REFIID

REFIID is a type used in Component Object Model (COM) programming on Windows. It represents a reference to an interface identifier (IID) and is used to specify which interface a caller wants to access.

Definition and typing: In C++, REFIID is defined as a reference to an IID, typically typedef const IID& REFIID. In C, where references do not exist, it resolves to a pointer to a const IID, typically typedef const IID* REFIID. The IID itself is a GUID that uniquely identifies a COM interface.

Usage: REFIID is most commonly seen as the type of the riid parameter in COM methods such as QueryInterface, for example: HRESULT QueryInterface(REFIID riid, void** ppvObject). The riid parameter identifies the requested interface. The implementation checks the IID and, if supported, returns a pointer to that interface through ppvObject; otherwise, it returns E_NOINTERFACE.

Relation to IID and GUID: IID stands for Interface Identifier and is defined as a GUID. REFIID simply describes passing that IID by reference (or by pointer in C) to functions, rather than passing the IID by value.

Examples: Well-known IIDs include IID_IUnknown and IID_IDispatch; many COM interfaces define their own IIDs, assigned as globally unique identifiers to ensure uniqueness across components.

Notes: REFIID is defined in Windows headers such as Objbase.h and OleIdl.h and is a common convention across COM APIs for identifying interfaces in a type-safe way.