Variadic parameter of length 1
I have several times seen a construct
template foo(T...) if(T.length == 1)
{
...
}
What is that good for?
Why using variadic parameter if anyway exactly one parameter is
required?!?
Re: Variadic parameter of length 1
On Wednesday, 20 August 2014 at 15:11:53 UTC, Dominikus Dittes
Scherkl wrote:
I have several times seen a construct
template foo(T...) if(T.length == 1)
{
...
}
What is that good for?
Why using variadic parameter if anyway exactly one parameter is
required?!?
AFAIK, it's a historical workaround to accept T as either alias
or not alias, as varargs have auto alias. EG:
foo!int //OK
foo!hello //OK too
Re: Variadic parameter of length 1
On Wednesday, 20 August 2014 at 15:26:14 UTC, monarch_dodra wrote: AFAIK, it's a historical workaround to accept T as either alias or not alias, as varargs have auto alias. EG: foo!int //OK foo!hello //OK too Ah, ok. And why historical? Is that not necessary anymore? What better solution is there today?
Re: Variadic parameter of length 1
On Wednesday, 20 August 2014 at 15:11:53 UTC, Dominikus Dittes
Scherkl wrote:
I have several times seen a construct
template foo(T...) if(T.length == 1)
{
...
}
What is that good for?
Why using variadic parameter if anyway exactly one parameter is
required?!?
That's because template alias parameters cannot take builtin
types like int, and type parameters can only take types. But
tuple elements can be anything.
struct SomeType {}
template a(X) {enum a = 0;} /* type parameter */
enum a1 = a!int; /* ok */
enum a2 = a!SomeType; /* ok */
version(none) enum a3 = a!42; /* Error: template instance a!42
does not match template declaration a(X) */
template b(alias x) {enum b = 0;} /* alias parameter */
version(none) enum b1 = b!int; /* Error: template instance b!int
does not match template declaration b(alias x) */
enum b2 = b!SomeType; /* ok */
enum b3 = b!42; /* ok */
template c(x ...) if(x.length == 1) {enum c = 0;} /* tuple
parameter */
enum c1 = c!int; /* ok */
enum c2 = c!SomeType; /* ok */
enum c3 = c!42; /* ok */
Re: Variadic parameter of length 1
On Wed, Aug 20, 2014 at 5:34 PM, Dominikus Dittes Scherkl via Digitalmars-d-learn digitalmars-d-learn@puremagic.com wrote: On Wednesday, 20 August 2014 at 15:26:14 UTC, monarch_dodra wrote: AFAIK, it's a historical workaround to accept T as either alias or not alias, as varargs have auto alias. EG: foo!int //OK foo!hello //OK too Ah, ok. And why historical? Is that not necessary anymore? What better solution is there today? No better solution that I know of. alias template parameters (alias a) match symbols (names, user-defined types) whereas type parameter (T) match only pure types. So when we need to match anything, we use (T...) if (T.length == 1)
Re: Variadic parameter of length 1
On Wednesday, 20 August 2014 at 15:37:18 UTC, Philippe Sigaud via Digitalmars-d-learn wrote: No better solution that I know of. alias template parameters (alias a) match symbols (names, user-defined types) whereas type parameter (T) match only pure types. So when we need to match anything, we use (T...) if (T.length == 1) Ok, now it's clear. Thank you very much. Sometimes D is really a little weird...
Re: Variadic parameter of length 1
On Wednesday, 20 August 2014 at 15:34:30 UTC, Dominikus Dittes Scherkl wrote: And why historical? Is that not necessary anymore? What better solution is there today? Historical in a sense that distinct can be anything template parameter is probably a better approach but it is too late to change such core language part.
Re: Variadic parameter of length 1
On Wednesday, 20 August 2014 at 17:02:59 UTC, Dicebot wrote: On Wednesday, 20 August 2014 at 15:34:30 UTC, Dominikus Dittes Scherkl wrote: And why historical? Is that not necessary anymore? What better solution is there today? Historical in a sense that distinct can be anything template parameter is probably a better approach but it is too late to change such core language part. Yeah, what he said. It's a language artifact.
Re: Variadic parameter of length 1
On Wed, 20 Aug 2014 17:47:36 + monarch_dodra via Digitalmars-d-learn digitalmars-d-learn@puremagic.com wrote: Yeah, what he said. It's a language artifact. by the way, it would be nice to have wiki page with such artifacts and their explanations. signature.asc Description: PGP signature
