dtl

ValidatedObject

Category: utilities	Component type: type

Description

ValidatedObject's maintain state information that allows them to determine whether they are in a valid state or have been corrupted. Once an operation is performed in one of these self-validating classes that detects that the object is no longer in a valid state, the operation will throw a ValidityException. To get validation facilties for an object, simply derive from ValidatedObject. You can choose whether a method needs to validate the object or not by calling validate()at the beginning of the method if you wish to validate the object's state. To define the critieria used to define whether the object is in a valid state or not, override the valid() method. Often, in your valid() method, you may wish to test whether the object is already valid to save some work as no further testing is required if the object is found to already be invalid. Unless you override the revalidate() method, once an object becomes invalid, it remains invalid permanently. Your revalidate() method tries to reconcile the state of the object, doing whatever processing or cleanup it needs to return the object to a valid state. This reconciling of the object can either always succeed, sometimes succeed (and either throwing ValidityException or doing something else in case of failure), or never succeed (also achievable just by not overriding revalidate(), which will throw ValidityException).

Definition

Defined in the validate.h header file.

Example: A validated BankAccount object

class MoneyAccount : public ValidatedObject
{
private:
   // if less than 0 money left, we're broke --> object corrupted!
   // (must then see MoneyAccount manager to revalidate() or go to jail:P)
   // (in this example though, once you're in debt, your account is frozen!)
   double balance;
public:
   MoneyAccount(unsigned double initialDeposit) : ValidatedObject(), balance(initialDeposit) { }

   // should be able to see balance even if account is frozen!
   double GetBalance() { return balance; }

   // account only valid if not in debt!
   virtual bool valid()
   {
      // if we know the object is invalid from a previous error,
      // object is still invalid (in this case, account still frozen)
      if (!ValidatedObject::valid())
         return false;

      // freeze account by invalidating object if in debt
      if (balance < 0)}
      {
         invalidate();
         return false;
      }
      else
         return true;
   }

   // these operations work only if the account is not frozen
   void deposit(unsigned double amt)
   {
      validate(); // call to validate will throw if object not valid
      balance += amt;
   }

   double withdraw(unsigned double amt)
   {
      validate(); // call to validate will throw if object not valid
    
      // can withdraw up to how much we have left in the account
      double amtWithdrawn = (balance - amt >= 0 ? amt : balance);
      balance -= amtWithdrawn;
      return amtWithdrawn; // return actual amount withdrawn
   }

   // called by creditors to make bank pay! Account could get frozen here!
   // assume bank always pays creditors even if account is frozen
   // calls to any other MoneyAccount methods should properly discover
   // any new debt and thus freeze account
   void pay_creditors(unsigned double amt) { balance -= amt; }
};

void IWillThrow()
{
   // create account and then perform some transactions
   MoneyAccount acct(200.00);
   acct.deposit(700.00);
   acct.withdraw(550.00);
   
   // uh oh ... now creditors take my money away, my account is frozen
   acct.pay_creditors(1000);

   // now trying to do something with my frozen account will throw ValidityException
   acct.withdraw(50.00);
}

int main()
{
   try
   {
       // call our method which throws
       IWillThrow();
   }
   catch (RootException &ex)
   {
       // can also say: cout << ex << endl;
       // operator<<() for RootExceptions just streams out what()
       cout << ex.what() << endl;
   }
   return 0; // won't reach here ... exception thrown above
}

Model of

None.

Public base classes

None.

Members

Notes

None.

See also

RootException, ValidityException.

Copyright © 2002, Michael Gradman and Corwin Joy.

Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appears in all copies and that both that copyright notice and this permission notice appear in supporting documentation. Corwin Joy and Michael Gradman make no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

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