Get struct template types

[evilrat]

On Tuesday, 4 February 2014 at 09:58:53 UTC, ed wrote:
> On Tuesday, 4 February 2014 at 09:46:01 UTC, evilrat wrote:
>> On Tuesday, 4 February 2014 at 09:39:48 UTC, evilrat wrote:
>>> On Tuesday, 4 February 2014 at 09:30:22 UTC, ed wrote:
>>>> Hi,
>>>>
>>>> given a struct like so:
>>>>
>>>> struct S(alias N, T) {...}
>>>>
>>>> is there a way to get the template parameters of S? 
>>>> Something like:
>>>>
>>>> S.typetuple[0] == N,
>>>> S.typetuple[1] == T
>>>>
>>>> I've had a look at std.typecons and std.typetuple but I 
>>>> don't see what I'm missing something and cannot see a way to 
>>>> do the above.
>>>>
>>>> Thanks,
>>>> ed
>>>
>>> typeof(N).stringof will return type of N as string
>>> for details read http://dlang.org/expression.html
>>
>> oops sorry, it's for another people.
>>
>> what are you trying to achieve? when passing types as template 
>> args it is still types, so again typeof should give you type 
>> for variables(in such cases there is typeof), otherwise type 
>> is type. also, afaik it is not possible to store types in 
>> tuple, but i think you can store variables in tuple and 
>> retrieve their types at runtime
>
>
> I am trying to set up an isS, isSOfN or isSOfT so I can create 
> functions that allow restricted types of S based on all, or 
> only one of the template parameters.
>
> Like so (pseudo-D code)
> ---
> struct S(alias N, T) {}
>
> //
> // Hopefully it is clear what I am trying to do here
> enum isSOfN = template(alias N,STYPE) {
>     static assert(N == STYPET.typetuple[0]);
> }
>
> enum isSOfT = template(T,STYPE) {
>     static assert(T == STYPET.typetuple[1]);
> }
> //
>
>
> void f(T)(T t) if(isSOfN!(3, T)) { // allow S with N=3 and any 
> T (see below)
> }
>
> ...
> ...
>
> auto s3i = S!(3,int);
> auto s3f = S!(3,float);
>
> s3i.f(); // Succeeds
> s3f.f(); // Succeeds
>
> auto S4i = S!(4,int);
> s4i.f(); // compile time error
> --
>
> I'm looking into T.stringof now, I might be able to get it 
> working with some compile-time string magic of D :)
>
> If this is a bad way to go about it please let me know. I'm 
> trying to learn idiomatic D as I go.
>
> Thanks,
> ed

for this simple case static if is pretty straightforward, but 
then if only purpose is to construct type it is better use 
regular generic function and typecons i think.

------------------------------
import std.stdio;

struct S(alias N, T)
{
   T x;
   void f()
   {
      static if(N==3)
	 {
		writeln("n = 3, type = ", T.stringof);
	 }
	 else static if (N == 2)
	 {
		writeln("n = 2, type = ", T.stringof);
	 }
	 else
		static assert(0, "NO!");
   }
}

void main()
{
S!(3, int) a; // ok
S!(2, float) b; // ok
S!(4, double) c; // oops
a.f();
b.f();
}

------------------------------