Bomb Pattern

Bomb

• called a "mini pattern"
• problem: want event to execute when a block exits
• solution: define class with code to be executed in destructor; declare instance of class at top of block
• examples:
  • check postconditions for method
  • set cursor to hourglass and restore when exit function (from BCB help notes):

        // in .h file:
    
        enum MouseCursor;
    
        // from Borland C++ Builder help notes
        class WaitCursor {
        public:
            WaitCursor();
           ~WaitCursor();
        private:
            MouseCursor saved_cursor;
        };
    
        // in .cpp file:
    
        WaitCursor::WaitCursor()
        {
            saved_cursor = Screen->Cursor;
            Screen->Cursor = crHourGlass;
        }
    
        WaitCursor::~WaitCursor()
        {
            Screen->Cursor = saved_cursor;
        }
    

  • Usage:

        void read_database()
        {
           WaitCursor just_a_second;
           ... code to open database, read items, close connection
        }
    

• Relies on C++ "destructors" - cannot do this in Java