Re: Why internal linkage variables can't be used to instantialize an template?
On Mar 26, 11:51 am, "h.yuz...@gmail.com" <h.yuz...@gmail.com> wrote:
Compiler: VC8.0
External linkage variable can be used as template's non-type
parameters. But internal not even if the value of it can be
evaluated at compiling time. Why?
Because the standard says so.
Historically, when the standard was being written, templates
were very, very new, and there had been very little experience
with them. There was some doubt as to whether support for
instantiating the template with something which didn't have
external linkage would be implementable, and also as to whether
it would really be that useful. So the committee wisely decided
to ban it, knowing that if there was a demand, and it were
proven that it could be implemented, it would be trivial to add
it later (where as removing something from the standard is
almost impossible).
I believe that there is currently a proposal to loosen the
restrictions before the committee.
Thank you all. The code
snipet like follow:
int i1 = 1;
static int i2 = 2;
template<const int& i> void f() {}
int main()
{
f<i1>();
f<i2>(); /*Error: an expression involving objects with internal
linkage cannot be used as a non-type argument*/
}
I can observe that static i2 was initialized to 2 at compiling time.
The initialization has nothing to do with it, since you're
binding a reference. Initialization or not, there's no
technical reason to ban this.
Of course, In this particular case, there is a simple and
elegant work-around: put i2 in an anonymous namespace, rather
than declaring it static. It is generally recommended to avoid
static at namespace scope anyway. The problem shows up,
however, with locals, and in particular, local types.
--
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