[PATCH 15/15] api-ms-win-core-winrt-string: Implement a few HSTRING functions.

[Martin Storsjö]

Hi Sebastian,

On Mon, 1 Dec 2014, Sebastian Lackner wrote:

>>> * http://msdn.microsoft.com/en-us/library/br224632(v=vs.85).aspx
>>> mentions: """Calling WindowsDeleteString decrements the reference
>>> count of the backing buffer, and if the reference count reaches 0,
>>> the Windows Runtime de-allocates the buffer."""
>>>
>>> I don't see the concept of refcounted backing buffers anywhere in
>>> your code, which is most likely wrong. Its difficult to fix that
>>> later.
>>
>> I had a really hard time understanding the MSDN part here - I don't
>> find any corresponding function for increasing a refcount, so I don't
>> see how the HSTRING itself would be refcounted. So I think that only
>> matters once you pass those string handles into the full Windows
>> Runtime itself.
>
> I think the main purpose of that is to implement fast versions for 
> functions like SubString(...). If its not created with 
> WindowsCreateStringReference(...) both strings can use the same backing 
> buffer, which is faster than copying part of the string.
>
> Normally strings are immutable, but as it is also possible to get a raw 
> pointer to the buffer ( 
> http://msdn.microsoft.com/en-us/library/br224636(v=vs.85).aspx ) an 
> application might depend on this specific implementation detail. Tests 
> would really help here to confirm my theory that changing the substring 
> also modifies the original one.

I did test this now, and no, a string created with WindowsSubstring 
doesn't share the same backing buffer as the original string it was based 
on (created with WindowsCreateString), so changes to either of them via 
the pointer from WindowsGetStringRawBuffer doesn't affect the other one.

So therefore I don't really see where reference counting would come into 
play with this API at all, unless the Windows Runtime hooks into HSTRING 
objects via some other (non-public) functions. Or is CloseHandle supposed 
to be able to handle HSTRINGs, and is there any corresponding function 
that would increase the reference count of any generic HANDLE?

// Martin