Going from CTFE-land to Template-land

[Denis Koroskin]

On Tue, 17 Nov 2009 18:07:03 +0300, Don <nospam at 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!