Using fold expressions to simplify variadic function templates

In this chapter, we are discussing folding several times; this is an operation that applies a binary function to a range of values to produce a single value. We have seen this when we discussed variadic function templates and will see it again with higher-order functions. It turns out there is a significant number of cases where the expansion of a parameter pack in variadic function templates is basically a folding operation. To simplify writing such variadic function templates C++17 introduced fold expressions that fold an expansion of a parameter pack over a binary operator. In this recipe, we will see how to use fold expressions to simplify writing variadic function templates.

Getting ready

The examples in this recipe are based on the variadic function template add() that we wrote in the previous recipe, Writing a function template with a variable number of arguments. That implementation is a left-folding operation. For simplicity...