Question

I just took a look at the Expressions grammar for D 2.0 (NewExpression) and something caught my attention:

NewExpression:

NewArguments ClassArguments BaseClasslistopt { DeclDefs } 

ClassArguments:

class ( ArgumentList )

class ( )

class

What exactly are these ClassArguments and DeclDefs? Is there an example somewhere that demonstrates their use?

Answer 1

ClassArguments is the keyword class followed by the constructor arguments. DeclDefs are the declarations inside the class.

---

This syntax is to create an instance of an anonymous nested class, e.g.

import std.stdio;

void main() {
  class K {
    this() { writeln("K.__ctor"); }
  }

  auto f = new class (1, "4", 7.0) K {
    this(int a, string b, double c) {
      super();
      writefln("anon.__ctor: %s %s %s", a, b, c);
    }
  };
}

(See http://ideone.com/cA1qo.)

The above can be rewritten into the less obscure form

import std.stdio;

void main() {
  class K {
    this() { writeln("K.__ctor"); }
  }

  class AnonymousClass : K {
    this(int a, string b, double c) {
      super();
      writefln("anon.__ctor: %s %s %s", a, b, c);
    }
  }
  auto f = new AnonymousClass(1, "4", 7.0);
}