Null references (oh no, not again!)

[Max Samukha]

On Thu, 05 Mar 2009 09:09:42 +0100, Don <nospam at nospam.com> wrote:

>Denis Koroskin wrote:
>> On Wed, 04 Mar 2009 19:39:06 +0300, Don <nospam at nospam.com> wrote:
>> 
>>> Andrei Alexandrescu wrote:
>>>> Don wrote:
>>>>> Andrei Alexandrescu wrote:
>>>>>> And there is no reference type with two subtypes. It's one type in 
>>>>>> the language and one in the library. Maybe-null (the library) is a 
>>>>>> supertype of non-null (the default).
>>>>>
>>>>> One problem I can see is with extern(C),(Windows) functions, since 
>>>>> pointers are maybe-null in C. The name-mangling has to work out.
>>>>> I can't see how this can be done without the compiler knowing 
>>>>> SOMETHING about both nullable and non-nullable types.
>>>>> At the bare minimum, you need to deal with maybe-null returns and 
>>>>> reference parameters from C functions.
>>>>  Walter is thinking of making only references non-null and leaving 
>>>> pointers as they are. (I know, cry of horror.) But say pointers are 
>>>> also non-null. Then:
>>>>  extern(C) MaybeNull!(void*) malloc(size_t s);
>>>>  will work, provided that MaybeNull has no size overhead and that 
>>>> word-sized structs are returned in the same register as word returns 
>>>> (I seem to remember Walter told me that's the case already).
>>>
>>> Here's a typical annoying Windows API function
>>> --------
>>> int GetTextCharsetInfo(
>>>    HDC hdc,                // handle to DC
>>>    LPFONTSIGNATURE lpSig,  // data buffer
>>>    DWORD dwFlags           // reserved; must be zero
>>> );
>>>
>>> lpSig
>>> [out] Pointer to a FONTSIGNATURE data structure that receives 
>>> font-signature information. The lpSig parameter can be NULL if you do 
>>> not need the FONTSIGNATURE information.
>>> ---------
>>> How do you do this?
>>>
>>> Don.
>> 
>> extern(System) int GetTextCharsetInfo(
>>     HDC hdc,
>>     MaybeNull!(FONTSIGNATURE*) lpSig, // or whatever
>>     DWORD dwFlags);
>> 
>> GetTextCharsetInfo(hdc, null, flags); // fine
>> GetTextCharsetInfo(hdc, &sig, flags); // also ok
>> 
>But it needs to have the type name mangled as LPFONTSIGNATURE, not as 
>MaybeNull!(FONTSIGNATURE*). Otherwise it can't link to Windows.

Parameter names are not mangled for stacall, only their total size.
IIRC, the above will get mangled into _GetTextCharsetInfo at 12 just like
it would without MaybeNull