Book Image

Template Metaprogramming with C++

By : Marius Bancila
Book Image

Template Metaprogramming with C++

By: Marius Bancila

Overview of this book

Learn how the metaprogramming technique enables you to create data structures and functions that allow computation to happen at compile time. With this book, you'll realize how templates help you avoid writing duplicate code and are key to creating generic libraries, such as the standard library or Boost, that can be used in a multitude of programs. The introductory chapters of this book will give you insights into the fundamentals of templates and metaprogramming. You'll then move on to practice writing complex templates and exploring advanced concepts such as template recursion, template argument deduction, forwarding references, type traits, and conditional compilation. Along the way, you'll learn how to write variadic templates and how to provide requirements to the template arguments with C++20 constraints and concepts. Finally, you'll apply your knowledge of C++ metaprogramming templates to implement various metaprogramming patterns and techniques. By the end of this book, you'll have learned how to write effective templates and implement metaprogramming in your everyday programming journey.
Table of Contents (16 chapters)
Part 1: Core Template Concepts
Part 2: Advanced Template Features
Part 3: Applied Templates
Appendix: Closing Notes

Exploring requires expressions

A requires expression may be a complex expression, as seen earlier in the example with the container concept. The actual form of a requires expression is very similar to function syntax and is as follows:

requires (parameter-list) { requirement-seq }

The parameter-list is a comma-separated list of parameters. The only difference from a function declaration is that default values are not allowed. However, the parameters that are specified in this list do not have storage, linkage, or lifetime. The compiler does not allocate any memory for them; they are only used to define requirements. However, they do have a scope, and that is the closing curly brace of the requires expression.

The requirements-seq is a sequence of requirements. Each such requirement must end with a semicolon, like any statement in C++. There are four types of requirements:

  • Simple requirements
  • Type requirements
  • Compound requirements
  • Nested requirements
  • ...