www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - How to determine constness at compile time?

reply "Janice Caron" <caron800 googlemail.com> writes:
I'm trying to write a template to determine the constness (or not) of
array members. The following seemed the obvious thing to do, but it
doesn't work. Anyone know how to do it right?

template constID(T)
{
    static if(is(T:invariant(T)[]))
    {
        static const int constID = 2;
    }
    else static if(is(T:const(T)[]))
    {
        static const int constID = 1;
    }
    else
    {
        static const int constID = 0;
    }
}

void main()
{
    writefln(constID!(string));
}

This prints 0. I would have expected (hoped for) it to print 2.
Nov 27 2007
next sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
"Janice Caron" wrote
 I'm trying to write a template to determine the constness (or not) of
 array members. The following seemed the obvious thing to do, but it
 doesn't work. Anyone know how to do it right?

import std.stdio; template realConstID(T) { static if(is(T X : X[])) alias X Y; else static if(is(T X : X*)) alias X Y; else alias T Y; static if(is(Y == invariant(Y))) { static const int constID = 2; } else static if(is(Y == const(Y))) { static const int constID = 1; } else { static const int constID = 0; } } template constID(T) { static const int constID = realConstID!(T).constID; } void main() { writefln(constID!(char[])); writefln(constID!(const(char)[])); writefln(constID!(invariant(char)[])); writefln(constID!(char*)); writefln(constID!(const(char)*)); writefln(constID!(invariant(char)*)); } outputs: 0 1 2 0 1 2 -Steve
Nov 27 2007
prev sibling parent reply "Janice Caron" <caron800 googlemail.com> writes:
That is genius! Sheer genius! Thank you.

I did ponder for a while why constID!() invoked realConstID!(),
instead of doing it in a single template. So I did the experiment and
found it wouldn't compile if you did it in a single template. I don't
know why though!

Anyway, cheers!
Nov 27 2007
parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
"Janice Caron" wrote
 That is genius! Sheer genius! Thank you.

 I did ponder for a while why constID!() invoked realConstID!(),
 instead of doing it in a single template. So I did the experiment and
 found it wouldn't compile if you did it in a single template. I don't
 know why though!

I think it has something to do with the alias Y statement, because now constID is not the only member of the template. I know you said this worked for you in the past, but maybe it was just luck that it worked :) -Steve
Nov 27 2007