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digitalmars.D.learn - is expression for template structs/classes instances?

reply d coder <dlang.coder gmail.com> writes:
Greetings

I want to find if a given struct type is instantiated from a
particular template struct type. For example:

struct S (T)  {
  alias T Type;
  T t;
}

And later I want to find out if a given type is of type S(*)
(basically any type instantiated from template struct S). In fact I do
not know the type value T used at the time of instantiating S!(T).

I was looking at "is ( Type Identifier : TypeSpecialization ,
TemplateParameterList )" expression at
http://www.digitalmars.com/d/2.0/expression.html#IsExpression .
Thought there would be some way using that, but I could not find any.

Regards
Cherry
Dec 20 2010
next sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
d coder:


 I want to find if a given struct type is instantiated from a
 particular template struct type. For example:
 
 struct S (T)  {
   alias T Type;
   T t;
 }
 
 And later I want to find out if a given type is of type S(*)
 (basically any type instantiated from template struct S). In fact I do
 not know the type value T used at the time of instantiating S!(T).



I remember someone has shown here the URL of a module that given an
instantiated template, it returns the typetuple of its instantiation arguments.
This code maybe returns the alias of the template before instantiation too. But
I don't remember the name of the module.
The need expressed in this post is very common, so some standard way to do it
is necessary in Phobos or in __traits/meta.

Bye,
bearophile
Dec 21 2010
parent bearophile <bearophileHUGS lycos.com> writes:
 The need expressed in this post is very common, so some standard way to do it
is necessary in Phobos or in __traits/meta.



I have added this:
http://d.puremagic.com/issues/show_bug.cgi?id=5361

Bye,
bearophile
Dec 21 2010
prev sibling next sibling parent "Simen kjaeraas" <simen.kjaras gmail.com> writes:
d coder <dlang.coder gmail.com> wrote:


 Greetings

 I want to find if a given struct type is instantiated from a
 particular template struct type. For example:

 struct S (T)  {
   alias T Type;
   T t;
 }

 And later I want to find out if a given type is of type S(*)
 (basically any type instantiated from template struct S). In fact I do
 not know the type value T used at the time of instantiating S!(T).

 I was looking at "is ( Type Identifier : TypeSpecialization ,
 TemplateParameterList )" expression at
 http://www.digitalmars.com/d/2.0/expression.html#IsExpression .
 Thought there would be some way using that, but I could not find any.



If you know the template you want to check for, isExpression works:

S!int foo;
static if ( is( typeof(foo) f == S!T, T ) ) {
     // Here, T == int, f == typeof(foo)
}

Note that the syntax "is ( Type Identifier : TypeSpecialization ,
TemplateParameterList )" is only usable inside static if.

-- 
Simen
Dec 21 2010
prev sibling next sibling parent d coder <dlang.coder gmail.com> writes:
 S!int foo;
 static if ( is( typeof(foo) f =3D=3D S!T, T ) ) {
 =A0 =A0// Here, T =3D=3D int, f =3D=3D typeof(foo)
 }

 Note that the syntax "is ( Type Identifier : TypeSpecialization ,
 TemplateParameterList )" is only usable inside static if.



Thanks Simen

I do know the template. I will try out your solution. Will let you
know if I face issues.

Regards
- Cherry
Dec 21 2010
prev sibling next sibling parent d coder <dlang.coder gmail.com> writes:
 I do know the template. I will try out your solution. Will let you
 know if I face issues.



Simen

It works perfect, And this is exactly what I was looking for. If you
see my original post, I also thought this form of "is" expression
should work. Just could not get around to the right syntax.

With your help it is working now. I am using a slightly more elaborate
check which is obvious but I am writing it here to just let the list
know.

static if ( is( typeof(foo) f == S!T, T : int) ) {
  // foo is an object of type S!T
  // where T is convertible to int
}

Thanks once more
Warm Regards
- Cherry
Dec 21 2010
prev sibling next sibling parent "Simen kjaeraas" <simen.kjaras gmail.com> writes:
d coder <dlang.coder gmail.com> wrote:


 I do know the template. I will try out your solution. Will let you
 know if I face issues.



 Simen

 It works perfect, And this is exactly what I was looking for. If you
 see my original post, I also thought this form of "is" expression
 should work. Just could not get around to the right syntax.

 With your help it is working now. I am using a slightly more elaborate
 check which is obvious but I am writing it here to just let the list
 know.

 static if ( is( typeof(foo) f == S!T, T : int) ) {
   // foo is an object of type S!T
   // where T is convertible to int
 }

 Thanks once more
 Warm Regards
 - Cherry



Glad to be of service.

-- 
Simen
Dec 21 2010
prev sibling parent Jonathan M Davis <jmdavisProg gmx.com> writes:
On Tuesday, December 21, 2010 02:57:45 d coder wrote:

 I do know the template. I will try out your solution. Will let you
 know if I face issues.



 Simen
 
 It works perfect, And this is exactly what I was looking for. If you
 see my original post, I also thought this form of "is" expression
 should work. Just could not get around to the right syntax.
 
 With your help it is working now. I am using a slightly more elaborate
 check which is obvious but I am writing it here to just let the list
 know.
 
 static if ( is( typeof(foo) f == S!T, T : int) ) {
   // foo is an object of type S!T
   // where T is convertible to int
 }



Yes, that sort of thing works. The problem is when you want to know whether an 
arbitrary type is an instantiation of a particular template. For instance, if 
you have

struct S(T)
{
}

struct Q(T)
{
}

and you pass S!int to Q - Q(S!int) - Q sees S!int is a specific type, not an 
instantiation of S, so it's difficult to have a template constraint checking
that 
the type passed to Q is an instantiation of S. However, if you know exactly 
which instantiation that you're checking for, then it's relatively easy.

- Jonathan M Davis
Dec 21 2010