## digitalmars.D.learn - is(Type:Type) fails to evaluate to true

- silverclaw <post_message newgroup.com> Oct 08 2007
- Frits van Bommel <fvbommel REMwOVExCAPSs.nl> Oct 08 2007
- Daniel Keep <daniel.keep.lists gmail.com> Oct 08 2007

I picked up the Matrix class I abandoned some months ago and updated it to the latest DMD (2.022). However, on the opMulAssign the following check fails where it should pass: const Matrix!(Type) opMul(T)(const T multiplier) { static if (is(T:Matrix)) //multiplication by a Matrix { (....) } else //multiplication by a scalar { (....) } } It used to work on some other compiler version. So, on the main() I made the following code: (static) if (is(Matrix:Matrix)) writefln("true"); else writefln("false"); always evaluates to false (prints "false") with or without the "static". As you may have noticed, it is a template class: public class Matrix(Type) As I said, it used to work, I'm going to try to find out with which DMD. Any suggestions?

Oct 08 2007

silverclaw wrote:I picked up the Matrix class I abandoned some months ago and updated it to the latest DMD (2.022). However, on the opMulAssign the following check fails where it should pass: const Matrix!(Type) opMul(T)(const T multiplier) { static if (is(T:Matrix)) //multiplication by a Matrix { (....) } else //multiplication by a scalar { (....) } }

I assume this is a member of your Matrix template?It used to work on some other compiler version. So, on the main() I made the following code: (static) if (is(Matrix:Matrix)) writefln("true"); else writefln("false"); always evaluates to false (prints "false") with or without the "static". As you may have noticed, it is a template class: public class Matrix(Type) As I said, it used to work, I'm going to try to find out with which DMD. Any suggestions?

Are you sure it used to work like this? Notice that inside the Matrix template definition, 'Matrix' refers to the current instantiation of the template, not the template itself. For a matrix multiplication, you may not want either of those behaviors. The current instantiation would only allow matrices of the exact same type (not other, compatible types. consider Matrix!(int) * Matrix!(short) for example). If Matrix referred to the template itself it would simply not work (causing the is() to return false) since a template is not a class (even though an *instance* of that template may be). One simple method to detect whether a type is an instance of Matrix would be to make all template instances derive from a common (non-templated) base class (or interface), and test for that. (Using a class would be most efficient, since interfaces require an extra vtable pointer) Note that you don't even need to declare any members for the base class/interface since you know that any class derived from it should have certain members, and your code uses the actual type T of that class. It's just a tag to tell you "This thing is a Matrix". So then the declaration would start with e.g. --- class MatrixBase {} public class Matrix(Type) : MatrixBase { --- and the test would be something like --- static if (is(T:MatrixBase)) //multiplication by a Matrix ---

Oct 08 2007

silverclaw wrote:I picked up the Matrix class I abandoned some months ago and updated it to the latest DMD (2.022). However, on the opMulAssign the following check fails where it should pass: const Matrix!(Type) opMul(T)(const T multiplier) { static if (is(T:Matrix)) //multiplication by a Matrix { (....) } else //multiplication by a scalar { (....) } } It used to work on some other compiler version. So, on the main() I made the following code: (static) if (is(Matrix:Matrix)) writefln("true"); else writefln("false"); always evaluates to false (prints "false") with or without the "static". As you may have noticed, it is a template class: public class Matrix(Type) As I said, it used to work, I'm going to try to find out with which DMD. Any suggestions?

Does is(T:Matrix!(Type)) work? Considering that a template doesn't exist until you instantiate it, I find it odd that is(T:Matrix) ever worked at all. -- Daniel

Oct 08 2007