The template feature in C++ is one of the great features of modern C++. A template is a simple and very powerful statement in C++ that defines the operations of a class or function. In this article, we will explain what is typename and how you can use them with templates in C++ that you can use in any modern, professional or free C++ IDE and compiler that supports C++11, C++14, C++17, and over standards.
Table of Contents
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 typename in C++?
A typename (Type Name) is used to specify a type parameter in template parameter lists. The typename provides an unknown identifier, it is a type as a hint to the compiler in template definitions. Since C++17, a typename can be used as an alternative to a class to declare type template parameters and template template parameters.
In general, a type parameter can be one of the following types.
- typename
- class
- concept (since C++20)
For example, we can declare a template as below.
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template<typename T> typename my_temp { }; |
How can we use typename in C++?
Here is a typename usage in template syntax.
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template< typename name(optional) = default > declaration |
In general, a typename can be used as follows.
typenamecan be used to declare that a dependent qualified name inside a declaration or a definition of a template.- Until C++11,
typenamewas being used as a non-dependent qualified type name, used inside a declaration or a definition of a template. This feature is not available in C++14, C++17 and above - Since C++17,
typenamecan be used in the template parameter list of a template declaration as an alternative to class to declare type template parameters and template template parameters - Since C++20,
typenamecan be used inside a requirement for type requirements.
Is there a simple example of how to use typename in C++?
Here is a simple example of how we can use typename in templates.
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template<typename T> class my_temp { T a; }; int main() { class my_temp<char> o1; // private "T a" is now "char a" } |
Is there a full example of how to use typename in C++?
Here is a full example that shows how we can use typename in templates.
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#include <iostream> template<typename T> class my_temp { private: T a; public: void print_size() { std::cout << sizeof(a) << " bytes" <<std::endl; } }; int main() { class my_temp<char> o1; // private "T a" is now "char a" class my_temp<short int> o2; // private "T a" is now "short int a" class my_temp<int> o3; // private "T a" is now "int a" o1.print_size(); o2.print_size(); o3.print_size(); system("pause"); return 0; } |
Here is the output of each typenames that we give there (char, short int, and int),
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1 bytes 2 bytes 4 bytes Press any key to continue . . . |
Where can we not use typename in C++?
Note that, according to standard C++98, and C++11, there are some restrictions (source) when using typename. Here are some of these:
typenameis prohibited in these conditions,- Outside of a template definition
- Before an unqualified type, like
int,float, ormy_special_type. - When naming a base class.
- In a constructor initialization list.
typenameis optional in other scenarios.typenameis mandatory before a qualified, dependent name which refers to a type.
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