self-referential invariant member

[Neil Vice]

"Denton Cockburn" <diboss at hotmail.com> wrote in message 
news:pan.2008.02.08.12.52.58.37333 at hotmail.com...
> On Wed, 06 Feb 2008 22:57:53 +0900, Neil Vice wrote:
>
>> I am attempting to provide a constant instance of a struct as a member of
>> the struct (in 2.010) and am getting unexpected compiler errors. Can 
>> someone
>> explain what the compiler thinks I'm trying to do here and/or how I'd go
>> about achieving what I want if it's possible?
>>
>> The following code:
>>
>>     struct Test
>>     {
>>         public invariant Test Default;
>>     }
>>
>>     void main()
>>     {
>>         Test test = Test.Default;
>>     }
>>
>> produces the following compiler output:
>>
>>     test.d(3): struct test.Test unable to resolve forward reference in
>> definition
>>
>> Similarly if Test is declared as a class not a struct the compiler 
>> produces
>> the following:
>>
>>     test.d(9): Error: 'this' is only allowed in non-static member 
>> functions,
>> not main
>>     test.d(9): Error: this for Default needs to be type Test not type int
>>     test.d(9): Error: cannot implicitly convert expression (this.Default) 
>> of
>> type invariant(Test) to test.Test
>>
>> If Default is declared as a manifest constant using the enum keyword the
>> compiler gets a stack overflow (bug #1800).
>>
>> If I attempt to initialise Default (where Test is a struct), the compiler
>> makes reference to opCall:
>>
>>     public invariant Test Default = {};
>>
>> yields:
>>
>>     test.d(9): Error: no property 'opCall' for type 'Test'
>>     test.d(9): Error: function expected before (), not 1 of type int
>>     test.d(9): Error: cannot implicitly convert expression (1(Default)) 
>> of
>> type int to Test
>
> You need to define your struct with a static opCall to handle an invariant
> Test:

I can't assign an invariant struct to a mutable struct variable without 
explicitly implementing a copy operator (opCall)?!

> Also, Default has to be declared static.

Yeh, I had incorrectly assumed invariants were static.

> I think that opCall will have to be set up to copy that Default;
> Isn't what you want a ref to that Default struct?  I thought assigning
> a struct is equivalent to copying it.

The struct only wraps a ulong so a copy is just fine. I also thought 
assigning a struct should be equivalent to copying it, but from this 
exercise it would seem that that is only the case if you explicitly 
implement opCall to perform the task.

Given the nature of structs, copying using a method rather than a straight 
binary copy strikes me as not only appallingly slow but a significant 
inconvenience to the developer =( Again I could be missing something...

Incidently, DMD often spits out the most confusing error messages that (to 
me) are seemingly unrelated to the actualy problem even when explained to 
me. With no other language/compiler have I had so much trouble tracking down 
errors of this nature.