Hello gentlemen:

I'm trying to translate the newest SQLite C header to D, and I stumbled unto this gem:
> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(void);

What's that? A function pointer that takes another function pointer as its name? I'm stuck at this and I don't know how to convert it to a D function pointer. Certainly, the inner pointer is easy:
> void (* function(sqlite3_vfs*,void*, const(char) *zSymbol) xDlSym)();

But what about the outer one? I am missing something?

Thanks in advance.

-- 
Yao G.

On Wed, 15 Sep 2010 17:05:24 -0400, Yao G. <yao.gomez@spam.gmail.com> wrote:

> Hello gentlemen:
>
> I'm trying to translate the newest SQLite C header to D, and I stumbled unto this gem:
>> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(void);
>
> What's that? A function pointer that takes another function pointer as its name? I'm stuck at this and I don't know how to convert it to a D function pointer. Certainly, the inner pointer is easy:
>> void (* function(sqlite3_vfs*,void*, const(char) *zSymbol) xDlSym)();
>
> But what about the outer one? I am missing something?
>
> Thanks in advance.

D supports C-style function pointers.  See here:

typedef char sqlite3_vfs;

// note you can't use void as a parameter type in D
void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(/*void*/);
pragma(msg, typeof(xDlSym).stringof);

outputs:

void function() function(sqlite3_vfs*, void*, const const(char*) zSymbol)

so I think it's a function pointer that takes those parameters and returns a function pointer that takes no parameters and returns nothing.

-Steve

On Wed, 15 Sep 2010 17:15:12 -0400, Steven Schveighoffer <schveiguy@yahoo.com> wrote:

> On Wed, 15 Sep 2010 17:05:24 -0400, Yao G. <yao.gomez@spam.gmail.com> wrote:
>
>> Hello gentlemen:
>>
>> I'm trying to translate the newest SQLite C header to D, and I stumbled unto this gem:
>>> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(void);
>>
>> What's that? A function pointer that takes another function pointer as its name? I'm stuck at this and I don't know how to convert it to a D function pointer. Certainly, the inner pointer is easy:
>>> void (* function(sqlite3_vfs*,void*, const(char) *zSymbol) xDlSym)();
>>
>> But what about the outer one? I am missing something?
>>
>> Thanks in advance.
>
> D supports C-style function pointers.  See here:
>
> typedef char sqlite3_vfs;
>
> // note you can't use void as a parameter type in D
> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(/*void*/);
> pragma(msg, typeof(xDlSym).stringof);
>
> outputs:
>
> void function() function(sqlite3_vfs*, void*, const const(char*) zSymbol)

BTW, not sure why the const const(char*)...

-Steve

On Wed, 15 Sep 2010 16:15:12 -0500, Steven Schveighoffer <schveiguy@yahoo.com> wrote:

> D supports C-style function pointers.  See here:
>
> typedef char sqlite3_vfs;
>
> // note you can't use void as a parameter type in D
> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(/*void*/);
> pragma(msg, typeof(xDlSym).stringof);
>
> outputs:
>
> void function() function(sqlite3_vfs*, void*, const const(char*) zSymbol)
>
> so I think it's a function pointer that takes those parameters and returns a function pointer that takes no parameters and returns nothing.
>
> -Steve

Thanks Steve!

-- 
Yao G.

Hello Steven,

> // note you can't use void as a parameter type in D
> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(/*void*/);
> pragma(msg, typeof(xDlSym).stringof);
> outputs:
> 
> void function() function(sqlite3_vfs*, void*, const const(char*)
> zSymbol)

D, now with C type un-garbleing!

-- 
... <IXOYE><

On Thu, 16 Sep 2010 10:06:24 -0400, BCS <none@anon.com> wrote:

> Hello Steven,
>
>> // note you can't use void as a parameter type in D
>> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(/*void*/);
>> pragma(msg, typeof(xDlSym).stringof);
>> outputs:
>>  void function() function(sqlite3_vfs*, void*, const const(char*)
>> zSymbol)
>
> D, now with C type un-garbleing!

I'd have to say, if I wasn't able to use D to do this, it would have taken me hours to figure this one out.  Even knowing what it is now, I still can't read it :)

D is a huge leap ahead of C in this regard!

-Steve

On 16/09/2010 15:06, BCS wrote:
> Hello Steven,
>
>> // note you can't use void as a parameter type in D
>> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(/*void*/);
>> pragma(msg, typeof(xDlSym).stringof);
>> outputs:
>>
>> void function() function(sqlite3_vfs*, void*, const const(char*)
>> zSymbol)
>
> D, now with C type un-garbleing!

Perhaps the only excuse for keeping C-style function pointer declarations in D.

But since we have htod, we could just as well use it and leave D free to get rid of this fossil that leads to a syntactic ambiguity.  Except that I've just found htod has a bug meaning it rejects this code.

Stewart.

Hello Steven,

> On Thu, 16 Sep 2010 10:06:24 -0400, BCS <none@anon.com> wrote:
> 
>> Hello Steven,
>> 
>>> // note you can't use void as a parameter type in D
>>> void (*(*xDlSym)(sqlite3_vfs*,void*, const char
>>> *zSymbol))(/*void*/);
>>> pragma(msg, typeof(xDlSym).stringof);
>>> outputs:
>>> void function() function(sqlite3_vfs*, void*, const const(char*)
>>> zSymbol)
>> D, now with C type un-garbleing!
>> 
> I'd have to say, if I wasn't able to use D to do this, it would have
> taken  me hours to figure this one out.  Even knowing what it is now,
> I still  can't read it :)
> 

The trick is that function pointers are best read from the inside out.
-- 
... <IXOYE><

On 16/09/2010 15:37, Steven Schveighoffer wrote:
> On Thu, 16 Sep 2010 10:06:24 -0400, BCS <none@anon.com> wrote:
>
>> Hello Steven,
>>
>>> // note you can't use void as a parameter type in D
>>> void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(/*void*/);
>>> pragma(msg, typeof(xDlSym).stringof);
>>> outputs:
>>> void function() function(sqlite3_vfs*, void*, const const(char*)
>>> zSymbol)
>>
>> D, now with C type un-garbleing!
>
> I'd have to say, if I wasn't able to use D to do this, it would have
> taken me hours to figure this one out. Even knowing what it is now, I
> still can't read it :)
<snip>

It took me a moment as well.  Basically, the C declaration

ReturnType (*Name)(Parameters);

becomes in D

ReturnType function(Parameters) Name;

Where you've got one inside another, you try to apply the same principle.  So

void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(void);

starting with the outermost level, you end up with (note (void) becomes ())

void function() (*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol);

instead of Name, you've got this funny thing.  You're left with a function pointer declaration where void function() is the ReturnType, so transforming it again you get

void function() function(sqlite3_vfs*, void*, char* zSymbol) xDlSym;
or in D2,
void function() function(sqlite3_vfs*, void*, const(char)* zSymbol) xDlSym;

Whoever wrote the C declaration must have had an even harder job getting it right!  I wouldn't have hesitated to use a typedef, if ever I had reason to do C stuff as complicated as this.

Stewart.

BCS Wrote:

> The trick is that function pointers are best read from the inside out.
> -- 
All C declarations are read from inside out, postfixes take precedence, that's why you have to use braces to give pointer higher precedence. One of the earlier books by Stroustroup gives a nice monster of arrays, pointers and functions to master understanding of declarations.