fun with properties

[Kai Meyer]

On 03/23/2011 06:48 AM, teo wrote:
> How can I use properties with increment/decrement and +=/-= operators?
>
> I did following tests (the errors are from dmd v2.052):
>
> class T
> {
>      private int _x;
>      @property
>      public int x() { return _x; }
> }
>
> void main()
> {
>      int[] a;
>      // case #1.1
>      a.length++;    // Error: a.length is not an lvalue
>      // case #1.2
>      a.length += 1; // Ok
>
>
>      auto t = new T();
>      // case #2.1
>      t.x++;      // Error: t.x() is not an lvalue
>      // case #2.2
>      t.x += 1;   // Error: 't.x' is not a scalar, it is a @property int()
>                  // Error: incompatible types for ((t.x) += (1)):
> '@property int()' and 'int'
>      // case #2.3
>      t.x()++;    // Error: t.x() is not an lvalue
>      // case #2.4
>      t.x() += 1; // Error: t.x() is not an lvalue
> }
>
> Basically I want to change the value of a member variable, which is
> accessed only through a property.
> It looks like that is partially possible with the length property of
> dynamic arrays although they probably are implemented in a different way.

You need a "write" property:

     @property
     {
         public int x() { return _x; } // Read property
         public void x(int x1) { _x = x1; } // Write property
     }

As for the dynamic array length property:

void main()
{
     int[] a;
     ++a.length; // Works
     a.length += 1; // Works
     a.length++; // Error: a.length is not an lvalue
}

I don't get why the error on a.length++ either. I'm curious about the 
answer.