We are now going to look at an alternative to the policy-based design we have been studying so far. It is not as general, but when it works, it can provide all the advantages of the policies, in particular, the composability, without some of the problems. To introduce this new approach, we will consider the problem of designing a custom value type.
A value type, to put it simply, is a type that behaves mostly like an int. Often, these types are numbers. While we have a set of built-in types for that, we may want to operate on rational numbers, complex numbers, tensors, matrices, or numbers that have units associated with them (meters, grams, and so on). These value types support a set of operations such as arithmetic operations, comparisons, assignment, and copying. Depending on what the value represents, we may need only a limited subset of these...
