allocating-and-releasing-memory-for-a-bstr.md

title	Allocating and Releasing Memory for a BSTR
ms.date	11/04/2016
f1_keywords	bstr
helpviewer_keywords	BSTRs, memory allocation memory deallocation, string memory memory [C++], releasing memory allocation, BSTRs memory deallocation, BSTR memory strings [C++], releasing
ms.assetid	98041e29-3442-4a02-b425-7a4a13e9cc84

Allocating and Releasing Memory for a BSTR

When you create BSTRs and pass them between COM objects, you must take care in treating the memory they use in order to avoid memory leaks. When a BSTR stays within an interface, you must free its memory when you are done with it. However, when a BSTR passes out of an interface, the receiving object takes responsibility for its memory management.

In general, the rules for allocating and releasing memory allocated for BSTRs are as follows:

• When you call into a function that expects a BSTR argument, you must allocate the memory for the BSTR before the call and release it afterwards. For example:

  [!code-cppNVC_ATLMFC_Utilities#192]

  [!code-cppNVC_ATLMFC_Utilities#193]

• When you call into a function that returns a BSTR, you must free the string yourself. For example:

  [!code-cppNVC_ATLMFC_Utilities#194]

  [!code-cppNVC_ATLMFC_Utilities#195]

• When you implement a function that returns a BSTR, allocate the string but do not free it. The receiving the function releases the memory. For example:

  [!code-cppNVC_ATLMFC_Utilities#196]

See also

Strings (ATL/MFC)
CStringT::AllocSysString
SysAllocString
SysFreeString