Check template parameter whether it has "length"

[John Colvin via Digitalmars-d-learn]

On Thursday, 8 October 2015 at 09:29:30 UTC, tcak wrote:
> I am "trying" to write a function that takes an array of items, 
> and returns the length of longest item.
>
> [code]
> size_t maxLength(A)( const A[] listOfString ) if( __traits( 
> hasMember, A, "length" ) )
> {
> 	return 0; // not implemented yet
> }
> [/code]
>
> I tried it with
>
> if( __traits( compiles, A.length ) )
>
> as well. But compiler doesn't match it.
>
> writeln("Max Length: ", maxLength( ["foo", "123456789"] ));
>
> Compilers says it cannot deduce function from argument types ...
>
> I do not want to check whether the type "A" is string, char[], 
> etc. As long as it has length (please do not put me into 
> ranges, library functions etc as much as possible), I want the 
> function to accept it.

I'm 99% sure something like __traits(hasMember, int[], "length" ) 
should evaluate to true. Please file a bug at issues.dlang.org   
I notice it also doesn't work for "ptr".

The correct workaround:
__traits(compiles, A.init.length ));
or
__traits(compiles, listOfStrings.length ));

A.length doesn't work because length is not a static member, so 
it's only accessible from an instance.

The __traits(compiles, ...) solution is actually more general 
because it will work if .length is implemented via UFCS and 
opDispatch.

FYI:
If you want to check whether a statement will compile, as opposed 
to an expression, make a function/delegate out of it, e.g.:
__traits(compiles, { size_t n = A.init.length; });
to check that A has a member length that can be assigned to 
size_t.

P.S. always check std.traits for solutions all your static 
reflection problems, there's a lot of good stuff in there.