Check template parameter whether it has "length"

[tcak via Digitalmars-d-learn]

On Thursday, 8 October 2015 at 09:50:12 UTC, John Colvin wrote:
> On Thursday, 8 October 2015 at 09:29:30 UTC, tcak wrote:
>> [...]
>
> 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.

__traits(compiles, { size_t n = A.init.length; });  did the trick.

[code]
size_t maxLength(A)( const A[] listOfString )
	if( __traits( compiles, { size_t len = A.init.length; } ) )
{
	size_t len = 0;

	foreach(A str; listOfString)
		if( str.length > len ) len = str.length;

	return len;
}
[/code]

BTW, there is nothing like std.traits.hasLength.