Understanding Templates in C++

Templates in C++?

Templates in C++ act as the foundation of generic programming. It is a simple yet powerful tool that acts as a blueprint for creating generic functions or classes. While using a template in C++, you pass a data type as a parameter. Thus, instead of maintaining multiple codes, you have to write one code and give the data type you want to use. C++ templates use two primary keywords, ‘template’ and ‘typename,’ where the ‘typename’ can be replaced with the ‘class’ keyword.

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How Do C++ Templates Work?

The templates in C++ are expanded during compile-time. At the very core, they work like macros except for a single significant difference. Unlike macros, the type checking in C++ templates is done before template expansion. This allows the compiled code to have multiple copies of the same class.

Function Templates in C++?

We use C++ function templates for creating functions that can be used with different data types. A function template is used to perform the same function on multiple data types. An alternative approach to this is using function overloading. However, using a function template is a better approach to writing less and more maintainable code. 

The basic syntax of a function template is:

template <class type> ret-type func-name(parameter_list) {

   // body of function

}

In the above syntax, type is just a placeholder that the compiler will automatically replace with the actual data type. The keyword class in the syntax is used to specify a generic type and be replaced by a typename.

Example: Declaring a Function Template in C++

In the below example, you must create a function template in C++ and use it to find the larger value for different data types.

#include <iostream>

using namespace std;

template <class Ttype>

Ttype MaxVal(Ttype i, Ttype j){

    return (i > j)? i: j;

}

int main(){

    cout << MaxVal<int>(5, 8) << endl; // Call myMax for int

    cout << MaxVal<double>(6.5, 4.0) << endl; // call myMax for double

    cout << MaxVal<char>('f', 'k') << endl; // call myMax for char

return 0;

}

Output

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Class Templates in C++

The class templates in C++ make the class using it generic. It operates similar to function templates except that it is used on the entire class instead of a single function.

The basic syntax of declaring a class template is:

template<class type>

class class_name

{

.

.

}

Once the class is created, you can declare the data type while declaring the class instance.

Example: Declaring a Class Template in C++

#include <iostream>

using namespace std;

template <class Ttype>

class Array{

private:

Ttype *pointr;

int size;

public:

Array(Ttype arr[], int s);

void print();

};

template <class Ttype>

Array<Ttype>::Array(Ttype arr[], int s) {

pointr = new Ttype[s];

size = s;

for(int i = 0; i < size; i++)

pointr[i] = arr[i];

}

template <class Ttype>

void Array<Ttype>::print() {

for (int i = 0; i < size; i++)

cout<<" "<<*(pointr + i);

cout<<endl;

}

int main(){

int arr[5] = {1, 2, 3, 4, 5};

Array<int> a(arr, 5);

a.print();

return 0;

}

Output

Passing Multiple Arguments to Templates in C++

Like normal functions and classes, you can pass multiple data types as arguments to templates. Now, let’s look at an example that takes multiple arguments.

#include<iostream>

using namespace std;

template<class Ttype, class Utype>

class A{

Ttype x;

Utype y;

public:

A(){ cout<<"The Constructor is Called"<<endl; }

};

int main() {

A<char, char> a;

A<int, double> b;

return 0;

}

Output

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How to Specify Default Arguments for Templates in C++?

Similar to normal parameters, you can specify default arguments for templates. Here’s an example to demonstrate how to specify a default argument for templates in C++.

#include<iostream>

using namespace std;

template<typename Ttype, typename Utype = char>

class Example{

public:

Ttype i;

Utype j;

Example() { cout<<"The Constructor is Called"<<endl; }

};

int main() {

Example<char> a;

return 0;

}

Output

Difference Between Function Overloading and Templates in C++

Like function overloading, templates in C++ also allow polymorphism. However, there is a fundamental difference between the two. You use the function overloading when you want to perform similar operations. On the other hand, templates in C++ are used to perform precisely identical operations on different data types.

Template Specialization?

Template specialization in C++ declares different codes or behavior for a specific data type. This means you can create a specialized version of a function when executed with a particular data type. The code depicted below demonstrates the use of template specialization in C++:

#include <iostream>

using namespace std;

template <typename Ttype>

void Example(Ttype t)

{

cout << "The standard example template: "

<< t << endl;

}

template<>

void Example(int t)

{

cout << "Specialized example template for int: "

<< t << endl;

}

int main()

{

Example<char>('x');

Example<int>(12);

Example<float>(15.30);

}

Output

As you can see in the output, the template is executed for char and float data types alike but differently for the int data type.

Conclusion

In this article, we learned about templates in C++. We also looked into examples for both types of templates: function and class. We learned how to create templates and used them to execute with different data types. 

If you are new to C++ programming and interested in learning about other fundamental concepts of C++, you can refer to our C++ Tutorial for Beginners. The course is specifically designed for beginners and contains guides for various basic concepts. This course will help you excel in the field of C++ programming and related concepts

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