On 6/7/16 9:42 PM, Carl Vogel wrote:
Hi,
What's the best way, when using a Callable as a template parameter, to
constrain it by signature?
For example, if you have a function that sorts an array like so:
T[] sortArray(alias less, T)(T[] arr) { ... }
Then you know that you'd want `less` to have signature (T, T) -> bool.
Now, I can use something like isCallable std.traits to make sure the
predicate is a Callable, and there are various function traits in the
module that I could combine with `is` clauses to enforce the signature
it seems, but that seems very clunky. Is there a better way?
I note that I don't find many examples of this in phobos, so I'm
wondering if there actually is a good solution...
The de-facto way to do this is to write a lambda, and check that you can
call the function inside the lambda, then check to see if that compiles.
e.g.:
if(is(typeof( { bool b = less(T.init, T.init)); } )))
The is(typeof( construct means "does this have a valid type". Should
only work if the call is sound.
You can also use __traits(compiles, but for some reason Phobos writers
prefer the is(typeof( mechanism (I remember someone saying there is a
difference, but I don't know what it is).
-Steve
