_bstr_t memory leak

Question

I have a c++ code. But it is not releasing memory properly. Tell me where I am wrong, here is my code

1 void MyClass::MyFunction(void)
2 {
3    for (int i=0; i<count; i++)
4    {
5        _bstr_t xml = GetXML(i);
6        // some work
7        SysFreeString(xml);
8    }
9 }

GetXML (line 5) returns me a BSTR. At this memory of program increases. But after SysFreeString (line 7) memory does not release. What I am doing wrong here?

Answer 1

First:

// This makes a copy.
// This is where the leak is. You are leaking the original string.
_bstr_t xml = GetXML();

// You want to use this, to attach the BSTR to the _bstr_t
_bstr_t xml = _bstr_t(GetXML(), false);

Second, don't do this:

SysFreeString(xml); 

The _bstr_t class will do that for you.

Third, BSTR will not release the memory to the OS immediately, it caches recently used strings in order to make SysAllocString faster. You shouldn't expect to see memory usage go straight down after SysFreeString.

You can control this behaviour for debugging purposes:

• http://support.microsoft.com/default.aspx?scid=kb;en-us;Q139071

Lastly, when viewing memory usage in Task Manager you need to look at the column "Commit Size" not "Working Set". Go to Menu->View->Select Columns to show the column. And also note that this really only helps over a period of time - the memory may not be released to the OS immediately, but if you have no leaks, it shouln't go up forever, over a course of hours.