.net pdf 417

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// assign a string str4 = str1; cout << str1 << "\n" << str3 << "\n"; // concatenate two strings str4 = str1 + str2; cout << str4 << "\n"; // concatenate a string with a C-string str4 = str1 + " to " + str3; cout << str4 << "\n"; // compare strings if(str3 > str1) cout << "str3 > str1\n"; if(str3 == str1+str2) cout << "str3 == str1+str2\n"; /* A string object can also be assigned a normal string */ str1 = "This is a null-terminated string\n"; cout << str1; // create a string object using another string object string str5(str1); cout << str5; // input a string cout << "Enter a string: "; cin >> str5; cout << str5; return 0; }

1-21

We will look at Windows Forms timers next. A timer is an object that raises an event after a configurable period of time has elapsed. There are, in fact, three Timer classes provided by the .NET Framework. There is one in the System.Threading namespace for use among multiple threads; one in the System.Timers namespace for serverbased recurring tasks; and one in the System.Windows.Forms namespace that is optimized for the single-threaded processing environment used to handle events in a Form object. Here we will concern ourselves with the Windows Forms timer. This timer object is normally associated and config- Figure 18.2 The slide show uses a TrackBar ured within a form. For our example, we control to track the current position within the album. will create a small slide show form that will flip through each photo in an album. Our new window is shown in figure 18.2, and will be accessible from a new menu item in the View menu of our MyPhotos MDI application. A quick summary of the Timer class we will use is given in .NET Table 18.2.

Alpha Omega AlphaBeta Alpha to Omega str3 > str1 This is a null-terminated string This is a null-terminated string

The Windows Forms Timer class represents a timer component that raises events at userdefined intervals. This timer is optimized for use in Windows Forms applications and is expected to occur within the processing thread for a Form object. This class is part of the System.Windows.Forms namespace, and inherits from the System.ComponentModel.Component class. Enabled Public Properties Interval Start Public Methods Stop Stops the timer. This is equivalent to setting the Enabled property to false. Occurs when the timer is enabled and the specified interval has elapsed. Gets or sets the time in milliseconds between timer ticks. Starts the timer. This is equivalent to setting the Enabled property to true. Gets or sets whether the time is currently active.

As you can see, objects of type string can be manipulated in ways similar to C++ s built-in data types Notice the ease with which the string handling is accomplished For example, the + is used to concatenate strings and the > is use to compare two strings To accomplish these operations using C-style, null-terminated strings, less convenient calls to the strcat( ) and strcmp( ) functions would be required Because C++ string objects can be freely mixed with C-style null-terminated strings, there is no disadvantage to using them in your program and there are considerable benefits to be gained There is one other thing to notice in the preceding program: the sizes of the strings are not specified string objects are automatically sized to hold the string that they are given Thus, when assigning or concatenating strings, the target string will grow as needed to accommodate the size of the new string It is not possible to overrun the end of the string This dynamic aspect of string objects is one of the ways that they are better than standard null-terminated strings (which are subject to boundary overruns)