## digitalmars.D.learn - Cross product template

- Diggory (15/15) May 14 2013 I have a vector struct, Vector(T, uint N) templated on the type T
- Diggory (15/30) May 14 2013 Well, after much trial and error I ended up with this, which
- Simen Kjaeraas (5/17) May 14 2013 auto cross(T, uint N, U...)(Vector!(T,N) a, U b) { return 0; }

I have a vector struct, Vector(T, uint N) templated on the type T and number of components, N. I'm trying to write a function "cross" which will calculate the cross product of a number of vectors. For a given number of components, N, the cross function should take N-1 arguments, each one a Vector!(?, N) and will return a vector perpendicular to the vectors passed in. The ? means the type is free to be anything. The problem is that however I try to write it, the template argument deduction isn't powerful enough to work out which instantiation to use. I thought something like this would work to deduce the parameters and then I could use constraints to enforce the other rules, but no: auto cross(T, N, U...)(Vector!(T, N) a, U b) { return 0; }

May 14 2013

On Wednesday, 15 May 2013 at 01:31:43 UTC, Diggory wrote:I have a vector struct, Vector(T, uint N) templated on the type T and number of components, N. I'm trying to write a function "cross" which will calculate the cross product of a number of vectors. For a given number of components, N, the cross function should take N-1 arguments, each one a Vector!(?, N) and will return a vector perpendicular to the vectors passed in. The ? means the type is free to be anything. The problem is that however I try to write it, the template argument deduction isn't powerful enough to work out which instantiation to use. I thought something like this would work to deduce the parameters and then I could use constraints to enforce the other rules, but no: auto cross(T, N, U...)(Vector!(T, N) a, U b) { return 0; }Well, after much trial and error I ended up with this, which seems to work: template VectorN(T : Vector!(T, N), uint N) { enum VectorN = N; } template isVectorN(uint N) { template isVectorN(T) { enum isVectorN = VectorN!(T) == N; } } auto cross(T...)(T args) if (allSatisfy!(isVectorN!(T.length+1), T) && T.length) { return 0; }

May 14 2013

On Wed, 15 May 2013 03:31:40 +0200, Diggory <diggsey googlemail.com> wrote:I have a vector struct, Vector(T, uint N) templated on the type T and number of components, N. I'm trying to write a function "cross" which will calculate the cross product of a number of vectors. For a given number of components, N, the cross function should take N-1 arguments, each one a Vector!(?, N) and will return a vector perpendicular to the vectors passed in. The ? means the type is free to be anything. The problem is that however I try to write it, the template argument deduction isn't powerful enough to work out which instantiation to use. I thought something like this would work to deduce the parameters and then I could use constraints to enforce the other rules, but no: auto cross(T, N, U...)(Vector!(T, N) a, U b) { return 0; }auto cross(T, uint N, U...)(Vector!(T,N) a, U b) { return 0; } Oughta work. -- Simen

May 14 2013