static alias this

[Ali Çehreli via Digitalmars-d-learn]

On 02/07/2015 04:46 AM, Mike wrote:

 > B)-----------------------------------------
 > struct StaticRegister {
 >      static private uint _value;
 >      @property static uint value() { return _value; }
 >      @property static void value(uint v) { _value = v; }
 >
 >      alias value this;
 > }
 >
 > void main(string[] s) {
 >      StaticRegister = 1;
 >      assert(StaticRegister == 1);
 > }
 > -------------------------------------------
 >
 > ... the assignment error is eliminated, but the read is not.

I bet it's an unintentional implementation artifact. Assigning to a type 
doesn't make sense to me.

 > I argue that either both errors should be eliminated, or neither should
 > be eliminated.

It is not hard to create strange D code. I recommend the following 
lightning talk for fun. :) Brian Schott at DConf 2014:

   http://www.youtube.com/watch?v=oF8K4-bieaw#t=1620

 > Now, in the example below, `this` is referring to the type itself in a
 > static context

Not exactly "type itself" because you apply typeof() to it later below. 
I this 'this' means "the type of the object if it were not a static 
function". :/

 > C)-------------------------------------------
 > import std.stdio;
 >
 > struct StaticRegister {
 >      static string GetType() { return typeof(this).stringof; }
 > }
 >
 > void main(string[] s) {
 >      writeln(StaticRegister.GetType());
 > }
 > -------------------------------------------
 >
 > So, it follows that the example below should work... and it does
 > D)-------------------------------------------
 > struct StaticRegister {
 >      static private uint _value = 0;
 >      @property static uint value() { return _value; }
 >      @property static void value(uint v) { _value= v; }
 >
 >      static uint GetValue() {
 >          return this.value;

That works by accident. I am surprised that 'this' works in a static 
function. In any case, it should better be null.

Ok, it's good that 'this' is not available:

         assert(this is null);

Error: 'this' is only defined in non-static member functions, not foo

Phew... :) So, 'this.value' works by accident because the compiler 
reaches for *static* value() without evaluating 'this'.

Ali