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digitalmars.D - static `this`

reply "Mike" <none none.com> writes:
I'm elevating this from D.Learn [1] because I think it needs some 
input from the language designers.

This code compiles and executes:
-----------------------------------
import std.stdio;

struct StaticRegister {
     static private uint _value;
      property static uint value() { return _value; }
      property static void value(uint v) { _value = v; }

     static alias value this;

     static void test() {
         writeln(this.stringof);
         writeln(typeof(this).stringof);
         writeln(this.value);
     }
}

void main(string[] s) {
     // works due to `alias value this`
     StaticRegister = 1;

     StaticRegister.test();
}
-----------------------------------

Output:
StaticRegister
StaticRegister
1



However, this code fails to compile:
-----------------------------------
struct StaticRegister {
     static void test() {
         assert(this is null);
     }
}

void main(string[] s) {
     StaticRegister.test();
}
-----------------------------------

Output:
test.d(5): Error: 'this' is only defined in non-static member 
functions, not test

To make things more confusing, Bug 380 [2] apparently provided a 
fix for `this` not evaluating in a static context, but it was a 
D1 bug and may not apply to D2.

Is `this` overloaded to mean "this class" in a static context or 
is `this` only valid in a non-static context.  Please clarify, 
and if it's a bug, help me understand so I can make an accurate 
and actionable bug report.

Thanks for the help,
Mike

[1] - "static alias this" thread on D.Learn - 
http://forum.dlang.org/post/xcnwuneclebuyqcjbkwu forum.dlang.org
[2] - Bug 380 - https://issues.dlang.org/show_bug.cgi?id=380
Feb 07 2015
parent reply "Meta" <jared771 gmail.com> writes:
On Sunday, 8 February 2015 at 00:31:42 UTC, Mike wrote:

 I'm elevating this from D.Learn [1] because I think it needs 
 some input from the language designers.

 This code compiles and executes:
 -----------------------------------
 import std.stdio;

 struct StaticRegister {
     static private uint _value;
      property static uint value() { return _value; }
      property static void value(uint v) { _value = v; }

     static alias value this;

     static void test() {
         writeln(this.stringof);
         writeln(typeof(this).stringof);
         writeln(this.value);
     }
 }

 void main(string[] s) {
     // works due to `alias value this`
     StaticRegister = 1;

     StaticRegister.test();
 }
 -----------------------------------

 Output:
 StaticRegister
 StaticRegister
 1


`this` should not exist within a static member function. The fact 
that this code compiles is probably a bug. `StaticRegister = 1` 
is almost definitely not intended behaviour, and I don't know 
about `static alias value this`.



 However, this code fails to compile:
 -----------------------------------
 struct StaticRegister {
     static void test() {
         assert(this is null);
     }
 }

 void main(string[] s) {
     StaticRegister.test();
 }
 -----------------------------------

 Output:
 test.d(5): Error: 'this' is only defined in non-static member 
 functions, not test


This is correct behaviour.



 Is `this` overloaded to mean "this class" in a static context 
 or is `this` only valid in a non-static context.  Please 
 clarify, and if it's a bug, help me understand so I can make an 
 accurate and actionable bug report.


`this` should only be valid in a non-static context, as far as I 
know. That's pretty much what static means: "there is no `this`".
Feb 07 2015
parent reply "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm gmx.net> writes:
On Sunday, 8 February 2015 at 01:57:55 UTC, Meta wrote:

 On Sunday, 8 February 2015 at 00:31:42 UTC, Mike wrote:

 Is `this` overloaded to mean "this class" in a static context 
 or is `this` only valid in a non-static context.  Please 
 clarify, and if it's a bug, help me understand so I can make 
 an accurate and actionable bug report.


 `this` should only be valid in a non-static context, as far as 
 I know. That's pretty much what static means: "there is no 
 `this`".


The following is a useful idiom for static factory methods:

     struct S {
         this(int x) {
         }

         static auto make(int x) {
             return typeof(this)(x);
         }
     }
Feb 08 2015
next sibling parent "Meta" <jared771 gmail.com> writes:
On Sunday, 8 February 2015 at 13:13:18 UTC, Marc Schütz wrote:

 On Sunday, 8 February 2015 at 01:57:55 UTC, Meta wrote:

 On Sunday, 8 February 2015 at 00:31:42 UTC, Mike wrote:

 Is `this` overloaded to mean "this class" in a static context 
 or is `this` only valid in a non-static context.  Please 
 clarify, and if it's a bug, help me understand so I can make 
 an accurate and actionable bug report.


 `this` should only be valid in a non-static context, as far as 
 I know. That's pretty much what static means: "there is no 
 `this`".


 The following is a useful idiom for static factory methods:

     struct S {
         this(int x) {
         }

         static auto make(int x) {
             return typeof(this)(x);
         }
     }


Okay, I forgot about that one. `typeof` is somewhat of a special 
case, though, since you can also do typeof(1/0) and get `int`.
Feb 08 2015
prev sibling parent reply "Mike" <none none.com> writes:
On Sunday, 8 February 2015 at 13:13:18 UTC, Marc Schütz wrote:

 On Sunday, 8 February 2015 at 01:57:55 UTC, Meta wrote:

 On Sunday, 8 February 2015 at 00:31:42 UTC, Mike wrote:

 Is `this` overloaded to mean "this class" in a static context 
 or is `this` only valid in a non-static context.  Please 
 clarify, and if it's a bug, help me understand so I can make 
 an accurate and actionable bug report.


 `this` should only be valid in a non-static context, as far as 
 I know. That's pretty much what static means: "there is no 
 `this`".


 The following is a useful idiom for static factory methods:

     struct S {
         this(int x) {
         }

         static auto make(int x) {
             return typeof(this)(x);
         }
     }


Yes, there are a few cases where `this` seems to be allowed in a 
static context, but is that by accident, or by design?

If by accident, then then we have a bug that, unfortunately, 
people may be using as a feature.  If by design, then there's 
still a bug because it doesn't work consistently.

The grammar specification [1] is silent about the semantics of 
`this` in a static context, and the examples show usage only in a 
non-static context.

So, my qustion still remains:  Does `this` mean "this 
class/struct" in a static context, or does `this` have no 
semantic meaning in a static context?

Mike

[1] - http://dlang.org/expression.html#this
Feb 08 2015
parent Steven Schveighoffer <schveiguy yahoo.com> writes:
On 2/8/15 5:54 PM, Mike wrote:

 On Sunday, 8 February 2015 at 13:13:18 UTC, Marc Schütz wrote:

 On Sunday, 8 February 2015 at 01:57:55 UTC, Meta wrote:

 On Sunday, 8 February 2015 at 00:31:42 UTC, Mike wrote:

 Is `this` overloaded to mean "this class" in a static context or is
 `this` only valid in a non-static context.  Please clarify, and if
 it's a bug, help me understand so I can make an accurate and
 actionable bug report.


 `this` should only be valid in a non-static context, as far as I
 know. That's pretty much what static means: "there is no `this`".


 The following is a useful idiom for static factory methods:

     struct S {
         this(int x) {
         }

         static auto make(int x) {
             return typeof(this)(x);
         }
     }


 Yes, there are a few cases where `this` seems to be allowed in a static
 context, but is that by accident, or by design?


typeof(this) is supposed to work in a static context. I can't find the 
spec part that describes this, but it definitely is not a bug.

Everything else is a bug.

-Steve
Feb 09 2015