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)
1
Part 1: Core Template Concepts
5
Part 2: Advanced Template Features
9
Part 3: Applied Templates
Appendix: Closing Notes

Constraining class templates

Class templates and class template specializations can also be constrained just like function templates. To start, we’ll consider the wrapper class template again, but this time with the requirement that it should only work for template arguments of integral types. This can be simply specified in C++20 as follows:

template <std::integral T>
struct wrapper
{
   T value;
};
wrapper<int>    a{ 42 };    // OK
wrapper<double> b{ 42.0 };  // error

Instantiating the template for the int type is fine but does not work for double because this is not an integral type.

Requirements that also be specified with requires clauses and class template specializations can also be constrained. To demonstrate this, let’s consider the scenario when we want to specialize the wrapper class template but only for types whose size is 4 bytes. This can be implemented as follows...