Monday, June 23, 2008

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.

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 ...

5 comments :

  1. Very Useful Example

    ReplyDelete
  2. you should have the virtual functions private. see https://sourcemaking.com/design_patterns/template_method/cpp/1

    ReplyDelete

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