How made my to!MyStruct

[Jonathan M Davis]

On 2011-06-25 11:09, Zardoz wrote:
> Hi. I made a parametrized struct, a Vector, and I'm trying to make to! work
> to make conversion between Vectors with different parametrized type.
> 
> Code :
> 
> public struct Vector(T, size_t dim_)
> if (__traits(isFloating, T) ) {
>   static enum size_t dim = dim_; /// Vector Dimension
> 
>   static assert (dim >= 2 && dim <= 4, "Not valid dimension size.");
>   static assert (__traits(isFloating, T), "Type not is a Float Point
> type."); // Ok, redundant static assert (is(T : real), "Type not is like a
> Float Point type.");
> 
>   union {
>     private T[dim] coor; /// Vector coords like Array
> 
>     struct {
>       static if( dim >= 1) T x;
>       static if( dim >= 2) T y;
>       static if( dim >= 3) T z;
>       static if( dim >= 4) T w;
>     }
>   }
>   // Constructors and other methods/stuff....
> }
> 
> Actually I saw std.conv code and I see that to! call to a parametrized
> funtion called toImpl(T,S) (S s). I made my own toImpl for convert a
> Vector to another Vector and I checked that it works calling directly :
> /**
> * to converto a vector to other vector
> */
> T toImpl(T, S)(S s)
> if (!implicitlyConverts!(S, T) && isVector!T && isVector!S )
> {
>   static assert (T.dim >= S.dim, "Original Vector bigger that destiny
> Vector"); T newVector; auto i = 0;
>   static if (is (typeof(newVector.x) == typeof(s.x)))  {
>     for (; i < S.dim; i++)
>       newVector.coor[i] =  s.coor[i];
>   } else {
>     for (; i < S.dim; i++)
>       newVector.coor[i] =  to!(typeof(newVector.x))(s.coor[i]);
>   }
> 
>   for (; i< T.dim; i++) // Expands a small vector to a bigger dimension
> with 0 value in extra dimension newVector.coor[i] = 0;
> 
>   return newVector;
> }
> 
> But when I try to use to! (for example to!Vector(double, 4)
> (other_Vector)), I get a error from srd.conv :
> 
> /usr/include/d/dmd/phobos/std/conv.d(99): Error: template
> std.conv.toImpl(T,S) if (!implicitlyConverts!(S,T) && isSomeString!(T) &&
> isInputRange!(Unqual!(S)) && isSomeChar!(ElementType!(S))) does not match
> any function template declaration
> /usr/include/d/dmd/phobos/std/conv.d(99): Error: template
> std.conv.toImpl(T,S) if (!implicitlyConverts!(S,T) && isSomeString!(T) &&
> isInputRange!(Unqual!(S)) && isSomeChar!(ElementType!(S))) cannot deduce
> template function from argument types
> !(Vector!(double,4))(Vector!(float,2))
> /usr/include/d/dmd/phobos/std/conv.d(99): Error: template instance errors
> instantiating template src/vector.d(139): Error: template instance
> std.conv.to!(Vector!(double,4)).to!(Vector!(float,2)) error instantiating
> 
> What I'm doing wrong ???

std.conv.to does not currently work to convert between user-defined types. 
There has been some discussion about making it work for such cases, and 
there's currently a pull request on github that deals with it, so it may be 
possible to convert between user-defined types with std.conv.to in the next 
release, but you can't currently do it.

- Jonathan M Davis