Elements of constant array as template arguments.

Re: Elements of constant array as template arguments.

Andrzej 'Foxy' D. wrote:[color=blue]
> I cannot figure out, why is the following code incorrect:
>
> const int array[] = { 1 };
> const int var = 1;
>
> template<int I>
> struct Blah {};
>
> int main() {
> Blah<var> b1;
> Blah<array[0]> b2; // ERROR!
> }
>
> I know that template arguments can only be constant expressions. But
> I don't understand why isn't 'array[0]' treated as a constant
> expression.[/color]

Indexing is a run-time thing. 'array' is converted to a pointer and
then indexing is performed on that pointer. It does not qualify as
a compile-time constant expression which is defined in subclause 5.19.
Pointers just cannot be part of a constant expression except as
arguments of 'sizeof'.
[color=blue]
> It seems to me that if an array is const and the index is
> a constant expression (e.g. 0), then the whole expression is
> constant. I can't imagine, how a compiler can NOT know, what's the
> value of array[0] during compile-time.[/color]

The langauge just doesn't specify it that way. As to why it is so,
ask in 'comp.std.c++', they could explain the rationale behind
different parts of the Standard.

V
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