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++.
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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.
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template < parameters > declaration |
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++?
A 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’.
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template < type_parameter_key ... name(optional) > declaration |
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.
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template<class ... T> class my_temp { }; |
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.
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template<class ...> class my_temp { }; |
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.
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my_temp<> class_noparam; // the parameter list is empty my_temp<int> class_int; // the parameter list has one item my_temp<int, float, std::string> class_multiparam; // the parameter list has three items |
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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#include <iostream> template<typename ... Targs> class my_temp { }; my_temp<void> class_noparam; // the parameter list is empty my_temp<int> class_int; // the parameter list has one item my_temp<int, float, std::string> class_multiparam; // the parameter list has three items int main() { } |
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