a WeakRef object from another WeakRef object.

WeakRef::~WeakRef destructor

Deinitializes the current instance of the WeakRef class.

~WeakRef();

WeakRef::As

Sets the specified ComPtr pointer parameter to represent the specified interface.

template<typename U>
HRESULT As(
   _Out_ ComPtr<U>* ptr
);

template<typename U>
HRESULT As(
   _Out_ Details::ComPtrRef<ComPtr<U>> ptr
);

Parameters

U
An interface ID.

ptr
When this operation completes, an object that represents parameter U.

Return value

  • S_OK if this operation succeeds; otherwise, an HRESULT that indicates the reason the operation failed, and ptr is set to nullptr.

  • S_OK if this operation succeeds, but the current WeakRef object has already been released. Parameter ptr is set to nullptr.

  • S_OK if this operation succeeds, but the current WeakRef object isn't derived from parameter U. Parameter ptr is set to nullptr.

Remarks

An error is emitted if parameter U is IWeakReference, or isn't derived from IInspectable.

The first template is the form that you should use in your code. The second template is an internal, helper specialization; it supports C++ language features such as the auto type deduction keyword.

Starting in the Windows 10 SDK, this method doesn't set the WeakRef instance to nullptr if the weak reference couldn't be obtained, so you should avoid error-checking code that checks the WeakRef for nullptr. Instead, check ptr for nullptr.

WeakRef::AsIID

Sets the specified ComPtr pointer parameter to represent the specified interface ID.

HRESULT AsIID(
   REFIID riid,
   _Out_ ComPtr<IInspectable>* ptr
);

Parameters

riid
An interface ID.

ptr
When this operation completes, an object that represents parameter riid.

Return value

  • S_OK if this operation succeeds; otherwise, an HRESULT that indicates the reason the operation failed, and ptr is set to nullptr.

  • S_OK if this operation succeeds, but the current WeakRef object has already been released. Parameter ptr is set to nullptr.

  • S_OK if this operation succeeds, but the current WeakRef object isn't derived from parameter riid. Parameter ptr is set to nullptr. (For more information, see Remarks.)

Remarks

An error is emitted if parameter riid isn't derived from IInspectable. This error supersedes the return value.

The first template is the form that you should use in your code. The second template (not shown here, but declared in the header file) is an internal, helper specialization that supports C++ language features such as the auto type deduction keyword.

Starting in the Windows 10 SDK, this method doesn't set the WeakRef instance to nullptr if the weak reference couldn't be obtained, so you should avoid error-checking code that checks the WeakRef for nullptr. Instead, check ptr for nullptr.

WeakRef::CopyTo

Assigns a pointer to an interface, if available, to the specified pointer variable.

HRESULT CopyTo(
   REFIID riid,
   _Deref_out_ IInspectable** ptr
);

template<typename U>
HRESULT CopyTo(
   _Deref_out_ U** ptr
);

HRESULT CopyTo(
   _Deref_out_ IWeakReference** ptr
);

Parameters

U
Pointer an IInspectable interface. An error is emitted if U isn't derived from IInspectable.

riid
An interface ID. An error is emitted if riid isn't derived from IWeakReference.

ptr
A doubly indirect pointer to IInspectable or IWeakReference.

Return value

S_OK if successful; otherwise, an HRESULT that describes the failure. For more information, see Remarks.

Remarks

A return value of S_OK means that this operation succeeded, but doesn't indicate whether the weak reference was resolved to a strong reference. If S_OK is returned, test that parameter ptr is a strong reference; that is, parameter ptr isn't equal to nullptr.

Starting in the Windows 10 SDK, this method doesn't set the WeakRef instance to nullptr if the weak reference couldn't be obtained, so you should avoid error checking code that checks the WeakRef for nullptr. Instead, check ptr for nullptr.

WeakRef::operator&

Returns a ComPtrRef object that represents the current WeakRef object.

Details::ComPtrRef<WeakRef> operator&() throw()

Return value

A ComPtrRef object that represents the current WeakRef object.

Remarks

WeakRef::operator& is an internal helper operator that's not meant to be used in your code.