Convert CComPtr<IShelltem2> to LPWSTR*? - c++

Question

I'm using a variable of type CComPtr and I need to modify a LPWSTR* variable. The function I use extracts metadata about file description for executable files. I am not sure about how I should allocate memory for the LPWSTR* and how to change its value to the one of the CComPtr. lpszFileDesc must get the value of description.
BOOL ExeDescription(LPWSTR* lpszFileDesc, LPCWSTR filePath)
{
CComPtr<IShellItem2> item;
HRESULT hr = CoInitialize(nullptr);
*lpszFileDesc = NULL;
BOOL fResult = TRUE;
hr = SHCreateItemFromParsingName(filePath, nullptr, IID_PPV_ARGS(&item));
if (FAILED(hr))
{
fResult = FALSE;
}
else
{
CComPtr<WCHAR> description;
hr = item->GetString(PKEY_FileDescription, &description);
if (FAILED(hr))
{
fResult = FALSE;
}
else
{
if (!description)
{
*lpszFileDesc = PathFindFileNameW(filePath);
}
else
{
// here I want to copy the contents of description
// into lpszFileDesc but I don't know how
}
if (!*lpszFileDesc)
{
fResult = FALSE;
}
}
}
CoUninitialize();
return fResult;
}
Also, when I call this function how do I deallocate the memory for lpszFileDesc after calling the function?
For example if in wmain() I have:
LPWSTR* lpszFileDesc;
ExeDescription(LPWSTR* lpszFileDesc, LPCWSTR filePath);
How do I deallocate the memory if I don't need the file description after that?

Answer 1

Basic Errors
HRESULT hr = CoInitialize(nullptr);
...
CoUninitialize();
COM should be initialized only once at startup of the thread, because it defines the concurrency model of the thread (amongst other things). It's not up to your function to decide how COM will be initialized for the thread. Once COM is initialized for a thread, subsequent calls to CoInitialize[Ex] within that thread will fail anyway. So remove this code and put it into WinMain or the main function of the thread where you are using COM.
CComPtr<WCHAR> description;
Using CComPtr is wrong here, because IShellItem2::GetString() does not return an interface, but a simple C string. Such "raw" memory allocated by COM API must be freed using CoTaskMemFree(), which can be automated by using CComHeapPtr.
Preferred solution - change the interface
how do I deallocate the memory for lpszFileDesc
Do yourself a favor and use std::wstring instead of raw C string pointer to return a string from your function. The std::wstring destructor takes care of deallocation automatically. Manually managing the memory of C strings is too cumbersome and error-prone. When someone else reads your code and sees std::wstring, there will be no question about how the memory is managed.
I suggest to change your interface like this:
BOOL ExeDescription(std::wstring& fileDesc, LPCWSTR filePath);
... and the assignment within the function body becomes:
if (!description)
{
fileDesc = PathFindFileNameW(filePath);
}
else
{
fileDesc = description;
}
CComHeapPtr<WCHAR> has a conversion operator to WCHAR*, that's why the assignment to std::wstring simply works.
Call the function like this:
std::wstring fileDesc;
ExeDescription(fileDesc, filePath);
// No worries about deallocation of fileDesc!
Solution using original interface
That being said, here is a solution using your original interface. You can either use the COM allocator, as IShellItem2::GetString() already uses it (and there will be no copying in the common case) or use a different allocator (then you always have to copy). In both cases, the caller is responsible to call the right deallocation function, which you have to document (another reason why I would prefer the std::wstring solution).
Example of using the COM allocator:
BOOL ExeDescription(LPWSTR* lpszFileDesc, LPCWSTR filePath)
{
// ... other code ...
// GetString() uses CoTaskMemAlloc() internally
hr = item->GetString(PKEY_FileDescription, lpszFileDesc);
// ... other code ...
if (! *lpszFileDesc )
{
LPCWSTR fileName = PathFindFileNameW(filePath);
// Allocate buffer using the COM allocator and copy fileName to it.
std::size_t const len = wcslen(fileName);
*lpszFileDesc = reinterpret_cast<LPWSTR>(CoTaskMemAlloc(len * sizeof(WCHAR)));
if(*lpszFileDesc)
wcscpy_s(*lpszFileDesc, len, fileName);
}
// ... more code ...
}
Usage at the caller site:
LPWSTR fileDesc = nullptr;
ExeDescription(&fileDesc, filePath);
// ... use fileDesc ...
CoTaskMemFree(fileDesc);
Simplified usage with CComHeapPtr:
CComHeapPtr<WCHAR> fileDesc;
ExeDescription(&fileDesc, filePath);
// ... use fileDesc ...
// Deallocation happens automatically through CComHeapPtr's destructor