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digitalmars.D.learn - 3 variant questions

reply "Saaa" <empty needmail.com> writes:
How do I return a variant type in D1?
After assessing that a variadic argument is an array, how do I check its 
depth?
How do I set the variable given to me through the _arguments array?
thanks :)
May 11 2009
parent reply Christopher Wright <dhasenan gmail.com> writes:
Saaa wrote:

 How do I return a variant type in D1?
 After assessing that a variadic argument is an array, how do I check its 
 depth?
 How do I set the variable given to me through the _arguments array?
 thanks :) 



Variant func()
{
	return variant(5);
}

If you want to return something given from a runtime variadic argument 
list, neither Tango's Variant nor Phobos's Variant will work. Tango's 
Variant will get this capability soon, I hear.

There is also reflect.variant:
svn co http://felt-project.org/reflect

reflect.variant allows you to create a Variant from a void* and a 
TypeInfo, like you get with varargs.
May 11 2009
parent reply "Saaa" <empty needmail.com> writes:
"Christopher Wright" <dhasenan gmail.com> wrote in message 
news:gu8v1b$1uth$1 digitalmars.com...

 Saaa wrote:

 How do I return a variant type in D1?
 After assessing that a variadic argument is an array, how do I check its 
 depth?
 How do I set the variable given to me through the _arguments array?
 thanks :)



 Variant func()
 {
 return variant(5);
 }





 If you want to return something given from a runtime variadic argument 
 list, neither Tango's Variant nor Phobos's Variant will work. Tango's




 Variant will get this capability soon, I hear.




 There is also reflect.variant:
 svn co http://felt-project.org/reflect

 reflect.variant allows you to create a Variant from a void* and a 
 TypeInfo, like you get with varargs.



Shouldn't it be possible to set an variadic argument?

void func( ... )
{
//something like this
cast(int) _argptr = 10;
}
May 11 2009
parent reply Christopher Wright <dhasenan gmail.com> writes:
Saaa wrote:

 "Christopher Wright" <dhasenan gmail.com> wrote in message 
 news:gu8v1b$1uth$1 digitalmars.com...

 Saaa wrote:

 How do I return a variant type in D1?
 After assessing that a variadic argument is an array, how do I check its 
 depth?
 How do I set the variable given to me through the _arguments array?
 thanks :)



 {
 return variant(5);
 }






In D1/Phobos, you can use std.boxer.Box, which can work with variadic 
arguments.
May 11 2009
parent reply "Saaa" <empty needmail.com> writes:
 In D1/Phobos, you can use std.boxer.Box, which can work with variadic 
 arguments.




Sorry to ask, but do you have an answer to my other question as well?
Passing variadic arguments as ref I think is what I am asking for :)
May 11 2009
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
Saaa wrote:

 ...
 Passing variadic arguments as ref I think is what I am asking for :) 



You can't.  You have to explicitly take the address of the arguments.

  -- Daniel
May 12 2009
parent reply "Saaa" <empty needmail.com> writes:
 Saaa wrote:

 ...
 Passing variadic arguments as ref I think is what I am asking for :)



 You can't.  You have to explicitly take the address of the arguments.

  -- Daniel



Like this ?
*_argptr = 10;
:D

I don't know how to tell the compiler I want to write data there.
May 12 2009
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
Saaa wrote:

 Saaa wrote:

 ...
 Passing variadic arguments as ref I think is what I am asking for :)




  -- Daniel



 Like this ?
 *_argptr = 10;
 :D
 
 I don't know how to tell the compiler I want to write data there.



import std.stdarg;

assert( _arguments[0] is typeid(int*) );
auto arg = va_arg!(int*)(_argptr);
*arg = 10;

Probably.

  -- Daniel
May 12 2009
next sibling parent reply "Saaa" <empty needmail.com> writes:
I just noticed D1 does have std.stdarg.
I shouldn't just search on the website :(
(where it is missing on the phobos page)


 import std.stdarg;

 assert( _arguments[0] is typeid(int*) );
 auto arg = va_arg!(int*)(_argptr);
 *arg = 10;

 Probably.




  -- Daniel



So, you make arg point to the same as va_arg.
Why is this extra step necessary and why won't simple casting not work?
May 12 2009
parent reply Daniel Keep <daniel.keep.lists gmail.com> writes:
Saaa wrote:

 I just noticed D1 does have std.stdarg.
 I shouldn't just search on the website :(
 (where it is missing on the phobos page)
 

 import std.stdarg;

 assert( _arguments[0] is typeid(int*) );
 auto arg = va_arg!(int*)(_argptr);
 *arg = 10;

 Probably.




  -- Daniel



 So, you make arg point to the same as va_arg.



No, va_arg is a function.


 Why is this extra step necessary and why won't simple casting not work?



You should never directly work with _argptr.  It's not guaranteed to be
a simple pointer.  For example, I believe that GCC will sometimes use
registers (God only knows WHY).

var_arg!(T) will convert _argptr into the type you specify and it will
also advance _argptr to the next argument.

  -- Daniel
May 12 2009
next sibling parent reply "Saaa" <empty needmail.com> writes:
"Daniel Keep" <daniel.keep.lists gmail.com> wrote in message 
news:gubip2$187c$1 digitalmars.com...

 Saaa wrote:

 I just noticed D1 does have std.stdarg.
 I shouldn't just search on the website :(
 (where it is missing on the phobos page)


 import std.stdarg;

 assert( _arguments[0] is typeid(int*) );
 auto arg = va_arg!(int*)(_argptr);
 *arg = 10;

 Probably.




  -- Daniel



 So, you make arg point to the same as va_arg.



 No, va_arg is a function.





 Why is this extra step necessary and why won't simple casting not work?



 You should never directly work with _argptr.  It's not guaranteed to be
 a simple pointer.  For example, I believe that GCC will sometimes use
 registers (God only knows WHY).




 var_arg!(T) will convert _argptr into the type you specify and it will
 also advance _argptr to the next argument.



? :D


  -- Daniel
May 12 2009
parent Daniel Keep <daniel.keep.lists gmail.com> writes:
Saaa wrote:

 ...

 var_arg!(T) will convert _argptr into the type you specify and it will
 also advance _argptr to the next argument.



 ? :D



Same as would happen if you incorrectly cast anything.

i.e. anything.  Segfault if you're lucky.  If you're unlucky, random
crashes.

  -- Daniel
May 12 2009
prev sibling parent reply grauzone <none example.net> writes:
Dear Saaa, these varargs suck badly and you shouldn't use them. It's so 
simple to introduce portability errors or heisenbugs, and it's 
incredibly hard to get it right. You're better off with alternatives.

Alternative 1: Typesafe Variadic Functions
Useful if the variadic arguments should have only one type. And they're 
very easy to use. It's documented somewhere on 
http://www.digitalmars.com/d/1.0/function.html

Alternative 2: Tuples
http://www.digitalmars.com/d/1.0/template.html#TemplateTupleParameter
The variadic arguments can have any types, but such functions can't 
really be virtual methods or delegates.


 Why is this extra step necessary and why won't simple casting not work?



 You should never directly work with _argptr.  It's not guaranteed to be
 a simple pointer.  For example, I believe that GCC will sometimes use
 registers (God only knows WHY).



I think it's because GCC tries to keep the usual calling convention, 
where some arguments are passed as registers. Which is utterly pointless 
and stupid (ok, maybe it was easier to implement in gdc), and turns 
using varargs into hell.

It would be so simple if _argptr was just an array of pointers to the 
actual arguments. It'd be so simple it makes me mad.
May 12 2009
parent "Saaa" <empty needmail.com> writes:
 Dear Saaa, these varargs suck badly and you shouldn't use them. It's so
 simple to introduce portability errors or heisenbugs, and it's




 incredibly hard to get it right. You're better off with alternatives.

 Alternative 1: Typesafe Variadic Functions
 Useful if the variadic arguments should have only one type. And they're 
 very easy to use. It's documented somewhere on 
 http://www.digitalmars.com/d/1.0/function.html



It doesn't say much about setting/changing the passed arguments.

How do these differ?

int test1( int i ...) { return i;}
int test2( int i ) { return i;}
test1(3);  // returns 3
test2(3);  // returns 3


 Alternative 2: Tuples
 http://www.digitalmars.com/d/1.0/template.html#TemplateTupleParameter
 The variadic arguments can have any types, but such functions can't really 
 be virtual methods or delegates.



Well, I should learn about them templates anyways :)
Templates create the necessary appropriately typed instances of a function
 at compile-time.. seems more usefull as my get function should work with
any type.


 Why is this extra step necessary and why won't simple casting not work?



 You should never directly work with _argptr.  It's not guaranteed to be
 a simple pointer.  For example, I believe that GCC will sometimes use
 registers (God only knows WHY).



 I think it's because GCC tries to keep the usual calling convention, where 
 some arguments are passed as registers. Which is utterly pointless and 
 stupid (ok, maybe it was easier to implement in gdc), and turns using 
 varargs into hell.

 It would be so simple if _argptr was just an array of pointers to the 
 actual arguments. It'd be so simple it makes me mad.
May 12 2009
prev sibling parent reply "Saaa" <empty needmail.com> writes:
 import std.stdarg;

 assert( _arguments[0] is typeid(int*) );
 auto arg = va_arg!(int*)(_argptr);
 *arg = 10;

 Probably.

  -- Daniel



Calling the following returns an Access Violation Error after
correctly writing the two lines.

void main()
{
int i;
get( file, `i`, i);
}

public void get(in char[][] file, in char[] identifier, ...)
{
assert( _arguments[0] is typeid(int) );
writefln(`assert done`);
auto arg = va_arg!(int*)(_argptr);
writefln(`assign done`);
*arg = 7;
return;
}
May 12 2009
parent reply John C <johnch_atms hotmail.com> writes:
Saaa Wrote:


 import std.stdarg;

 assert( _arguments[0] is typeid(int*) );
 auto arg = va_arg!(int*)(_argptr);
 *arg = 10;

 Probably.

  -- Daniel



 Calling the following returns an Access Violation Error after
 correctly writing the two lines.
 
 void main()
 {
 int i;
 get( file, `i`, i);
 }
 
 public void get(in char[][] file, in char[] identifier, ...)
 {
 assert( _arguments[0] is typeid(int) );
 writefln(`assert done`);
 auto arg = va_arg!(int*)(_argptr);
 writefln(`assign done`);
 *arg = 7;
 return;
 } 
 



You get an AV because you're passing the argument by value. You need to pass
its address instead.

Try this:

void main() {
  int i;
  get(file, "i", &i);
  writeln(i);
}

It will print "7".
May 12 2009
parent "Saaa" <empty needmail.com> writes:


 You get an AV because you're passing the argument by value. You need to 
 pass its address instead.

 Try this:

 void main() {
  int i;
  get(file, "i", &i);
  writeln(i);
 }

 It will print "7".



Ah of course, thanks. 



 May 12 2009