One of the great features of modern C++ is templates. A template is a simple and very powerful statement in C++ that defines the operations of a class or function, and Template Instantiation is to create a new definition of a function, class, or member of a class from a template declaration with template argument types. In this article, we will explain Template Instantiation in C++ examples which can be used by a professional C++ IDE and compiler.
First of all, let’s try to explain what template in C++ is.
Table of Contents
What Is 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 variable types in those template operations. Templates are like macros in C++, considered in compilation 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.
For example, here is a function template that adds two a and b parameters defined as type T:
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template <class T> T add (T a, T b) { return a+b; } |
for example, if a and b are int variables, this template can be used as below
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int i = add <int> (a, b); |
and in the next lines same template can be used for the x and y float variables as below too,
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float f = add <float> (x, y); |
Templates are a powerful entity that can be parameterized by one or more template parameters. These parameters can be a type template parameter, a non-type template parameter, or a template as a template parameter.
What Is template instantiation in C++?
Template Instantiation is to create a new definition of a function, class, or member of a class from a template declaration with template argument types. For example, a class template is not a type or an object, or any other entity, this does not generate a code from a source file that contains only a template definition. When instantiated, the code of a template appears.
When we define a template class where it will work with a couple of explicit types, we declare the template in the header file like a normal class declaration. We put template definitions in a source file as in normal class definitions. Then we explicitly instantiate only the version of our template that we want.
Syntax of a definition an explicit Instantiation of a template:
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template class_key template_name < arguments > ; |
Syntax of a declaration of an explicit instantiation of a template:
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extern template class_key template_name < arguments > ; // since C++11 |
Note that here the extern keyword in the specialization is applied to the member functions defined outside of the body of the class. Functions which are defined inside the class declaration are considered as inline functions and they are instantiated.
For example, if you want to define a template that has 20 integer variables – for example ages – you can declare ages like this:
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template class ages<int, 20>; |
Template instantiation can be done in two different ways:
- Explicit instantiation
- Implicit instantiation
Where is a simple template instantiation example in C++?
Here is a simple class template explicit instantiation example in C++.
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// Class Template Example template <class T> class Weight { }; // Template Instantiation template class Weight<int>; // explicit instantiation of Weight class template template class Weight<double>; // explicit instantiation of Weight class template |
Is there a full template instantiation example in C++?
Here is an example of a class template and template instantiation of this class template,
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#include <iostream> // Class Template Example template <class T> class Weight { private: // A variable for a value of type T T val; public: Weight(T n) : val(n) { } // constructor T value() // method to obtain value { return val; } }; // Template Instantiation template class Weight<int>; // explicit instantiation of Weight class template template class Weight<double>; // explicit instantiation of Weight class template int main() { // Class Template Usage Weight<double> car_weight(1650.2); std::cout << "Weight of the Car " << car_weight.value() << " kg" << std::endl; system("pause"); return 0; } |
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