Re: Exception handling?
On Tue, 22 Jun 2010 17:27:17 -0400, Joseph M. Newcomer
<newcomer@flounder.com> wrote:
Part of the problem is that not everything is encapsulable in RAII. For example, if I
should return a pointer to an object, the object must have a lifetime that exceeds scope.
But if there is a failure, I should free the storage up. RAII doesn't handle this
gracefully. Other examples include "either I will return a handle or no handle will be
allocated" but you have to allocate the handle to make progress. RAII is powerful,
partcularly with smart pointers, but it is always based on the premise that leaving scope
means deletion of whatever resources are consumed, and that is not always feasible.
That's really not a problem. Smart pointer classes can have a "release"
function, that returns the encapsulated pointer and relinquishes ownership.
For example, here's a function I wrote to get an IShellFolder interface
(encapsulated in a COM smart pointer) and optionally return a PIDL for an
absolute path:
IShellFolderPtr
GetFolder(
const wchar_t* path,
LPITEMIDLIST* pidlFolder = 0,
HWND hWnd = 0)
{
IShellFolder* pDesktop = GetDesktop();
nc_ITEMIDLIST_ptr pidl(GetPidl(pDesktop, path, hWnd));
IShellFolder* pFolder;
Co::Vet(pDesktop->BindToObject(
pidl.get(), 0, IID_IShellFolder, (void**) &pFolder));
if (pidlFolder)
*pidlFolder = pidl.release();
return IShellFolderPtr(pFolder, false);
}
I used the nc_ITEMIDLIST_ptr ("nc" means non-copyable i.e. not
reference-counted and not weirdly copyable like auto_ptr) to protect the
PIDL across the call to Co::Vet, which is my function to turn HRESULT
errors into exceptions. After the IShellFolder pointer is obtained, if
pidlFolder is non-NULL, the pointer is released from pidl and stored to it,
transferring ownership to the caller. If pidlFolder is NULL, the PIDL is
deleted when the function returns and pidl is destroyed. The creation of
the IShellFolderPtr cannot fail, so this is exception-safe. (The GetDesktop
result is a cached value that is allocated on first call, so it doesn't
leak if an exception is thrown.)
Using RAII like this can linearize a function that would otherwise be
cluttered with multiple try/catch blocks or deeply nested ifs or serial ifs
with multiple goto targets at the bottom of the function.
--
Doug Harrison
Visual C++ MVP