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 */
