In this chapter we explore a template programming technique called expression templates. It was originally invented in support of numeric array classes, and that is also the context in which we introduce it here.
A numeric array class supports numeric operations on whole array objects. For example, it is possible to add two arrays, and the result contains elements that are the sums of the corresponding values in the argument arrays. Similarly, a whole array can be multiplied by a scalar, meaning that each element of the array is scaled. Naturally, it is desirable to keep the operator notation that is so familiar for built-in scalar types:
Array<double> x(1000), y(1000); … x = 1.2*x ...