Templates in C++

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.

example : 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>.

Function and Class templates are two tupes ...

Function Template:

a source file that contains only template definitions. In order for any code to appear, a template must be instantiated: the template arguments must be determined so that the compiler can generate an actual function (or class, from a class template). 

example

template<typename T>
void f(T s)
{
    std::cout << s << '\n';
}

Explicit Instantiation : An explicit instantiation definition forces instantiation of the function or member function they refer to. It may appear in the program anywhere after the template definition, and for a given argument-list, is only allowed to appear once in the program.

template<typename T>
void f(T s)
{
    std::cout << s << '\n';
}
 
template void f<double>(double); // instantiates f<double>(double)
template void f<>(char); // instantiates f<char>(char), template argument deduced
template void f(int); // instantiates f<int>(int), template argument deduced

Implicit Instantiation : When code refers to a function in context that requires the function definition to exist, and this particular function has not been explicitly instantiated, implicit instantiation occurs. The list of template arguments does not have to be supplied if it can be deduced from context

#include <iostream>
 
template<typename T>
void f(T s)
{
    std::cout << s << '\n';
}
 
int main()
{
    f<double>(1); // instantiates and calls f<double>(double)
    f<>('a'); // instantiates and calls f<char>(char)
    f(7); // instantiates and calls f<int>(int)
    void (*ptr)(std::string) = f; // instantiates f<string>(string)
}

Class templates

Just like we can create function templates, we can also create class templates, allowing classes to have members that use template parameters as types. For example:

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:

Example 1.

#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

Example 2 : 

// class templates
#include <iostream>
using namespace std;

template <class T>
class mypair {
    T a, b;
  public:
    mypair (T first, T second)
      {a=first; b=second;}
    T getmax ();
};

template <class T>
T mypair<T>::getmax ()
{
  T retval;
  retval = a>b? a : b;
  return retval;
}

int main () {
  mypair <int> myobject (100, 75);
  cout << myobject.getmax();
  return 0;
}

Notice the syntax of the definition of member function getmax:



template <class T>
T mypair<T>::getmax ()