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<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 the following result −

7
hello
exception: stack<>::pop(): empty stack

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