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Learn To Use Type Template Parameter Pack In C++

Learn To Use Type Template Parameter Pack In C++

In modern C++, a template is a simple and very powerful statement that defines the operations of a class or function. Templates are parameterized features of C++ and they let us use a different number of parameters. In this article, we will explain how you can use a type template parameter pack in C++. The examples will work in any recent C++ Developer Tool that supports C++11, C++14, C++17, and over. 

First of all, let’s try to explain what templates are in C++.

What is a template in C++?

template is a very powerful statement in C++ that simply defines the operations of a class, a function, an alias, or a variable. It lets the user apply the same template on different types to increase code reuse. Templates are like macros in C++, except the compiler checks the types used before the template is expanded. In the compilation mechanism of a template in C++, the source code contains only a template for a function or class, but when it is compiled, the same template can be used on multiple data types.

Here is the syntax of a template.

The parameters of a template can be,

  • type template parameter,
  • non-type template parameter,
  • template template parameter (a template used as a parameter).

What is type template parameter pack in C++?

type template parameter (typename or class) is a type parameter key provided within a template parameters list and it is a typename or class. Thus, a user may use different type names or classes for the different declarations with templates.

Sometimes, when we use a template we may use optional template arguments. The number of arguments can vary when using these templates. A template parameter pack is a template parameter with … (3 dots) ellipsis symbol that can be used with more template arguments (non-types, types, or templates) or it can be zero. The parameter pack symbol can be used as a function parameter pack too, it can be zero or it may have more function arguments.

Note that, if a template has at least one parameter pack, this template is called as a variadic template.

Here is the syntax for the ‘type template parameter pack’.

A type parameter can be one of the following types,

  • typename
  • class
  • concept (since C++20).

For example, we can declare a template with parameter pack as shown below.

The name is optional so if we don’t need class name in your template we can use it with parameter pack as below too.

How to use a template that has a type template parameter pack in C++?

We can use many parameters if the template is variadic template as we showed above. We can use it without a parameter, with a single parameter, or more. Here is an example that shows how we can use a template that has type template parameter pack.

Is there an example of a type template parameter pack in C++?

Here is an example of the type template parameter pack in C++’.

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About author

Dr. Yilmaz Yoru has 35+ years of coding with more than 30+ programming languages, mostly C++ on Windows, Android, Mac-OS, iOS, Linux, and some other operating systems. He graduated and received his MSc and PhD degrees from the Department of Mechanical Engineering of Eskisehir Osmangazi University. He is the founder and CEO of ESENJA LLC Company. His interests are Programming, Thermodynamics, Fluid Mechanics, Artificial Intelligence, 2D & 3D Designs, and high-end innovations.
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