3 Exploring Functions

Functions are a fundamental concept in programming; regardless of the topic we discuss, we end up writing functions. Trying to cover functions in a single chapter is not only hard but also not very rational. Being a fundamental element of the language, functions are encountered in every recipe of this book. This chapter, however, covers modern language features related to functions and callable objects, with a focus on lambda expressions, concepts from functional languages such as higher-order functions, and type-safe functions with a variable number of arguments.

The recipes included in this chapter are as follows:

• Defaulted and deleted functions
• Using lambdas with standard algorithms
• Using generic and template lambdas
• Writing a recursive lambda
• Writing a function template with a variable number of arguments
• Using fold expressions to simplify variadic function templates
• Implementing the higher-order functions map and fold
• Composing functions into a higher-order function
• Uniformly invoking anything callable

We will start this chapter by learning about a feature that makes it easier for us to provide special class member functions or prevent any function (member or non-member) from being invoked.