Re: The is expression

[Philippe Sigaud]

On Sat, Apr 2, 2011 at 13:05, enuhtac <enuhtac_li...@gmx.de> wrote:
> This is the type I would like to check for:
>
> struct A( T, string s )
> { ... };

Hi,

the trick is to use a function do the work for you. Let's define isAnA:

void isAnA(T, string s)( A!(T,s) a) { }

isAnA can only be called (compiled) with your A.

A!(int, "abc") a;
A!(double, "") b;

isAnA(a); // OK
isAnA(b); // OK
isAnA(123); // does not compile.

So you can nest it in a template and check at compile-time if it compiles:

template isMyA(Type)
{
    static if (__traits(compiles,
                        {
                            void isAnA(T, string s)(A!(T,s) a) {}
                            isAnA(Type.init); // create a value of
type Type, see if isAnA accepts it.
                        }))
        enum bool isMyA = true;
    else
        enum bool isMyA = false;
}

Note that this is strictly tailored to A's with (T, string s) as
arguments. You can relax the constraints by adapting the test
function.

For a more generic way to test for this, you can have a look there:

http://svn.dsource.org/projects/dranges/trunk/dranges/docs/templates.html
      ("isInstanceOf" and "Template ParametersTypeTuple")
http://svn.dsource.org/projects/dranges/trunk/dranges/docs/typepattern.html
     (look for "isA")


Philippe