Boost.Any in Dlang (inspired by http://d.hatena.ne.jp/pknight/20100325/1269539749)

any.d

import std.traits,
       std.meta;

class Any {
  private {
    abstract class _AnyBase {
      abstract TypeInfo type();
      abstract _AnyBase clone();
    }

    class _Any(T) : _AnyBase {
      T value;

      this (T value) {
        this.value = value;
      }

      override TypeInfo type() const {
        return typeid(T);
      }

      override _AnyBase clone() {
        return new _Any!T(this.value);
      }

      ~this () {}
    }

    _AnyBase obj;
  }

public:
  this(T)(T value) {
    this.obj = new _Any!T(value);
  }

  this () {}

  this(Any any_obj) {
    if (any_obj.obj !is null) {
      this.obj = any_obj.obj.clone();
    } else {
      this.obj = null;
    }
  }

  Any opAssign(T)(T value) if (!is(T == Any)) {
    delete this.obj;
    this.obj = new _Any!T(value);
    return this;
  }

  T castTo(T)() {
    if (this.obj is null) {
      throw new Exception("this object has no value");
    }
    if ((cast(_Any!T)this.obj) is null) {
      throw new Exception("can not cast into incompatible type");
    } else {
      return (cast(_Any!T)this.obj).value;
    }
  }

  TypeInfo type() {
    return this.obj.type;
  }
}

import std.stdio;

class K {
  int value;
  this (int value) {
    this.value = value;
  }
}

void main() {
  Any a = new Any;

  a = 10;
  writeln(a.type);
  writeln(a.castTo!int);

  a = [1, 2, 3, 4, 5];
  writeln(a.type);
  writeln(a.castTo!(int[]));

  a = new K(123);
  writeln(a.type);
  writeln(a.castTo!(K).value);
}