Re: [fpc-pascal] Untyped var params

2017-01-08 Thread Sven Barth 
On 08.01.2017 11:36, Ryan Joseph wrote:
> 
>> On Jan 8, 2017, at 5:22 PM, Sven Barth  wrote:
>>
>> 1. Why are you trying to assign something to io when you already did
>> that with "obj := …"?
> 
> I used absolute here so I could know the type of io and not type cast within 
> the function. No good? The idea is I want “io” to be assigned with the value 
> from newObject or set to nil if the input is nil. I know io is a member of 
> TObject but I don’t want type checking when I return the value from the 
> function otherwise all my calls look like RetainObject(TObject(foo), newFoo). 
> It’s a small annoyance but I thought I would take the time to learn Free 
> Pascal more since it always pays off in the end.

The use of absolute is absolutely (ha!) fine. However the second
assignment using "io" is superfluous as you already did the assignment
through the absolute alias.

Regards,
Sven
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Re: [fpc-pascal] Untyped var params

2017-01-08 Thread Ryan Joseph 

> On Jan 8, 2017, at 5:22 PM, Sven Barth  wrote:
> 
> 1. Why are you trying to assign something to io when you already did
> that with "obj := …"?

I used absolute here so I could know the type of io and not type cast within 
the function. No good? The idea is I want “io” to be assigned with the value 
from newObject or set to nil if the input is nil. I know io is a member of 
TObject but I don’t want type checking when I return the value from the 
function otherwise all my calls look like RetainObject(TObject(foo), newFoo). 
It’s a small annoyance but I thought I would take the time to learn Free Pascal 
more since it always pays off in the end.

Casting TObject(input) := obj; worked but the value of “io” is still not 
getting set. Because it’s a var I expected the value to be returned to the 
caller. 

Regards,
Ryan Joseph

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Re: [fpc-pascal] Untyped var params

2017-01-08 Thread Ryan Joseph 

> On Jan 8, 2017, at 5:36 PM, Ryan Joseph  wrote:
> 
> Casting TObject(input) := obj; worked but the value of “io” is still not 
> getting set. Because it’s a var I expected the value to be returned to the 
> caller. 

Never mind, I got it working, stupid mistake in my testing.

yes just like that function. I just learned about absolute so I was making us 
of it. Seems to work the same as casting in FreeAndNil. Thanks for helping.

procedure FreeAndNil(var obj);
  var
temp: tobject;
  begin
temp:=tobject(obj);
pointer(obj):=nil;
temp.free;
  end;


Regards,
Ryan Joseph

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Re: [fpc-pascal] Untyped var params

2017-01-08 Thread Sven Barth 
On 08.01.2017 10:35, Ryan Joseph wrote:
> 
>> On Jan 8, 2017, at 2:37 AM, Andrew Hall  wrote:
>>
>> If you cast your “something” to a typed pointer, the compiler will do the 
>> work for you in the usual way:
>>
>> PDouble(something)^ := myDouble;
>> PAnsiString(something)^ := myAnsiString;
>> myRecordStructure  := PMyRecordStructure(something)^;
> 
> I’m not getting any of this to work. 
> 
> Here is the pattern I was attempting. It’s a “release existing, retain and 
> assign new” function which handles nil inputs safely. I thought I could use 
> untyped params so I don’t need to typecast “io” to TObject when I call 
> RetainObject every time.
> 
> procedure RetainObject (var io; newObject: TObject);
> var
>   obj: TObject absolute io;
> begin
>   if obj <> nil then
>   obj.Release;
>   if newObject <> nil then
>   begin
>   obj := newObject;
>   obj.Retain;
>   end
>   else
>   obj := nil;
>   
>   TObjectPtr(io)^ := obj; // crashes here
> end;

1. Why are you trying to assign something to io when you already did
that with "obj := ..."?
2. TObject is an implicit pointer, so "TObject(io) := ..." would be
correct (see the implementation of FreeAndNil() in
$fpc/rtl/objpas/sysutils/sysutils.inc) [otherwise your usage of "obj"
would already crash]

Regards,
Sven
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Re: [fpc-pascal] Untyped var params

2017-01-08 Thread Ryan Joseph 

> On Jan 8, 2017, at 2:37 AM, Andrew Hall  wrote:
> 
> If you cast your “something” to a typed pointer, the compiler will do the 
> work for you in the usual way:
> 
> PDouble(something)^ := myDouble;
> PAnsiString(something)^ := myAnsiString;
> myRecordStructure  := PMyRecordStructure(something)^;

I’m not getting any of this to work. 

Here is the pattern I was attempting. It’s a “release existing, retain and 
assign new” function which handles nil inputs safely. I thought I could use 
untyped params so I don’t need to typecast “io” to TObject when I call 
RetainObject every time.

procedure RetainObject (var io; newObject: TObject);
var
obj: TObject absolute io;
begin
if obj <> nil then
obj.Release;
if newObject <> nil then
begin
obj := newObject;
obj.Retain;
end
else
obj := nil;

TObjectPtr(io)^ := obj; // crashes here
end;


var
foo: TMyObject;
something: TMyObject;

foo := TMyObject.Create;
RetainObject(something, foo);


Regards,
Ryan Joseph

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Re: [fpc-pascal] Untyped var params

2017-01-08 Thread Sven Barth 
Am 08.01.2017 04:43 schrieb "Dmitry Boyarintsev" :
>
> On Sat, Jan 7, 2017 at 6:02 PM, Lars  wrote:
>>
>> Some brave soldiers once tried to reinvent generics using these tricks..
>
>
> Well, Pascal run-time has been using "hidden" generics forever.
>
> As an example:
> http://www.freepascal.org/docs-html/rtl/system/val.html
>
> Val() declares the second parameter as untyped var parameter.
> Unfortunately, it's not quite so. Compiler recognizes  the run-time
function and requires the parameter to be either an Integer or Float-point
type (or an enumerated type since 2.3.1).
>
> So it's more a generic-like behavior (with integer and float types
specialized)

Val() is a compiler intrinsic so there doesn't exist a declaration for it
anyway, so it's quite a bit outside of the scope of var parameters.

Regards,
Sven
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Dmitry Boyarintsev 
On Sat, Jan 7, 2017 at 6:02 PM, Lars  wrote:

> Some brave soldiers once tried to reinvent generics using these tricks..


Well, Pascal run-time has been using "hidden" generics forever.

As an example:
http://www.freepascal.org/docs-html/rtl/system/val.html

Val() declares the second parameter as untyped var parameter.
Unfortunately, it's not quite so. Compiler recognizes  the run-time
function and requires the parameter to be either an Integer or Float-point
type (or an enumerated type since 2.3.1).

So it's more a generic-like behavior (with integer and float types
specialized)

thanks,
Dmitry
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Dmitry Boyarintsev 
On Sat, Jan 7, 2017 at 6:53 PM, Bart  wrote:

> OK, I can imagine more complex examples where this may come in handy.
>
> And thanks for the rest of the explanation.


And only more complex examples should be used :)
There're much better ways to implement var-type variables.

I was always thinking that having untyped variables in Pascal is more a
syntactical advantage, whenever a Pascal need to target managed platforms
(i.e. Net or Java). These platforms don't any kind of pointers at all.
Thus a code written using pointers would not even compile on them.

Thus using open arrays (of bytes) and untyped variables would be a good
replacement for pointers whenever writing a cross-platform code (meaning a
code that would need to target both non-managed and managed platforms)

Not sure, but pascal JVM has its own means of dealing with pointers anyway.
My knowledge is poor on the subject of jvm, but it's well explained in the
wiki!

thanks,
Dmitry
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Bart 
On 1/8/17, Dmitry Boyarintsev  wrote:

> Think about something like a var-type variable, where the actual type of Y
> might vary.
>
> procedure XX(var Y; YType: integer);
> begin
>   case YType  of
> T_INT: PInteger(@Y)^:= 3;
> T_SINGLE: PSingle(@Y)^:=3;
> T_DOUBLE: PDouble(@Y)^:=3;
>   end;
> end;

OK, I can imagine more complex examples where this may come in handy.

And thanks for the rest of the explanation.

Bart
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Dmitry Boyarintsev 
On Sat, Jan 7, 2017 at 5:42 PM, Bart  wrote:

>
> procedure X(var Y);
> begin
>   PInteger(Y)^ := 3;  //syntax may be wrong, too lazy to test
> end;
>
> To me this completely defies the purpose of an untyped var parameter,
> since in this exmaple you do know, @design time, the type of Y, so why
> not write then
>
> procedure X(var Y: Integer);
> begin
>   Y := 3;
> end;
>

Think about something like a var-type variable, where the actual type of Y
might vary.

procedure XX(var Y; YType: integer);
begin
  case YType  of
T_INT: PInteger(@Y)^:= 3;
T_SINGLE: PSingle(@Y)^:=3;
T_DOUBLE: PDouble(@Y)^:=3;
  end;
end;

Again, earlier in the thread I gave a link to anySort() example, that could
sort any array-like structure (as long as elements of the structure are
placed in sequential order in memory).

As I said mentioned earlier. Untyped parameter, is an implicit pointer
(since the value is always passed by reference).
But there's a compile-time difference.
If you'd declare a parameter as a pointer
procedure X( Y:Pointer);

Then it would be possible to use the function as following:
X(nil)
which requires you, at least add additional sanity check into X procedure.

But if you'd declare it as X(var Y), then the compiler would not allow to
pass nil, always requiring a user to provide a valid variable to be passed
and writing something like X(nil) is no longer possible.

Here's an example:
http://www.freepascal.org/docs-html/rtl/system/iunknown.queryinterface.html

QueryInterface must return an interface variable. BUT:
* the actual returned interface type is unknown at compile time.
* "obj" parameter must not be nil, since the reference must be written
somewhere.

It's also more convenient from syntax point of view, since a developer
doesn't have to write "@" symbol when passing a parameter. The reference
would be resolved by the compiler itself.


thanks,
Dmitry
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Lars 
On Sat, January 7, 2017 2:52 pm, Bart wrote:
> On 1/7/17, Andrew Hall  wrote:
>
>
>> If you cast your “something” to a typed pointer, the compiler will
>> do the work for you in the usual way:
>>
>
> But if you know that at forehand (and you know, since you cast it),
> why then use an untyped var parameter at all?
>

Some brave soldiers once tried to reinvent generics using these tricks..


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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Bart 
On 1/7/17, Dmitry Boyarintsev  wrote:

>> But if you know that at forehand (and you know, since you cast it),
>> why then use an untyped var parameter at all?
>>
>
> http://www.freepascal.org/docs-html/rtl/system/fillchar.html ?

That does not realy answer my question.

It was suggested to do something like this:

procedure X(var Y);
begin
  PInteger(Y)^ := 3;  //syntax may be wrong, too lazy to test
end;

To me this completely defies the purpose of an untyped var parameter,
since in this exmaple you do know, @design time, the type of Y, so why
not write then

procedure X(var Y: Integer);
begin
  Y := 3;
end;

FillChar OTOH _is_ a good example of why you would use an untype var parameter.

Bart
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Dmitry Boyarintsev 
On Sat, Jan 7, 2017 at 4:52 PM, Bart  wrote:

>
> But if you know that at forehand (and you know, since you cast it),
> why then use an untyped var parameter at all?
>

http://www.freepascal.org/docs-html/rtl/system/fillchar.html ?

thanks,
Dmitry
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Bart 
On 1/7/17, Andrew Hall  wrote:

> If you cast your “something” to a typed pointer, the compiler will do the
> work for you in the usual way:
>

But if you know that at forehand (and you know, since you cast it),
why then use an untyped var parameter at all?

Bart
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Dmitry Boyarintsev 
On Sat, Jan 7, 2017 at 2:37 PM, Andrew Hall  wrote:

> If you cast your “something” to a typed pointer, the compiler will do the
> work for you in the usual way:
>
> PDouble(something)^ := myDouble;
>
or rather
PDouble(@something)^ := myDouble;
?

thanks,
Dmitry
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Andrew Hall 
If you cast your “something” to a typed pointer, the compiler will do the work 
for you in the usual way:

PDouble(something)^ := myDouble;
PAnsiString(something)^ := myAnsiString;
myRecordStructure  := PMyRecordStructure(something)^;

Or less elegantly:

AnsiString(Pointer(something)^) := myAnsiString;

Regards,

On 07 Jan 17, at 03:29 , Ryan Joseph  wrote:
> So I need to need to use Move to write memory directly to “var something”? 
> That makes sense I guess and could be useful is some cases but not like I was 
> planning. Thanks for helping.

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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Lars 
On Sat, January 7, 2017 4:29 am, Ryan Joseph wrote:
>

>> On Jan 4, 2017, at 11:07 PM, Dmitry Boyarintsev
>>  wrote:
>>
>>
>> Treat is as a pointer (it's an implicit pointer anyway)
>>
>>
>
> So I need to need to use Move to write memory directly to “var
> something”? That makes sense I guess and could be useful is some cases
> but not like I was planning. Thanks for helping.
>

Why do you need to deal with it directly rather than indirectly setting up
something to access it...

What is the advantage of writing directly in your case..
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Re: [fpc-pascal] Untyped var params

2017-01-07 Thread Ryan Joseph 

> On Jan 4, 2017, at 11:07 PM, Dmitry Boyarintsev  
> wrote:
> 
> Treat is as a pointer (it's an implicit pointer anyway) 
> 

So I need to need to use Move to write memory directly to “var something”? That 
makes sense I guess and could be useful is some cases but not like I was 
planning. Thanks for helping.


Regards,
Ryan Joseph

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Re: [fpc-pascal] Untyped var params

2017-01-04 Thread Dmitry Boyarintsev 
On Wed, Jan 4, 2017 at 10:24 AM, Ryan Joseph 
wrote:

> But how do you assign to “something" then if you don’t know the type? The
> param is “var” so shouldn’t I be able to assign to the param and return it
> back to the caller? In your example how could I return 5 to “i” inside
> GetSomething?
>
> Treat is as a pointer (it's an implicit pointer anyway)

function GetSomething (var something): boolean
var
  pi: PInteger;
begin
  pi:=@something;
  GetSomething:=pi^>=5;
end;

or an old-school way using "absolute".

function GetSomething (var something): boolean;
var
  i: Integer absolute something;
begin
  GetSomething:=i>=5;
end;

var
  i : integer;
begin
  i:=3;
  writeln(GetSomething(i));
end.

check out "anysort" function
http://wiki.freepascal.org/Array_sort

It's using untyped parameter to perform a sort on an arbitrary type array

thanks,
Dmitry
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Re: [fpc-pascal] Untyped var params

2017-01-04 Thread Ryan Joseph 

> On Jan 4, 2017, at 10:14 PM, Dmitry Boyarintsev  
> wrote:
> 
> var
>   i: integer;
> begin
>   i:=5;
>   GetSometing(i)
> 

But how do you assign to “something" then if you don’t know the type? The param 
is “var” so shouldn’t I be able to assign to the param and return it back to 
the caller? In your example how could I return 5 to “i” inside GetSomething?

Regards,
Ryan Joseph

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Re: [fpc-pascal] Untyped var params

2017-01-04 Thread Dmitry Boyarintsev 
On Wed, Jan 4, 2017 at 9:29 AM, Ryan Joseph 
wrote:

> I’ve seen functions that use the untyped params like:
>
> function GetSomething (var something): boolean;
>
> This seems like something useful I would like to use but how can you
> assign to the parameter? I tried but I got an error message which is
> confusing because it’s a “var” after all.
>

A parameter you pass, would be passed by a reference (an address in memory)

Thus
  GetSomething(5)
will not work.


var
  i: integer;
begin
  i:=5;
  GetSometing(i)

Be careful using this open parameter, since you don't have any type
information about "something". You either should not assume the type or
pass the additional information

thanks,
Dmitry
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