language_fan wrote:
Mon, 12 Oct 2009 12:07:19 +0200, Don thusly wrote:
language_fan wrote:
Sat, 10 Oct 2009 10:30:31 +0200, Don thusly wrote:
The more fundamental problem is that you can't instantiate a template
from inside CTFE. IE, you can cross from the "compile-time world" to
the "runtime world" only once -- you can never get back.
That's not exactly true. Also both templates and CTFE are compile time
features. You can compute a value with CTFE in the "value world" and
lift the result to the "type world" with a template.
Yes, but the problem is that variables inside a CTFE function, even
though they are known at compile-time, are not permitted to be used as
template value parameters. For example:
template A(int X) { enum int A = B(X)+1; } // OK, template can call CTFE
int B(int X) { return A!(X) + 1; }
// Not OK, CTFE cannot call template.
As far as I can tell there is no reason why you cannot call templates
from a CTFE code. Your code above has two problems: a) it never
terminates
It wasn't meant to be a compilable example.
b) due to some lookup problem the compiler gets confused, this
has nothing to do with CTFE not being able to call templates - for
instance this works:
template A(int X) { enum int A = 2+1; }
int B(int X) { return A!(X) + 1; }
You're seeing a few bugs there. In the template X isn't a constant, it's
an alias (that is NOT in the spec, bug 2962). If you try adding a
"static assert(X!=2)", you'll find it's not a constant.
It will let you write: enum int A = X + 1; but that's bug 2414.
The rule is, any CTFE function must also be evaluatable at run-time.
Templates cannot be instantiated at run-time.
Therefore templates cannot be instantiated in CTFE.
