Book Image

Template Metaprogramming with C++

By : Marius Bancila
5 (1)
Book Image

Template Metaprogramming with C++

5 (1)
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

The std::declval type operator

The std::declval is a utility type operation function, available in the <utility> header. It’s in the same category as functions such as std::move and std::forward that we have already seen. What it does is very simple: it adds an rvalue reference to its type template argument. The declaration of this function looks as follows:

template<class T>
typename std::add_rvalue_reference<T>::type declval() noexcept;

This function has no definition and therefore it cannot be called directly. It can only be used in unevaluated contextsdecltype, sizeof, typeid, and noexcept. These are compile-time-only contexts that are not evaluated during runtime. The purpose of std::declval is to aid with dependent type evaluation for types that do not have a default constructor or have one but it cannot be accessed because it’s private or protected.

To understand how this works, let’s consider a class template that does...