Null references (oh no, not again!)

[Denis Koroskin]

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