this() member function?

[Wei Li]

Hi:

The following code is a quick and dirty variant implemention inspired by boost.variant. It works, but why is the constructor(this(T)()) cannot to be a template function?

//variant.d
module variant;

import std.stdio;
import std.metastrings;
import std.typetuple;

private template MixinMembers(int L, T, V...)
{
    mixin(Format!("%s var%s;", T.stringof, L - 1 - V.length));

    static if(V.length > 0)
        mixin MixinMembers!(L, V);
}

//catenate strings at compile time....
private template MixinThisFunc(char[] S, T, V...)
{
    public const char[] Result = S ~ 
        Format!("public this(%s v) { assign!(%s)(v); } ", T.stringof, T.stringof);
    pragma(msg, Result ~ "\n");
    static if(V.length > 0)
        mixin MixinThisFunc!(Result, V);
}

class Variant(TList...)
{
    public alias TList  TypeList;
    public alias Variant!(TList)    SelfType;

    private union Holder
    {
        mixin MixinMembers!(TList.length, TList);
    }

    private Holder m_holder;
    private int m_which = -1;
    
    public mixin(MixinThisFunc!("",TypeList).Result);

    public this()
    {
        m_holder.var0 = m_holder.var0.init;
        m_which = 0;
    }

    public SelfType assign(T)(T rhs)
    {
        enum { index =  IndexOf!(T, TypeList) }
        static assert( index >= 0);
        mixin(Format!("m_holder.var%s = rhs;", index));
        m_which = index;
        return this;
    }

    public SelfType opAssign(T)(T rhs)
    {
        return assign(rhs);
    }

    public int which()
    {
        return m_which;
    }

    public T get(T)()
    {
        enum { index = IndexOf!(T, TypeList) }
        static assert(index >= 0);
        assert(index == which());
        mixin(Format!("return m_holder.var%s;", index));
    }
}

void main()
{
    scope auto v = new Variant!(int, double, char, char[])(1000);
    writefln("which: %d", v.which());
    v = 100.0;
    v = 'A';
    writefln("which: %d", v.which());
    char[] str = "foobar";
    v = str;
    writefln("which: %d", v.which());
    str = "";
    str = v.get!(char[]);
    writefln(str);
}
             - Wei Li

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