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