Rules for Freeing BSTRs in OLE Automation

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OLE Automation defines the BSTR data type to handle strings that are allocated by one component and freed by another. The rule for freeing a BSTR is as follows:

The callee frees a BSTR passed in as a by-reference parameter before assigning the parameter a new value. In all other cases, the caller frees the BSTR.

In other words, the caller handles BSTRs as follows:

1. Free BSTR returned by called function or returned through a by- reference parameter of called function.
2. Free BSTR passed as a by-value parameter to a function.

The callee handles BSTRs as follows:

1. Free BSTR passed in as a by-reference parameter before assigning a new value to the parameter. Do not free if a new value is not assigned to the by-reference parameter.
2. Do not free BSTR passed in as a by-value parameter.
3. Do not free BSTR that has been returned to caller.
4. If a BSTR passed in by the caller is to be stored by the callee, store a copy using SysAllocString(). The caller will release the BSTR that it passed in.
5. If a stored BSTR is to be returned, return a copy of the BSTR. The caller will release the BSTR that is returned.