Careful with & operator on CComBSTR variables.

Be careful when using the & operator on a CComBSTR variable. If this variable is not empty, using the & operator on it to point it to a new BSTR will lead to a memory leak.

For instance,

if you have a variable bsz1

declared and initialized as:

CComBSTR bsz1("HiWorld");

the member BSTR is allocated by issuing a call to SysAllocString. If you use somewhere after that in your code (&bsz1), you will cause a memory leak.

You can call bsz1.empty() before you use &bsz1 to prevent from the memory leak.

Cheers.

Elyasse

Comments

• Anonymous
  October 28, 2004
  What's really strange is that unlike CComPtr, CComBSTR doesn't even assert that the pointer is NULL in its operator&.
  
  I heard that this might be fixed in Whidbey, but in the meantime we manually added ATLASSERT(m_bstr == NULL) to our ATL sources. This single change has allowed us to find somewhere around 10-15 BSTR leaks, and generated only a few false positives.
• Anonymous
  October 29, 2004
  Im confused. Can you explain this a little more? Simply taking the address of a CComBSTR causes a memory leak? I dont get it.
  
• Anonymous
  October 29, 2004
  Taking the address does not cause the leak. Overwritng the pointer with something else does:
  
  CComPtr<IXMLNode> spNode1, spNode2;
  
  CComBSTR bstr;
  spNode1->get_text (&bstr);
  spNode2->get_text (&bstr);
  
  The second get_text() call leaks the BSTR allocated by the first one.