Re: Simple template constraint question

[Daniel Kozak via Digitalmars-d-learn]

WhatMeWorry píše v So 19. 09. 2015 v 17:09 +0000:
> Does D provide complete template constraint granularity?
> 
> In other words, I want to only accept structs in the template 
> below.
> I've find the isAggregateType which is close but no cigar. Am I 
> missing
> some other filters?
> 
> And a more open ended question.  Is there a more elegant solution 
> for the
> below function?  Maybe a one-liner?  I have a knack for making 
> simple solutions
> complex :)
> 
> 
> 
> // Calculate the number of components for openGL generic
> // vertex attribute. Must be 1, 2, 3, 4.
> 
> // isAggregateType constraint accepts class, union, struct, and 
> interface
> 
> int getNumberComponents(T)(T someStruct) if (isAggregateType!(T))
> {
>      int members = 0;
>      foreach (i, member; someStruct.tupleof)
>      {
>          //writefln("Member %s:", i);
>          //writefln("  type : %s", typeof(member).stringof);
>          //writefln("  value: %s", member);
>          members++;
>      }
>      return members;
> }

http://dlang.org/expression.html#IsExpression

3. is ( Type == TypeSpecialization )

import std.stdio;

struct S
{
}

class C
{
}

void f(T)(T someStruct) if (is (T == struct)) {
    writeln("struct");
}

void f(T)(T notStruct) if (!is(T == struct)) {
    writeln("not a struct");
}

void main() {
    auto c = new C();
    auto s = S();

   f(c); // not a struct
   f(s); // struct
}