How would you do this in D?

[Don]

Jens Mueller wrote:
> Jonathan M Davis wrote:
>> On 2011-05-10 18:06, Jose Armando Garcia wrote:
>>> Thanks. I should have read
>>> http://www.digitalmars.com/d/2.0/template.html more carefully.
>>>
>>> "Multiple instantiations of a TemplateDeclaration with the same
>>> TemplateArgumentList, before implicit conversions, all will refer to
>>> the same instantiation."
>>>
>>> The default values which are evaluated at the call site are part of
>>> the TemplateArgumentList.
>> Yes, default arguments are evaluated at the call site, not where the code is 
>> declared. I believe that C++ has the opposite behavior, so it's not uncommon 
>> that people miss that. It's _very_ useful though - particularly when dealing 
>> with __FILE__ and __LINE__.
> 
> I did some little testing (see attached file). The bar() example shows
> that default arguments are evaluated at call site (even in C++). If it
> was only evaluated when declared it would always return 1.
> But __LINE__ does not work as in D. I believe that is due to __LINE__
> being a macro in C whereas in D it is evaluated at instantiation time
> (i.e. it is not text replacement). So it seems that instantiation of
> default arguments is the same in D and C++. The difference is that
> __LINE__ and __FILE__ are different things in C++ and D. In C++ there is
> no such thing as evaluating __LINE__. Or let's say they are constant
> expressions (due to textual replacement) where in D they are evaluated
> differently depending on the context.
> Does this make sense?

__LINE__ and __FILE__ are magical when used as default arguments in D, 
for exactly this use case, since it is *extremely* useful. In every 
other case they behave as in C++. (IIRC the magic is not even 
implemented for default arguments of __LINE__ + 0, only for __LINE__).