Null references (oh no, not again!)

[Don]

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.