Monday, June 23, 2008

C++ Template Pattern

By SourceTricks 6:53 AM C++ Template Pattern 1 comment

What is template design pattern?

Template pattern is a behavioral design pattern.
This has nothing to do with C++ templates as such.
Template patterns is a common form in object oriented programming. Having an abstract base class (one or more pure virtual functions) is a simple example of template design pattern.
In the template pattern, parts of program which are well defined like an algorithm are defined as a concrete method in the base class. The specifics of implementation are left to the derived classes by making these methods as abstract in the base class.
The method which implements the algorithm is also refered as template method and the class which implements this methods as the template class.

EXAMPLE: Demonstrate the usage of template design pattern

#include <iostream>
using namespace std;

// Base class
// Template class
class Account {
    public:
        // Abstract Methods
        virtual void Start() = 0;

        virtual void Allow() = 0;

        virtual void End() = 0;

        virtual int MaxLimit() = 0;

        // Template Method
        void Withdraw(int amount) {

            Start();

            int limit = MaxLimit();
            if ( amount < limit ) {
            Allow();
            }
            else {
            cout << "Not allowed" << endl;
            }

            End();
        }
};

// Derived class
class AccountNormal : public Account {
    public:
        void Start() {
            cout << "Start ..." << endl;
        }

        void Allow() {
            cout << "Allow ..." << endl;
        }

        void End() {
            cout << "End ..." << endl;
        }

        int MaxLimit() {
            return 1000;
        }
};

// Derived class
class AccountPower : public Account {
    public:
        void Start() {
            cout << "Start ..." << endl;
        }

        void Allow() {
            cout << "Allow ..." << endl;
        }

        void End() {
            cout << "End ..." << endl;
        }

        int MaxLimit() {
            return 5000;
        }
};

int main() {
    AccountPower power;
    power.Withdraw(1500);

    AccountNormal normal;
    normal.Withdraw(1500);
}

OUTPUT:-
Start ...
Allow ...
End ...

Start ...
Not allowed
End ...