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C++Template Tutorial




Templates are the foundation of generic programming

Function Template

The general form of a template function definition
template <class type> ret-type func-name(parameter list) {
   // body of function
} 
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 = 40;
   int j = 25;
   cout << "Max(i, j): " << Max(i, j) << endl; 

   double f1 = 12; 
   double f2 = 18; 
   cout << "Max(f1, f2): " << Max(f1, f2) << endl; 

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

   return 0;
}

Run Example »
$g++ -o main *.cpp
$main
Max(i, j): 40 Max(f1, f2): 18 Max(s1, s2): World

Class Template

We also have the possibility to write class templates, so that a class can have members that use template parameters as types.
template <class type> class class-name {
   .
   .
   .
}
type is the placeholder type name, which will be specified when a class is instantiated. 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;
   } 
} 

Run Example »
it will be following result:-
7
hello
Exception: Stack<>::pop(): empty stack