Call method if declared only

[mipri]

On Friday, 28 February 2020 at 08:08:59 UTC, Виталий Фадеев wrote:
> Searching for beauty readable code...

The pattern throughout Phobos is static tests, like isInputRange!R

So something like this:

   import std.stdio;
   import std.traits;

   template canOnKey(T) {
       static if (__traits(hasMember, T, "onKey"))
           enum canOnKey = isCallable!(__traits(getMember, T, 
"onKey"));
       else
           enum canOnKey = false;
   }

   struct A { void onKey() { writeln("called A"); } }
   struct B { }
   struct C { bool onKey = false; }

   void maybeKey(T)(T o) if (canOnKey!T) {
       o.onKey();
   }
   void maybeKey(T)(T o) if (!canOnKey!T) { }

   void main() {
       auto a = A(), b = B(), c = C();
       maybeKey(a);
       maybeKey(b);
       maybeKey(c);
   }

output:

   called A

(and no other output)

Of course in this exact instance it would be simpler to write

   void maybeKey(T)(T o) {
       static if (canOnKey!T)
           o.onKey();
   }

And rather than lots of specific hasThisExactMethod!T tests, it
might be nice if in your actual program there's a 'class' of
grouped properties that you can test for all at once, again
like isInputRange!T and friends.