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What is a template class in c++ ?

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What is a template class in c++ ?
posted Jan 22, 2016 by anonymous

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2 Answers

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Class templates are generally used to implement containers. A class template is instantiated by passing a given set of types to it as template arguments. The C++ Standard Library contains many class templates, in particular the containers adapted from the Standard Template Library, such as vector .

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 or vector .

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> 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

answer Jan 22, 2016 by Mohammed Hussain
0 votes

C++ templates enable you to define a family of functions or classes that can operate on different types of information. The Microsoft implementation of C++ templates is based on the ISO/ANSI C++ Standard.
Use templates in situations that result in duplication of the same code for multiple types. For example, you can use function templates to create a set of functions that apply the same algorithm to different data types. You can also use class templates to develop a set of typedef classes. Templates are sometimes a better solution than C macros and void pointers, and they are especially useful when working with collections.
example:

template<>
class Array {
public:
int len() const;
// ...
};
template<>
inline
int Array::len() const
{
// ...
}

answer Feb 26, 2016 by Josita Sarwan
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