2 Templates Issues (possibly bugs)

[John Kiro]

Thanks BCS,

I still have questions below..

BCS Wrote:

> Reply to John,
> 
> > Hello there
> > 
> > I've been experimenting with templates & template mixins, and I have
> > some questions & notes, here are they one by one:
> > 
> > 1- I define a template like this:
> > 
> > template Template1(T, alias Var, alias Func)
> > {
> > T abc = Func();		//OK.
> > T xyz = Var;		//#1 Error: non-constant expression i1
> > //Question 1: Why do I have this error?
> > }
> > The above error is issued in case I instantiate the template like this
> > for ex:
> > 
> > Template1!(int, i1, Func1).abc = 3;
> > Or
> > Template1!(int, i1, Func1).xyz = 3;
> 
> what is i1 in this case.
> 
> 
i1 is defined inside main() like this:
  int i1=900;


> > 2- Let's define 2 functions to be used with the previous template
> > (template2):
> > 
> > template Template2(T, alias TFunc1, alias TFunc2)
> > {
> > 	T var1 = TFunc1();
> > 	T var2 = TFunc2();
> > }

> > int Func1()
> > {
> > return 100;
> > }
> > int Func2()
> > {
> > return 200;
> > }
> > Now the next instantiation fails:
> > 
> > Template2!(int,Func1,Func2).var1;	//Error: var has no effect in
> > expression (var1)
> 
> 
> that is correct, referencing the variable var1 but not using it for anything 
> has no effect and is disallowed.
> 
What I understand is that "Template2!(int,Func1,Func2).var1;" is equivalent to
    int var1 = TFunc1();
Isn't it? If so, then what's wrong with it?

> The template you defined contains two global variables that are initialized 
> with the return from two function that are passed in.
> 
> The two issues you mention may be related. What I think is happening is that 
> the initializer for a variable must be a constant. Due to compile time function 
> evaluation (CTFE) the trivial function you used get converted to there return 
> values.
> 
Yes it seems to be so, but I don't see any restriction in the documentation that the initialization must be with a constant.

> If I had to guess, I'd say that the "i1" from above is a non trivial function 
> that can't be run through CTFE. 
> 
I tried something: I referenced a global var inside Func2(), and I got the error: "Error: cannot evaluate Func2() at compile time", thus confirming your thoughts. But as I said, why isn't this allowed, is it a template restriction? Where is it stated in the document?
Strangely, adding a writefln() call inside Func2() doesn't cause any error like this, although this call cannot be done in compile-time!
> 
> > 
> > The next one succeeds:
> > 
> > Template2!(int,Func1,Func2).var1=10;
> > 
> > Next one succeeds as well (even if the instance is not defined before
> > it):
> > 
> > writefln("var1 = ",Template2!(int,Func1,Func2).var1);	//value is 100
> > (as returned by Func1() )
> > Template2!(int,Func1,Func2).var1+=30;	//value is now 130
> > The question here is why the 1st one fails?
> > 
> > Also I see a confusion in the expression
> > "Template2!(int,Func1,Func2).var1", it's sometimes equivalent to
> > variable declaration (as in the previous writefln), and sometimes it's
> > equivalent to a symbol (as in the "+=30" statement).. I hope this note
> > would be added to the documentation.
> 
> as noted above, the expression "Template2!(int,Func1,Func2).var1" is a symbol 
> in all cases. it is actually a reference to a global variable declared in 
> the template.
> 
This is not what I understand; what I know is that template definition doesn't define any variables, it's same as class definition. No object is defined until the class is instantiated.
A global var would be declared if I instantiate the template or use a template mixin.

> I hope that helps, If not, please do ask more questions.
> 

> > 
> > Regards
> > John
> 
> 
Regards,
John