C++ Templates


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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 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> elements;

  public:
    void push(T const &);
    void pop();
    T top();
    bool empty();
};

template <class T>
void Stack<T>::push(T const &elem) {
    elements.push_back(elem);
}

template <class T>
void Stack<T>::pop() {
    if (elements.empty()) {
        throw out_of_range("Stack<>::pop(): empty stack");
    } else {
        elements.pop_back();
    }
}

template <class T>
T Stack<T>::top() {
    if (empty()) {
        throw out_of_range("Stack<>::top(): empty stack");
    }
    return elements.back();
}

template <class T>
bool Stack<T>::empty() {
    return elements.empty();
}


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

        // manipulate integer 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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