Re: Equivalents to policy classes in D

[Joseph Rushton Wakeling]

On 03/04/12 02:24, Cristi Cobzarenco wrote:
Mixins templates would be the answer:

OK, I've had a go at writing up some mixin template-based policy code of my own.

However ... it winds up with compilation errors that I can't decipher:

  mixin.d(55): Error: type MyStruct!(ulong,GenOne,WriteOne) has no value
  mixin.d(56): Error: type MyStruct!(double,GenOne,WriteTwo) has no value
  mixin.d(57): Error: type MyStruct!(double,GenThree,WriteOne) has no value
  mixin.d(58): Error: type MyStruct!(long,GenThree,WriteTwo) has no value
  /usr/local/include/d2/std/conv.d(244): Error: template std.conv.toImpl does 
not match any function template declaration
  /usr/local/include/d2/std/conv.d(252): Error: template std.conv.toImpl cannot 
deduce template function from argument types !(string)(ubyte)
  /usr/local/include/d2/std/conv.d(244): Error: template instance 
toImpl!(string) errors instantiating template


Here's the code.  What am I doing wrong ... ?

/////////////////////////////////////////////////////////////////////////////////
import std.stdio;

mixin template GenOne(T)
{
        void generate()
        {
                foreach(size_t i, ref T x; this.array)
                        x = i;
        }
}

mixin template GenThree(T)
{
        void generate()
        {
                foreach(size_t i, ref T x; this.array)
                        x = 3*i;
        }
}

mixin template WriteOne(T)
{
        void myWrite()
        {
                foreach(T x; this.array)
                        writeln(x);
        }
}

mixin template WriteTwo(T)
{
        void myWrite()
        {
                foreach(T x; this.array)
                        writeln(2*x);
        }
}

struct MyStruct(T, alias GeneratePolicy, alias WritePolicy)
{
        private T[] array;

        this(size_t n)
        {
                array.length = n;
                generate;
        }

        mixin GeneratePolicy!T;
        mixin WritePolicy!T;
}

void main()
{
        auto oneOne = MyStruct!(size_t, GenOne, WriteOne);
        auto oneTwo = MyStruct!(double, GenOne, WriteTwo);
        auto threeOne = MyStruct!(double, GenThree, WriteOne);
        auto threeTwo = MyStruct!(long, GenThree, WriteTwo);

        oneOne.myWrite;
        writeln;
        oneTwo.myWrite;
        writeln;
        threeOne.myWrite;
        writeln;
        threeTwo.myWrite;
        writeln;
}

/////////////////////////////////////////////////////////////////////////////////