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# C++ Templates

Templates are the foundation of generic programming which involves writing code in a way that is independent of any particular type.

A template is a blueprint or formula for creating a generic class or a function. The library containers like iterators, and algorithms are examples of generic programming and have been developed using template concept.

There is a single definition of each container, such as vector, but we can define many different kinds of vectors for example, vector <int> or vector <string>.

You can use templates to define functions as well as classes, let us see how do they work:

# Function Template:

The general form of a template function definition is shown here:

    template <class type> ret-type func-name(parameter list)
    {
       // body of function
    }

Here, type is a placeholder name for a data type used by the function. This name can be used within the function definition.

The following is the example of a function template that returns the maximum of two values:

    #include <iostream>
    #include <string>

    using namespace std;

    template <typename T>
    inline T const& Max (T const& a, T const& b)
    {
        return a < b ? b:a;
    }
    int main ()
    {

        int i = 39;
        int j = 20;
        cout << "Max(i, j): " << Max(i, j) << endl;

        double f1 = 13.5;
        double f2 = 20.7;
        cout << "Max(f1, f2): " << Max(f1, f2) << endl;

        string s1 = "Hello";
        string s2 = "World";
        cout << "Max(s1, s2): " << Max(s1, s2) << endl;

       return 0;
    }

If we compile and run above code, this would produce following result:

    Max(i, j): 39
    Max(f1, f2): 20.7
    Max(s1, s2): World

# Class Template:

Just as we can define function templates, we can also define class templates. The general form of a generic class declaration is shown here:

    template <class type> class class-name {
    .
    .
    .
    }

Here, type is the placeholder type name, which will be specified when a class is instantiated. You can define more than one generic data type by using a comma-separated list.

Following is the example to define class Stack<> and implement generic methods to push and pop the elements from the stack:

    #include <iostream>
    #include <vector>
    #include <cstdlib>
    #include <string>
    #include <stdexcept>

    using namespace std;

    template <class T>
    class Stack {
      private:
        vector<T> elems;     // elements

      public:
        void push(T const&);  // push element
        void pop();               // pop element
        T top() const;            // return top element
        bool empty() const{       // return true if empty.
            return elems.empty();
        }
    };

    template <class T>
    void Stack<T>::push (T const& elem)
    {
        // append copy of passed element
        elems.push_back(elem);
    }

    template <class T>
    void Stack<T>::pop ()
    {
        if (elems.empty()) {
            throw out_of_range("Stack<>::pop(): empty stack");
        }
    	// remove last element
        elems.pop_back();
    }

    template <class T>
    T Stack<T>::top () const
    {
        if (elems.empty()) {
            throw out_of_range("Stack<>::top(): empty stack");
        }
    	// return copy of last element
        return elems.back();
    }

    int main()
    {
        try {
            Stack<int>         intStack;  // stack of ints
            Stack<string> stringStack;    // stack of strings

            // manipulate int stack
            intStack.push(7);
            cout << intStack.top() <<endl;

            // manipulate string stack
            stringStack.push("hello");
            cout << stringStack.top() << std::endl;
            stringStack.pop();
            stringStack.pop();
        }
        catch (exception const& ex) {
            cerr << "Exception: " << ex.what() <<endl;
            return -1;
        }
    }

If we compile and run above code, this would produce following result:

    7
    hello
    Exception: Stack<>::pop(): empty stack

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