The problem with @properties in D

[Jonathan M Davis]

On Monday, December 12, 2011 16:25:33 Mehrdad wrote:
> In every language I've seen that has "properties" (C#, Python), they are:
> 
> - _Defined_ like methods
> - _Used_ like variables
> 
> The trouble is, this isn't true with D.
> 
> Consider:
> 
> struct Struct
> {
> int delegate() randgen1 = ...;
> @property
> int delegate() randgen2() { ... }
> }
> 
> Struct s;
> auto result = s.randgen2(); // This doesn't do the user expects
> 
> It is *not* possible, in D, to transparently use either one -- you have
> to treat properties, like methods, not like variables. Except that this
> is inconsistent -- in most other cases, you don't need to do that.
> 
> Or for example:
> 
> Struct s;
> auto a = &s.randgen1;
> auto b = &s.randgen2; // Should be an error
> 
> IMO, properties should not be callable with parentheses at all.
> Something like C# -- they should generate getter and setter methods
> instead, or the like.
> Furthermore, taking the address of a property should only work if you
> can take the address of its _value_. If you need the address of the
> actual function, then I think a corresponding getter method might be
> easier to use.
> 
> It gets even /worse/ in templated code, because you have no idea whether
> an alias is referring to a property or to a variable or whatever.
> 
> Making this change would obviously break code, but the break is
> obviously _trivial_ to fix: just remove extra parentheses. It won't
> exactly be the kind of breakage that causes headaches.
> 
> So should this be fixed?

Originally, it was the case that there was no @property, and any function with 
no arguments could be used like a property. It was decided to add @property to 
enforce that certain functions be treated as properties and some not. But the 
fact that @property was not how properties were initially implemented has 
caused issues in migrating.

-property makes it so that properties must be used like variables and non-
property functions cannot be. In theory, that will become the default behavior 
of the compiler eventually, but we couldn't enable it immediately due to a 
combination of bugs with @property and the fact that it would break code. For 
now, people can build with -property to make sure that their code uses 
properties properly.

- Jonathan M Davis