The C++ I/O System in Visual Studio .NET

Generator Data Matrix in Visual Studio .NET The C++ I/O System

#include <iostream> #include <iomanip> using namespace std; istream &prompt(istream &stream) { cin >> hex; cout << "Enter number using hex format: "; return stream; } int main() { int i; cin >> prompt >> i; cout << i; return 0; }

Remember that it is crucial that your manipulator return stream If this is not done, then your manipulator cannot be used in a compound input or output expression

You can use the C++ I/O system to perform file I/O In order to perform file I/O, you must include the header <fstream> in your program It defines several important classes and values

In C++, a file is opened by linking it to a stream As you know, there are three types of streams: input, output, and input/output To open an input stream, you must declare the stream to be of class ifstream To open an output stream, it must be declared as class ofstream A stream that will be performing both input and output operations must be declared as class fstream For example, this fragment creates one input stream, one output stream, and one stream capable of both input and output:

ifstream in; // input ofstream out; // output fstream both; // input and output open( ) opens a file

Once you have created a stream, one way to associate it with a file is by using open( ) This function is a member of each of the three stream classes The prototype for each is shown here: void ifstream::open(const char *filename, ios::openmode mode = ios::in);

void ofstream::open(const char *filename, ios::openmode mode = ios::out | ios::trunc); void fstream::open(const char *filename, ios::openmode mode = ios::in | ios::out); Here, filename is the name of the file; it can include a path specifier The value of mode determines how the file is opened It must be one or more of the following values defined by openmode, which is an enumeration defined by ios ios::app ios::ate ios::binary ios::in ios::out ios::trunc You can combine two or more of these values by ORing them together

OTE: Depending on your compiler, the mode parameter for fstream::open( ) may not default to in | out Therefore, you might need to specify this explicitly

Including ios::app causes all output to that file to be appended to the end This value can be used only with files capable of output Including ios::ate causes a seek to the end of the file to occur when the file is opened Although ios::ate causes a seek to the end of the file, I/O operations can still occur anywhere within the file The ios::in value specifies that the file is capable of input The ios::out value specifies that the file is capable of output The ios::binary value causes a file to be opened in binary mode By default, all files are opened in text mode In text mode, various character translations may take place, such as carriage return, linefeed sequences being converted into newlines When a file is opened in binary mode, no such character translations will occur Keep in mind that any file, whether it contains formatted text or raw data, can be opened in either binary or text mode The only difference is whether character translations take place The ios::trunc value causes the contents of a preexisting file by the same name to be destroyed, and the file is truncated to zero length When creating an output stream using ofstream, any preexisting file by that name is automatically truncated The following fragment opens a normal output file:

ofstream out; outopen("test");

Since the mode parameter to open( ) defaults to a value appropriate to the type of stream being opened, there is no need to specify its value in the preceding example If open( ) fails, the stream will evaluate to false when used in a Boolean expression You can make use of this fact to confirm that the open operation succeeded, by using a statement like this:

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

In general, you should always check the result of a call to open( ) before attempting to access the file 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( ); 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:

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

Although it is entirely proper to use the open( ) function for opening a file, most of the time you will not do so because the ifstream, ofstream, and fstream classes have constructors that automatically open the file The constructors have the same parameters and defaults as the open( ) function Therefore, the most common way you will see a file opened is shown in this example: