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:

1. 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:

HRESULT CMyWebBrowser::put_StatusText(BSTR bstr);

// shows using the Win32 function // to allocate memory for the string: BSTR bstrStatus = ::SysAllocString(L"Some text"); if (bstrStatus != NULL) { pBrowser->put_StatusText(bstrStatus); // Free the string: ::SysFreeString(bstrStatus); }

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

HRESULT CMyWebBrowser::get_StatusText(BSTR* pbstr);

BSTR bstrStatus; pBrowser->get_StatusText(&bstrStatus); // shows using the Win32 function // to free the memory for the string: ::SysFreeString(bstrStatus);

3. 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:

HRESULT CMyClass::get_StatusText(BSTR* pbstr) { try { //m_str is a CString in your class *pbstr = m_str.AllocSysString(); } catch (...) { return E_OUTOFMEMORY; } // The client is now responsible for freeing pbstr. return(S_OK); } 

 

 

 

http://msdn.microsoft.com/en-us/library/xda6xzx7.aspx