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C++ Templates
Using a class template

Using a class template is easy. Create the required classes by plugging in the actual type for the type parameters. This process is commonly known as "Instantiating a class". Here is a sample driver class that uses the Stack class template.

#include <iostream>
#include "stack.h"
using namespace std ;
void main()
{
	typedef Stack<float> FloatStack ;
	typedef Stack<int> IntStack ;

	FloatStack fs(5) ;
	float f = 1.1 ;
	cout << "Pushing elements onto fs" << endl ;
	while (fs.push(f))
	{
		cout << f << ' ' ;
		f += 1.1 ;
	}
	cout << endl << "Stack Full." << endl
	<< endl << "Popping elements from fs" << endl ;
	while (fs.pop(f))
		cout << f << ' ' ;
	cout << endl << "Stack Empty" << endl ;
	cout << endl ;

	IntStack is ;
	int i = 1.1 ;
	cout << "Pushing elements onto is" << endl ;
	while (is.push(i))
	{
		cout << i << ' ' ;
		i += 1 ;
	}
	cout << endl << "Stack Full" << endl
	<< endl << "Popping elements from is" << endl ;
	while (is.pop(i))
			cout << i << ' ' ;
	cout << endl << "Stack Empty" << endl ;
}

Program Output

Pushing elements onto fs
1.1 2.2 3.3 4.4 5.5 
Stack Full.

Popping elements from fs
5.5 4.4 3.3 2.2 1.1 
Stack Empty

Pushing elements onto is
1 2 3 4 5 6 7 8 9 10 
Stack Full

Popping elements from is
10 9 8 7 6 5 4 3 2 1 
Stack Empty

In the above example we defined a class template Stack. In the driver program we instantiated a Stack of float (FloatStack) and a Stack of int(IntStack). Once the template classes are instantiated you can instantiate objects of that type (for example, fs and is.)

A good programming practice is using typedef while instantiating template classes. Then throughout the program, one can use the typedef name. There are two advantages:

  • typedef's are very useful when "templates of templates" come into usage. For example, when instantiating an STL vector of int's, you could use:
    typedef vector<int, allocator<int> > INTVECTOR;
    	
  • If the template definition changes, simply change the typedef definition. For example, currently the definition of template class vector requires a second parameter.
    typedef vector<int, allocator<int> > INTVECTOR; INTVECTOR vi1;
    	
    In a future version, the second parameter may not be required, for example,
    typedef vector<int> INTVECTOR; INTVECTOR vi1;
    	
Imagine how many changes would be required if there was no typedef!

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