On 04/10/2011 04:10 PM, spir wrote:
> Hello,
>
> I need a trick to allow a function template parameter be optional.
> The following (reduced case) fails because D wants to call f:
>
> uint f(uint i) { return i; }
> struct S (alias func=null) {
> enum bool hasFunc = !(func == null); // *** error line ***
> }
> unittest {
> // ok
> auto s1 = S!()();
> writeln(s1.hasFunc);
> // not ok
> auto s2 = S!(f)();
> writeln(s2.hasFunc);
> }
> ==>
> Error: function ___.f (uint i) is not callable using argument types ()
> (I consider that a bug due to "f" implicitely meaning "f()" --but this is
> another story.)
>
> The following also fails:
>
> // not ok
> auto s3 = S1!(&f)();
> writeln(s3.hasFunc);
> ==>
> Error: expression & f is not a valid template value argument

PS: I also cannot pass a typed func:

struct S (uint delegate (uint) func=null) {
    enum bool hasFunc = !(func == null);
}

==>
	Error: arithmetic/string type expected for value-parameter, not uint delegate(uint)

-- 
_________________
vita es estrany
spir.wikidot.com

On 04/10/2011 04:10 PM, spir wrote:
> Hello,
>
> I need a trick to allow a function template parameter be optional.
> The following (reduced case) fails because D wants to call f:
>
> uint f(uint i) { return i; }
> struct S (alias func=null) {
> enum bool hasFunc = !(func == null); // *** error line ***
> }
> unittest {
> // ok
> auto s1 = S!()();
> writeln(s1.hasFunc);
> // not ok
> auto s2 = S!(f)();
> writeln(s2.hasFunc);
> }
> ==>
> Error: function ___.f (uint i) is not callable using argument types ()
> (I consider that a bug due to "f" implicitely meaning "f()" --but this is
> another story.)

Found one solution using isCallable! Other tricks welcome...

uint f(uint i) { return i; }
struct S (alias func=null) {
    enum bool hasFunc = isCallable!(typeof(func));
}
unittest {
    auto s1 = S!()();
    writeln(s1.hasFunc);
    auto s2 = S!(f)();
    writeln(s2.hasFunc);
}

I'd also like to know why pointer cannot be template *alias* parameters, like in:
    auto s2 = S!(&f)();
==>
	Error: expression & f is not a valid template value argument

Denis
-- 
_________________
vita es estrany
spir.wikidot.com

On 10/04/2011 15:31, spir wrote:
> I'd also like to know why pointer cannot be template *alias* parameters,
> like in:
> auto s2 = S!(&f)();
> ==>
> Error: expression & f is not a valid template value argument
>
> Denis

First of all, that error is useless, you should probably report a bug for that (particularly if it doesn't include a line number, I'm not sure if you've omitted that or not).

Secondly, alias parameters are used to pass a symbol to a template, &f is not a symbol, it's a pointer to the value held in symbol f.

--
Robert
http://octarineparrot.com/