idup class

[Steven Schveighoffer via Digitalmars-d-learn]

On Fri, 16 May 2014 16:28:41 -0400, Joshua Niehus <jm.niehus at gmail.com>  
wrote:

> trying to follow:
> http://ddili.org/ders/d.en/class.html
>
> //--- OSX 10.9 DMD 2.065
> module test;
>
> class Foo {
>       int num;
>
>       this(int num) {
>           this.num = num;
>       }
>
>       Foo dup() const {
>           return new Foo(this.num);
>       }
>
>       immutable(Foo) idup() const {
>           return new immutable(Foo)(this.num);
>       }
> }
>
> void main() {
>       auto  foo = new Foo(1);
>       auto mfoo = foo.dup();
>       auto ifoo = foo.idup();
> }
>
> * test.d(15): Error: mutable method test.Foo.this is not callable
> using a immutable object
> * test.d(15): Error: no constructor for Foo
> * Failed: ["dmd", "-v", "-o-", "test.d", "-I."]
> //---
>
> What am i missing?

your constructor needs the immutable tag.

this(int num) immutable ...

However, you can avoid much of this by tagging things as pure:

(untested, but just keep the internals the same)

class Foo
{
    int num;
    this(int num) pure {...}
    Foo dup() const pure {...}
    immutable(Foo) idup() const pure {...}
}

void main()
{
    ... // same implementation
}

Note that doing this, you do not need to have an idup, this should work:

immutable ifoo = foo.dup();

-Steve