winrt::Windows::Foundation::IUnknown struct (C++/WinRT)
Every C++/WinRT runtime class (whether a Windows or a third party runtime class) derives from winrt::Windows::Foundation::IUnknown. It represents the COM IUnknown interface, and it provides facilities such as querying for a different interface, abi functions, and comparison operators.
Syntax
struct IUnknown
Requirements
Minimum supported SDK: Windows SDK version 10.0.17134.0 (Windows 10, version 1803)
Namespace: winrt
Header: %WindowsSdkDir%Include<WindowsTargetPlatformVersion>\cppwinrt\winrt\base.h (included by default)
Constructors
| Constructor | Description |
|---|---|
| IUnknown::IUnknown constructor | Initializes a new instance of the IUnknown struct, optionally with a copy or move of the input data. |
Member functions
| Function | Description |
|---|---|
| IUnknown::as function | Returns the requested interface, if it is supported. Throws if it is not. |
| IUnknown::try_as function | Returns the requested interface, if it is supported. Returns null, or false, if it is not. |
Member operators
| Operator | Description |
|---|---|
| IUnknown::operator bool | Checks whether or not the IUnknown object is referencing an interface. |
| IUnknown::operator= (assignment operator) | Assigns a value to the IUnknown object. |
Free functions
| Function | Description |
|---|---|
| attach_abi function | Attaches an IUnknown object to a raw pointer that owns a reference to its target; an additional reference is not added. |
| copy_from_abi function | Copies to an IUnknown object from another pointer. Decrements the reference count on any currently referenced interface or object, copies the raw pointer parameter, and begins managing the lifetime of the interface or object pointed to by it. |
| copy_to_abi function | Copies to another pointer from an IUnknown object. Increments the reference count on any currently referenced interface or object, and copies that interface or object's memory address into the parameter. |
| detach_abi function | Detaches from the raw IUnknown interface without decrementing the reference count, perhaps to return it to a caller. |
| get_abi function | Returns the underlying raw IUnknown interface pointer should you need to pass it to a function. |
| get_unknown function | A helper function that returns the address of the underlying raw IUnknown interface of an object of a projected type. |
| put_abi function | Returns the address of the underlying raw IUnknown interface pointer as a pointer to a pointer to void; this function helps you call methods (such as COM methods) that return references as out parameters via a pointer to a pointer to void. |
| swap function | Swaps the contents of the two IUnknown parameters so that they point at one another's target. |
Free operators
| Function | Description |
|---|---|
| operator!= (inequality operator) | Returns a value indicating whether the two parameters refer to different targets. |
| operator< (less-than operator) | Returns a value indicating whether the first parameter's target occurs earlier in memory than that of the second parameter. |
| operator<= (less-than-or-equal-to operator) | Returns a value indicating whether the first parameter's target occurs earlier in memory than, or at the same location as, that of the second parameter. |
| operator== (equality operator) | Returns a value indicating whether the two parameters refer to the same interface and/or object. |
| operator> (greater-than operator) | Returns a value indicating whether the first parameter's target occurs later in memory than that of the second parameter. |
| operator>= (greater-than-or-equal-to operator) | Returns a value indicating whether the first parameter's target occurs later in memory than, or at the same location as, that of the second parameter. |
IUnknown::IUnknown constructor
Initializes a new instance of the IUnknown struct, optionally with a copy or move of the input data.
Syntax
IUnknown() noexcept;
IUnknown(std::nullptr_t) noexcept;
IUnknown(winrt::Windows::Foundation::IUnknown const& other) noexcept;
IUnknown(winrt::Windows::Foundation::IUnknown&& other) noexcept;
Parameters
other
Another IUnknown that initializes the IUnknown object.
IUnknown::as function
Returns the requested interface, if it is supported. Throws if it is not. This function is useful if you want to query for an interface that you don't need to pass back to your caller.
Syntax
template <typename To> auto as() const;
template <typename To> void as(To& to) const;
Template parameters
typename To
The type of the requested interface.
Parameters
to
A reference to a value to receive the requested interface.
Return value
A com_ptr referencing the requested interface, or a strongly-typed smart pointer for the requested interface (either declared by C++/WinRT or by a third party).
IUnknown::try_as function
Returns the requested interface, if it is supported. Returns null (the auto-returning overload), or false (the bool-returning overload), if it is not. This function is useful if you want to query for an interface that you don't need to pass back to your caller.
Syntax
template <typename To> auto try_as() const noexcept;
template <typename To> bool try_as(To& to) const noexcept;
Template parameters
typename To
The type of the requested interface.
Parameters
to
A reference to a value to receive the requested interface. Can be a null reference.
Return value
A com_ptr referencing the requested interface, or a strongly-typed smart pointer for the requested interface (either declared by C++/WinRT or by a third party), if the requested interface is supported, otherwise null (the auto-returning overload) or false (the bool-returning overload).
If to is a null reference, returns null or false.
IUnknown::operator bool
Checks whether or not the IUnknown object is referencing an interface. If the IUnknown object is not referencing an interface, then it is logically null; otherwise it is logically not null.
Syntax
explicit operator bool() const noexcept;
Return value
true if the IUnknown object is referencing an interface (logically not null), otherwise false (logically null).
IUnknown::operator= (assignment operator)
Assigns a value to the IUnknown object.
Syntax
winrt::Windows::Foundation::IUnknown& operator=(winrt::Windows::Foundation::IUnknown const& other) noexcept;
winrt::Windows::Foundation::IUnknown& operator=(winrt::Windows::Foundation::IUnknown&& other) noexcept;
winrt::Windows::Foundation::IUnknown& operator=(std::nullptr_t) noexcept;
Parameters
other
An IUnknown value to assign to the IUnknown object, either by copy or by move.
Return value
A reference to the IUnknown object.
attach_abi function
Attaches an IUnknown object to a raw pointer that owns a reference to its target; an additional reference is not added. If needed, you can use this function to coalesce references.
Syntax
void attach_abi(winrt::Windows::Foundation::IUnknown& object, void* value) noexcept;
Parameters
object
An IUnknown value to operate on.
value
A raw pointer that owns a reference to its target.
copy_from_abi function
Copies to an IUnknown object from another pointer. Decrements the reference count on any currently referenced interface or object, copies the raw pointer parameter, and begins managing the lifetime of the interface or object pointed to by it.
Syntax
void copy_from_abi(winrt::Windows::Foundation::IUnknown& object, void* value) noexcept;
Parameters
object
An IUnknown value to operate on.
value
A raw pointer to a target whose lifetime should be managed by the IUnknown object.
copy_to_abi function
Copies to another pointer from an IUnknown object. Increments the reference count on any currently referenced interface or object, and copies that interface or object's memory address into the parameter. This function lets you hand out a reference to the same interface without calling QueryInterface.
Syntax
void copy_to_abi(winrt::Windows::Foundation::IUnknown const& object, void*& value) noexcept;
Parameters
object
An IUnknown value to operate on.
value
A raw pointer reference; via which to copy the pointer to the IUnknown object's target.
detach_abi function
Detaches an IUnknown object from its raw IUnknown interface without decrementing the reference count, perhaps to return it to a caller.
Syntax
void* detach_abi(winrt::Windows::Foundation::IUnknown& object) noexcept;
void* detach_abi(winrt::Windows::Foundation::IUnknown&& object) noexcept;
Parameters
object
An IUnknown value to operate on.
Return value
A pointer to the raw IUnknown interface referenced by the IUnknown object.
get_abi function
Returns the underlying raw IUnknown interface pointer should you need to pass it to a function. You may call AddRef, Release, or QueryInterface on the returned pointer.
Syntax
void* get_abi(winrt::Windows::Foundation::IUnknown const& object) noexcept;
Parameters
object
An IUnknown value to operate on.
Return value
A pointer to the raw IUnknown interface referenced by the IUnknown object.
operator!= (inequality operator)
Returns a value indicating whether the two parameters refer to different targets.
Syntax
bool operator!=(winrt::Windows::Foundation::IUnknown const& left, winrt::Windows::Foundation::IUnknown const& right) noexcept;
Parameters
left right
An IUnknown value whose target's memory address to compare with that of the other parameter.
Return value
true if the two parameters point to different targets, otherwise false.
operator< (less-than operator)
Returns a value indicating whether the first parameter's target occurs earlier in memory than that of the second parameter.
Syntax
bool operator<(winrt::Windows::Foundation::IUnknown const& left, winrt::Windows::Foundation::IUnknown const& right) noexcept;
Parameters
left right
An IUnknown value whose target's memory address to compare with that of the other parameter.
Return value
true if the first parameter's target's memory address is less than that of the second parameter, otherwise false.
operator<= (less-than-or-equal-to operator)
Returns a value indicating whether the first parameter's target occurs earlier in memory than, or at the same location as, that of the second parameter.
Syntax
bool operator<=(winrt::Windows::Foundation::IUnknown const& left, winrt::Windows::Foundation::IUnknown const& right) noexcept;
Parameters
left right
An IUnknown value whose target's memory address to compare with that of the other parameter.
Return value
true if the first parameter's target's memory address is less than or equal to that of the second parameter, otherwise false.
operator== (equality operator)
Returns a value indicating whether the two parameters refer to the same interface and/or object.
Syntax
bool operator==(winrt::Windows::Foundation::IUnknown const& left, winrt::Windows::Foundation::IUnknown const& right) noexcept;
Parameters
left right
An IUnknown value whose target's memory address to compare with that of the other parameter.
Return value
true if the two parameters point to the same target, otherwise false.
operator> (greater-than operator)
Returns a value indicating whether the first parameter's target occurs later in memory than that of the second parameter.
Syntax
bool operator>(winrt::Windows::Foundation::IUnknown const& left, winrt::Windows::Foundation::IUnknown const& right) noexcept;
Parameters
left right
An IUnknown value whose target's memory address to compare with that of the other parameter.
Return value
true if the first parameter's target's memory address is greater than that of the second parameter, otherwise false.
operator>= (greater-than-or-equal-to operator)
Returns a value indicating whether the first parameter's target occurs later in memory than, or at the same location as, that of the second parameter.
Syntax
bool operator>=(winrt::Windows::Foundation::IUnknown const& left, winrt::Windows::Foundation::IUnknown const& right) noexcept;
Parameters
left right
An IUnknown value whose target's memory address to compare with that of the other parameter.
Return value
true if the first parameter's target's memory address is greater than or equal to that of the second parameter, otherwise false.
put_abi function
Returns the address of the underlying raw IUnknown interface pointer as a pointer to a pointer to void; this function helps you call methods (such as COM methods) that return references as out parameters via a pointer to a pointer to void.
Syntax
void** put_abi(winrt::Windows::Foundation::IUnknown& object) noexcept;
Parameters
object
An IUnknown value to operate on.
Return value
The address of the underlying raw IUnknown interface pointer.
swap function
Swaps the contents of the two IUnknown parameters so that they point at one another's target.
Syntax
void swap(winrt::Windows::Foundation::IUnknown& left, winrt::Windows::Foundation::IUnknown& right) noexcept;
Parameters
left right
An IUnknown value whose pointer to mutually swap with that of the other parameter.