www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - Traits of variadic templates

reply Jeff <massung gmail.com> writes:
Let's say I have...

void foo(T...)(T xs)
{
     foreach(x; xs)
     {
         if (typeid(x) == typeid(int))
             writeln("int: ", x);
         else
             writeln("str: ", x);
     }
}

 From the body, it's obvious I really only want int or string to 
be passed in to foo. Ideally, this check would be done at 
compile-time.

Obviously, I could modify foo to something like:

void foo(Algebraic!(int, string)[] xs)

I could also put checks in the body at runtime. But, for the sake 
of this thread, let's not. ;-)

What would be ideal (IMO) would be something along the lines of:

void foo(T...)(T xs) if (isIntegral!T || isSomeString!T)

But, those don't work because T is a Tuple of the types. Is there 
some trait combination I can use to do this? Something like 
(obviously made up)...

all(TemplateArgsOf!T, t => isIntegral!t || isSomeString!t)

Thanks!
Feb 09 2021
parent reply Paul Backus <snarwin gmail.com> writes:
On Tuesday, 9 February 2021 at 16:22:16 UTC, Jeff wrote:

 But, those don't work because T is a Tuple of the types. Is 
 there some trait combination I can use to do this? Something 
 like (obviously made up)...

 all(TemplateArgsOf!T, t => isIntegral!t || isSomeString!t)

 Thanks!


import std.meta: allSatisfy, Or = templateOr;
allSatisfy!(Or!(isIntegral, isSomeString), T);

http://phobos.dpldocs.info/std.meta.allSatisfy.html
http://phobos.dpldocs.info/std.meta.templateOr.html
Feb 09 2021
parent Jeff <massung gmail.com> writes:
On Tuesday, 9 February 2021 at 16:25:46 UTC, Paul Backus wrote:

 On Tuesday, 9 February 2021 at 16:22:16 UTC, Jeff wrote:

 But, those don't work because T is a Tuple of the types. Is 
 there some trait combination I can use to do this? Something 
 like (obviously made up)...

 all(TemplateArgsOf!T, t => isIntegral!t || isSomeString!t)

 Thanks!


 import std.meta: allSatisfy, Or = templateOr;
 allSatisfy!(Or!(isIntegral, isSomeString), T);

 http://phobos.dpldocs.info/std.meta.allSatisfy.html
 http://phobos.dpldocs.info/std.meta.templateOr.html


Thanks so much!
Feb 09 2021