Re: Going from CTFE-land to Template-land

[Denis Koroskin]

On Tue, 17 Nov 2009 18:07:03 +0300, Don <nos...@nospam.com> wrote:

Bill Baxter wrote:

Currently this doesn't work, because the CTFE function doesn't "know"
that it's running compile-time:
 int templ_incr(int x)() {
    return x+1;
}
 int ctfe_incr(int x) {
    return templ_incr!(x);
}
 Seems common to write a function that you know is only intended to be
used compile-time.
But it can't compile because the compiler doesn't know you only plan
to call it at compile-time.
 Is something version(__ctfe) might help with?  E.g.
version(__ctfe) {

// only allow cfte_incr to be called at compile-time so it can use templates

    int ctfe_incr(int x) {
        return templ_incr!(x);
    }
}

No. Here's the only functionality you'll get. This works by exploiting bug 1330. It's inefficient: the inCTFE function gets called all the time. Should just be a bool value, which will be constant-folded away. Otherwise, it's the same as this:

// true if evaluated in CTFE, false if called at runtime.
bool inCTFE()
{
      int [1] x = [1];
      int [] y = x;
      y[0] = 2;
      return x[0]!=2;
}

static assert(inCTFE());

void main()
{
   assert(!inCTFE());
}

Haha, nice one!