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 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 the 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&lt;T&gt; elems;    // elements 
   public: 
  void push(T const&amp;);  // 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&lt;&gt;::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&lt;&gt;::top(): empty stack"); 
   }
   
   // return copy of last element 
   return elems.back();      
} 

int main() { 
   try {
  Stack&lt;int&gt;         intStack;  // stack of ints 
  Stack&lt;string&gt; stringStack;    // stack of strings 
  // manipulate int stack 
  intStack.push(7); 
  cout &lt;&lt; intStack.top() &lt;&lt;endl; 
  // manipulate string stack 
  stringStack.push("hello"); 
  cout &lt;&lt; stringStack.top() &lt;&lt; std::endl; 
  stringStack.pop(); 
  stringStack.pop(); 
   } catch (exception const& ex) { 
  cerr &lt;&lt; "Exception: " &lt;&lt; ex.what() &lt;&lt;endl; 
  return -1;
   } 
} 

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

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