使用COM进行异常安全的内存处理

Question

When using COM, I typically rely on ATL smart pointers, like ATL::CComPtr and ATL::CComBSTR , for resource management. But some of the methods I'm calling use output parameters to return pointers to allocated storage that I have to free. For example:

WCHAR *pszName = nullptr;
if (SUCCEEDED(pShellItem->GetDisplayName(SIGDN_FILESYSPATH, &pszName))) {
  DoSomething(pszName);
  CoTaskMemFree(pszName);
}

Note that GetDisplayName allocates memory for the string and returns a pointer to it through the output parameter. It's the caller's responsibility to free that memory with CoTaskMemFree .

If DoSomething throws an exception, then the above code will leak. I'd like to use some sort of smart pointer for pszName to avoid such leaks, but the API takes WCHAR** , so I don't see how I can pass anything but the address of a dumb pointer. Since I'm not the one allocating, I can't use RAII.

I can use RRID if I can make a deleter like this:

struct CoTaskMemDeleter {
  void operator()(void *p) { ::CoTaskMemFree(p); }
};

And then immediately assign the returned pointer to a standard smart pointer like this:

WCHAR *pszName = nullptr;
if (SUCCEEDED(pShellItem->GetDisplayName(SIGDN_FILESYSPATH, &pszName))) {
  std::unique_ptr<WCHAR, CoTaskMemDeleter> guard(pszName);
  DoSomething(pszName);
}

That works, but it seems error-prone to introduce an extra guard variable. For example, this approach leaves pszName pointing at the released memory, so it would be easy to accidentally use it again.

Is there a cleaner way to use a smart pointer or an RAII wrapper for COM-server allocated memory returned by output parameter? Am I missing something that ATL provides?

Answer 1

ATL already has this one out of the box:

CComHeapPtr<WCHAR> pszName;
const HRESULT nResult = pShellItem->GetDisplayName(..., &pszName);
// Hooray, pszName will be CoTaskMemFree'd for you on scope leave 
// via ~CComHeapPtr

CHeapPtr can be derived to implement other resource releasers in a similar way. CComHeapPtr is a MSDN documented class .