Summary  
This chapter introduces the concept of pure functions and shows how to structure code to avoid side effects—such as unexpected state changes or I/O—thereby improving predictability and testability.  

General domain of usage  
Unit testing

Master the functional programming tools and patterns available in modern C++. Learn to use lambdas, std::function, callable objects, predicates, and higher-order utilities to write expressive, reusable, and efficient code with the STL.

Explore the basics of functional programming in C++ and the building blocks for functional utilities.

Apply functional utilities with STL algorithms and predicates for expressive code.

Delve into advanced functional programming techniques and patterns in C++.

Apply functional programming tools to solve real-world problems using the STL.

Explore idiomatic patterns and best practices for functional programming in modern C++.

Avoiding Side Effects

Avoiding Side Effects