• Tobias Pankrath (24/24) May 26 2012 I am writing a mixin template that uses alias parameters and
• Philippe Sigaud (86/107) May 26 2012 th

"Tobias Pankrath" <tobias pankrath.net> writes:

I am writing a mixin template that uses alias parameters and 
should me instantiated in a class. With only one parameter, it 
works. But I fail with
using multiple aliases as a tuple.

This works:

mixin template print(alias x) {
     void doprint() { writeln(x); }
}

class A { int x; mixin print!x; }

Now I would like to do the same, but with several attributes of 
my class at once. Thus I tried tuple parameters:

mixin template print(alias b...) { ... } // seem not to be legal 
syntax.

My second try was this:

mixin template print(b...)
{
     void doprint() {
         foreach(mem; b)
             writeln(b);
     }
}

class A { int x,y; mixin print!(x, y); }

Now DMD says:  need this to access member

How can I do this? Thank you in advance :)

Philippe Sigaud <philippe.sigaud gmail.com> writes:

On Sat, May 26, 2012 at 12:39 PM, Tobias Pankrath <tobias pankrath.net> wro=
te:
 I am writing a mixin template that uses alias parameters and should me
 instantiated in a class. With only one parameter, it works. But I fail wi=

th
 using multiple aliases as a tuple.

 This works:

 mixin template print(alias x) {
 =C2=A0 =C2=A0void doprint() { writeln(x); }
 }

 class A { int x; mixin print!x; }

 Now I would like to do the same, but with several attributes of my class =

at
 once. Thus I tried tuple parameters:

 mixin template print(alias b...) { ... } // seem not to be legal syntax.

No, the legal syntax is indeed b..., as you use below. Normally, all
members of b should be aliases. Seems like a bug to me, but perhaps
people knowing the compiler internals better than us could answer.


 My second try was this:

 mixin template print(b...)
 {
 =C2=A0 =C2=A0void doprint() {
 =C2=A0 =C2=A0 =C2=A0 =C2=A0foreach(mem; b)
 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0writeln(b);
 =C2=A0 =C2=A0}
 }

 class A { int x,y; mixin print!(x, y); }

 Now DMD says: =C2=A0need this to access member

OK, here is a module that works:

module test;

import std.stdio;
import std.conv;

mixin template print(alias x)
{
   void doprint()
   {
       writeln(x);
    }
}

// same as you
class A
{
    int x;
    mixin print!x;
}

// extracting aliases one by one. No loop, but recursion
mixin template print2(alias a, rest...)
{
    void doprint()
    {
        writeln(a);
        static if (rest.length > 0) // still other aliases to extract
            mixin print2!(rest);
   }
}

class B
{
    int x,y;
    mixin print2!(x,y);
}

// Another solution, maybe more generic: string mixins
string print3(a...)()  property
{
    // We begin by assembling the desired final code, as a string
    string result =3D "
    void doprint()
    {
        writeln(";
    foreach(i,member;a)
        result ~=3D member ~ (i<a.length-1 ? ", " : "");
    return result ~ ");
    }";
}

class C
{
    int x,y;
    mixin(print3!("x","y")); // see the ( ) after mixin, and the
members are passed as strings
}

void main()
{
    auto a =3D new A();
    auto b =3D new B();
    auto c =3D new C();

    a.doprint();
    b.doprint();
    c.doprint();
}

I tried the string mixin version with (b...) (enabling a call like
this:  mixin(print3!(x,y)); ), but I got the same error as you.

Btw, it's possible to extract members of a class inside the mixin with
__traits(allMembers, typeof(this)) and to automate the process
somewhat:

mixin template print4()
{
    void doprint()
    {
        writeln([__traits(allMembers, typeof(this))]);
    }
}

class D
{
    int x,y;
    double z;
    mixin print4;
}

Philippe