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(C++) template

template is a keyword for templates. Templates 'provide direct support for generic programming in the form of programming using types as parameters' [1].

A template can take parameters [2]:

• Type parameters
• Value parameters
• Template parameters

templates can be used to:

• write template functions
• write template classes
• do template metaprogrammming (programs performed at compile time)

Templates make it possible to use one function or class to handle many different data types.

Examples

• Example 1: templated class of type T
• Example 2: templated class of type T, full specialisation to int and double
• Example 3: templated class with integer template argument, full specialisation to zero and one
• Example 4: templated class with enum template argument, full specialisation for each enum value

Advice

• There is no semantic difference between class and typename in a template-parameter [3,4]: 'template <class T>' and 'template<typename T>' are synonymous [5]
• Use templates to express algorithms that apply to many argument types [6]
• Use templates to express containers [7]
• When defining a template, first design and debug a non-template version; later generalize by adding parameters [8]
• Templates are type-safe, but checking happens too late [9]
• When designing a template, carefully consider the concepts assumed for its template arguments [10]
• If a class template should be copyable, give it a non-template copy constructor and a non-template copy assignment [11]
• If a class template should be movable, give it a non-template move constructor and a non-template move assignment [12]
• A virtual function member cannot be a template member function [13]
• Define a type as a member of a template only if it depends on all the class template's arguments [14]
• Use function templates to deduce class template argument types [15]
• Overload function templates to get the same semantics for a variety of argument types [16]
• Use argument substitution failure to provide just the right set of functions for a program [17]
• Use template aliases to simplify notation and hide implementation details [18]
• There is no seperate compilation of templates: #include template definitions in every translation unit that uses them [19]
• Use ordinary functions as interfaces to code that cannot deal with templates [20]
• Seperately compile large templates and templates with nontrivial context dependencies [21]

References

1. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.1, page 665: 'Templates provide direct support for generic programming in the form of programming using types as parameters
2. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 25.2, page 722: 'A template can take parameters: Type parameters [...], Value parameters [...], Template parameters [...]'
3. C++. International Standard. ISO/IEC 14882. Second edition. Paragraph 14.1.2.
4. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 25.2, page 722: 'A template argument is defined to be a type argument by prefixing it with typename or class. The result of using either is completely equivalent'
5. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[3] template <class T>' and 'template<typename T>' are synonymous'
6. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[1] Use templates to express algorithms that apply to many argument types'
7. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[2] Use templates to express containers'
8. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[4] When defining a template, first design and debug a non-template version; later generalize by adding parameters'
9. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[5] Templates are type-safe, but checking happens too late'
10. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[6] When designing a template, carefully consider the concepts (requirements) assumed for its template arguments'
11. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[7] If a class template should be copyable, give it a non-template copy constructor and a non-template copy assignment'
12. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[8] If a class template should be movable, give it a non-template move constructor and a non-template move assignment'
13. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[9] A virtual function member cannot be a template member function'
14. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[10] Define a type as a member of a template only if it depends on all the class template's arguments'
15. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[11] Use function templates to deduce class template argument types'
16. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[12] Overload function templates to get the same semantics for a variety of argument types'
17. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[13] Use argument substitution failure to provide just the right set of functions for a program'
18. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[14] Use template aliases to simplify notation and hide implementation details'
19. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[15] There is no seperate compilation of templates: #include template definitions in every translation unit that uses them'
20. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[16] Use ordinary functions as interfaces to code that cannot deal with templates'
21. Bjarne Stroustrup. The C++ Programming Language (4th edition). 2013. ISBN: 978-0-321-56384-2. Chapter 23.8, page 698: '[17] Seperately compile large templates and templates with nontrivial context dependencies'

Go back to Richel Bilderbeek's C++ page.

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