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C has one last storage specifier that originally applied only to variables of type int, char, or pointer types However, when C was standardized, its scope was broadened The register specifier requests the compiler to store a variable declared with this modifier in a manner that allows the fastest access time possible For integers, characters, and pointers, this typically means in a register of the CPU rather than in memory, where normal variables are stored For other types of variables, the compiler may use any other means to decrease their access time In fact, it can also simply ignore the request altogether In C++ Builder, the register specifier can be applied to local variables and to the formal parameters of a function You cannot apply register to global variables Also, because a register variable may be stored in a register of the CPU, you cannot obtain the address of a register variable (This restriction applies only to C, not to C++) In general, operations on register variables occur much faster than on variables stored in main memory In fact, when the value of a variable is actually held in the CPU, no memory access is required to determine or modify its value This makes register variables ideal for loop control Here is an example of how to declare a register variable of type int and use it to control a loop This function computes the result of Me for integers

private PhotoAlbum OpenTreeAlbum (TreeNode node) {

int int_pwr(register int m, register int e) { register int temp; temp = 1;

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string s = node.Tag as string; PhotoAlbum album = OpenAlbum(s);

for(; e; e--) temp *= m; return temp; }

In this example, m, e, and temp are declared to be register variables because they are all used within the loop In general practice, register variables are used where they will do the most good, that is, in places where many references will be made to the same variable This is important because not all variables can be optimized for access time It is important to understand that the register specifier is just a request to the compiler, which the compiler is free to ignore In general, you can count on at least two register variables of type char or int actually being held in a CPU register for any one function Additional register variables will be optimized to the best ability of the compiler

if (album == null) { // Unable to open album node.ImageIndex = ErrorIndex; node.SelectedImageIndex = ErrorIndex; } else { // Album opened successfully node.ImageIndex = AlbumIndex; node.SelectedImageIndex = SelectedAlbumIndex; } return album; }

If open( ) fails, the stream will evaluate to false when used in a Boolean expression Therefore, before using a file, you should test to make sure that the open operation succeeded You can do so by using a statement like this:

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a. If the album cannot be opened, use the error icon for both images. b. Otherwise, use the standard album images.

if(!mystream) { cout << "Cannot open file\n"; // handle error }

Although it is entirely proper to open a file by using the open( ) function, most of the time you will not do so because the ifstream, ofstream, and fstream classes have

With this method in place, we can create a BeforeExpand event handler for our TreeView control. The following table continues the previous steps to create this handler.

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constructor functions that automatically open the file The constructor functions have the same parameters and defaults as the open( ) function Therefore, you will most commonly see a file opened as shown here:

HANDLE THE BEFOREEXPAND EVENT Action 5 In the MainForm.cs [Design] window, add a BeforeExpand event handler for the TreeView control. Result

C++ Builder also supplies an extra, nonstandard, parameter to the stream constructors which specifies a UNIX permission code This parameter defaults to normal access Since this parameter is nonstandard, it is not described here As stated, if for some reason the file cannot be opened, the value of the associated stream variable will evaluate to false Therefore, whether you use a constructor function to open the file or an explicit call to open( ), you will want to confirm that the file has actually been opened by testing the value of the stream You can also check to see if you have successfully opened a file by using the is_open( ) function, which is a member of fstream, ifstream, and ofstream It has this prototype bool is_open( );

private void treeViewMain_BeforeExpand (object sender, System.Windows. Forms.TreeViewCancelEventArgs e) { TreeNode node = e.Node;

It returns true if the stream is linked to an open file and false otherwise For example, the following checks if mystream is currently open

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if(!mystreamis_open()) { cout << "File is not open\n"; //

HANDLE THE BEFOREEXPAND EVENT (continued) Action 6 To implement this handler, see if the expanding node is an album.

To close a file, use the member function close( ) For example, to close the file linked to a stream called mystream, use this statement:

string s = node.Tag as string; if (s == null || (Path.GetExtension(s) != ".abm")) { // Not an album node return; }