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++. In this article, we will explain what a variadic template is and how you can use parameter pack in the templates of modern C++. You can download C++ and follow along too.
First of all, let’s try to explain what a template is in C++,
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What is a template in C++?
A 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,
- a type template parameter,
- a non-type template parameter,
- a template template parameter (a template used as a parameter).
What is a variadic template in C++?
A type template parameter (typename or class) is a type passed as a parameter within the template parameters list. It is a typename or class. Thus, the user may use different type names or classes for the different declarations with templates.
A Variadic template is a template that has at least one parameter pack, that has a template parameter with the … (3 dots) ellipsis symbol that can be used to use templates with no arguments or with one or more arguments. These arguments can be non-types like (5, 9.81, “string”), or types like (int, char, float, string, class) or 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. Parameter pack symbol can be used as a function parameter pack too, it can be zero or it may have more function arguments.
Sometimes, when we use a template we may use more template arguments. declare a template we use type template parameter with a default, in this way, you can easily use templates with default option, or you can define your class name.
Here is the syntax for the ‘type template parameter pack’,
A type parameter can be one of the following types,
- a typename
- a class
- a concept (since C++20)
For example we can declare a template with class parameter pack as 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 variadic template in C++?
We can use many parameters if the template is a variadic template as below. We can use it without a parameter, with a single parameter, or more. Here is an example.
Is there an example of a variadic template in C++?
Here is an example to the ‘variadic template in C++’.
You can use variadic templates in C++ Builder.
C++ Builder is the easiest and fastest C and C++ IDE for building simple or professional applications on the Windows, MacOS, iOS & Android operating systems. It is also easy for beginners to learn with its wide range of samples, tutorials, help files, and LSP support for code. RAD Studio’s C++ Builder version comes with the award-winning VCL framework for high-performance native Windows apps and the powerful FireMonkey (FMX) framework for cross-platform UIs.
There is a free C++ Builder Community Edition for students, beginners, and startups; it can be downloaded from here. For professional developers, there are Professional, Architect, or Enterprise versions of C++ Builder and there is a trial version you can download from here