pExcepInfo

pExcepInfo is a parameter used in COM automation to convey detailed exception information from a called method back to the caller. It is a pointer to an EXCEPINFO structure and is most commonly involved when a method invoked via the IDispatch::Invoke function fails and returns the DISP_E_EXCEPTION error code. When this occurs, the callee may populate the EXCEPINFO structure pointed to by pExcepInfo with descriptive error information for the caller.

The EXCEPINFO structure contains several fields intended to describe the error in a language- and locale-independent way. Typical members include bstrSource (the component or source of the error), bstrDescription (a human-readable description), bstrHelpFile (an optional help file path), dwHelpContext (an optional help context identifier), and wCode (a numeric error code). There are additional reserved fields and a potential deferred-fill function for advanced scenarios. The exact layout and field names are defined in the Windows headers (OleAuto.h).

Usage considerations: when a method invocation uses pExcepInfo to report an error, the caller can inspect the provided strings to present a meaningful message to the user or log details for debugging. Any BSTR strings allocated in EXCEPINFO are the responsibility of the recipient to release (typically with SysFreeString). The pExcepInfo mechanism is distinct from, but related to, the IErrorInfo interface; IErrorInfo provides thread-level error information, while EXCEPINFO conveys per-call details.

In practice, pExcepInfo is mainly encountered by developers implementing or consuming automation interfaces that rely on IDispatch::Invoke, and it serves to deliver structured, localized error information across language and boundary gaps.