Hi,

Is it ok to have fields in @property ?

Implementing properties often require private fields to hold the values, and it seems natural to have them inside @property near the property methods.  But I couldn't find any mention in the docs about fields in @property.

This compiles with DMD 2.039, and runs fine:

---- cut here ----
import std.stdio;

class Foo
{
    this(string msg) { this._msg = msg; }

    @property {
	private string _msg; // <-- field in @property
	string msg() { return _msg; }
    }
}

void main()
{
    auto foo = new Foo("Hello world");
    writeln(foo.msg);
}
---- cut here ----

MIURA Masahiro wrote:

> Hi,
>
> Is it ok to have fields in @property ?
>
> Implementing properties often require private fields to hold the values, and it seems natural to have them inside @property near the property methods.  But I couldn't find any mention in the docs about fields in @property.
>
> This compiles with DMD 2.039, and runs fine:
>
> ---- cut here ----
> import std.stdio;
>
> class Foo
> {
>     this(string msg) { this._msg = msg; }
>
>     @property {
> 	private string _msg; // <-- field in @property
> 	string msg() { return _msg; }
>     }
> }
>
> void main()
> {
>     auto foo = new Foo("Hello world");
>     writeln(foo.msg);
> }
> ---- cut here ----

@property is fairly new so documentation is lacking and no one really knows exactly what is allowed. But if you are familiar with the purpose of properties and what @property is replacing (functions not requiring parentheses), your code makes since to be valid.

Currently D2 still allows calling of functions without parentheses, this I believe is supposed to change. You can also have properties that are overloaded.

@property {
void foo(string num)...
void foo(int num)...
int foo() { return _num; }
}

On Wed, 13 Jan 2010 08:30:39 +0100, MIURA Masahiro <echochamber@gmail.com> wrote:

> Hi,
>
> Is it ok to have fields in @property ?
>
> Implementing properties often require private fields to hold the
> values, and it seems natural to have them inside @property near
> the property methods.  But I couldn't find any mention in the docs
> about fields in @property.
>
> This compiles with DMD 2.039, and runs fine:
>
> ---- cut here ----
> import std.stdio;
>
> class Foo
> {
>     this(string msg) { this._msg = msg; }
>
>     @property {
> 	private string _msg; // <-- field in @property
> 	string msg() { return _msg; }
>     }
> }
>
> void main()
> {
>     auto foo = new Foo("Hello world");
>     writeln(foo.msg);
> }
> ---- cut here ----

@property {
  //stuff
}

means that anything within the curly braces has the @property attribute. For fields, @property does nothing, so

class foo {
  @property {
    int bar;
    int baz( ) { return bar; };
  }
}

is equivalent to

class foo {
  int bar;
  @property {
    int baz( ) { return bar; }
  }
}

In other words, this is not (or at least should not be) possible:

class foo {
  @property {
    int bar;
    int baz( ) { return bar; };
  }
  @property {
    int bar; // Duplicate foo.bar.
    int qux( ) { return bar; } // Will not return the latter bar.
  }
}

-- 
Simen