When using CoTaskMemAlloc, should I always call CoTaskMemFree?
atl, com, memory-management
Solution
Using the CRT's provided new/malloc and delete/free is a problem in COM interop. To make them work, it is very important that the same copy of the CRT both allocates and releases the memory. That's impossible to enforce in a COM interop scenario, your COM server and the client are practically guaranteed to use different versions of the CRT. Each using their own heap to allocate from. This causes undiagnosable memory leaks on Windows XP, a hard exception on Vista and up.
Which is why the COM heap exists, a single predefined heap in a process that's used both by the server and the client. IMalloc is the generic interface to access that shared heap, CoTaskMemAlloc() and CoTaskMemFree() are the system provided helper functions to use that interface.
That said, this is only necessary in a case where the server allocates memory and the client has to release it. Or the other way around. Which should always be rare in an interop scenario, the odds for accidents are just too large. In COM Automation there are just two such cases, a BSTR and a SAFEARRAY, types that are already wrapped. You avoid it in other cases by having the method caller provide the memory and the callee fill it in. Which also allows a strong optimization, the memory could come from the caller's stack.
Review the code and check who allocates the memory and who needs to release it. If both exist in the same module then using new/malloc is fine because there's now a hard guarantee that the same CRT instance takes care of it. If that's not the case then consider fixing it so the caller provides the memory and releases it.
Problem
I'm writing some COM and ATL code, and for some reason all the code uses `CoTaskMemAlloc` to allocate memory instead of `new` or `malloc`. So I followed along this coding style and I also use `CoTaskMemAlloc`. My teachers taught me to always `delete` or `free` when allocating memory. However I'm not sure if I should always be calling `CoTaskMemFree` if I use `CoTaskMemAlloc`?