C++: The Complete Reference in .NET framework

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C++: The Complete Reference
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// write all characters to disk for(i=0; i<256; i++) output((char) i); outclose(); return 0; }
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You might find it interesting to examine the contents of the CHARS file to see what extended characters your computer has available
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read( ) and write( )
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Another way to read and write blocks of binary data is to use C++'s read( ) and write( ) functions Their prototypes are istream &read(char *buf, streamsize num); ostream &write(const char *buf, streamsize num); The read( ) function reads num characters from the invoking stream and puts them in the buffer pointed to by buf The write( ) function writes num characters to the invoking stream from the buffer pointed to by buf As mentioned in the preceding chapter, streamsize is a type defined by the C++ library as some form of integer It is capable of holding the largest number of characters that can be transferred in any one I/O operation The next program writes a structure to disk and then reads it back in:
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#include <iostream> #include <fstream> #include <cstring> using namespace std; struct status { char name[80]; double balance; unsigned long account_num; }; int main() { struct status acc;
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strcpy(accname, "Ralph Trantor"); accbalance = 112323; accaccount_num = 34235678; // write data ofstream outbal("balance", ios::out | ios::binary); if(!outbal) { cout << "Cannot open file\n"; return 1; } outbalwrite((char *) &acc, sizeof(struct status)); outbalclose(); // now, read back; ifstream inbal("balance", ios::in | ios::binary); if(!inbal) { cout << "Cannot open file\n"; return 1; } inbalread((char *) &acc, sizeof(struct status)); cout << accname << endl; cout << "Account # " << accaccount_num; coutprecision(2); coutsetf(ios::fixed); cout << endl << "Balance: $" << accbalance; inbalclose(); return 0; } C++
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As you can see, only a single call to read( ) or write( ) is necessary to read or write the entire structure Each individual field need not be read or written separately As this example illustrates, the buffer can be any type of object The type casts inside the calls to read( ) and write( ) are necessary when operating on a buffer that is not defined as a character array Because of C++'s strong type checking, a pointer of one type will not automatically be converted into a pointer of another type
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C++: The Complete Reference
If the end of the file is reached before num characters have been read, then read( ) simply stops, and the buffer contains as many characters as were available You can find out how many characters have been read by using another member function, called gcount( ), which has this prototype: streamsize gcount(); It returns the number of characters read by the last binary input operation The following program shows another example of read( ) and write( ) and illustrates the use of gcount( ):
#include <iostream> #include <fstream> using namespace std; int main() { double fnum[4] = {9975, -344, 17760, 2001}; int i; ofstream out("numbers", ios::out | ios::binary); if(!out) { cout << "Cannot open file"; return 1; } outwrite((char *) &fnum, sizeof fnum); outclose(); for(i=0; i<4; i++) // clear array fnum[i] = 00; ifstream in("numbers", ios::in | ios::binary); inread((char *) &fnum, sizeof fnum); // see how many bytes have been read cout << ingcount() << " bytes read\n"; for(i=0; i<4; i++) // show values read from file cout << fnum[i] << " ";
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C++ File I/O
inclose(); return 0; }
The preceding program writes an array of floating-point values to disk and then reads them back After the call to read( ), gcount( ) is used to determine how many bytes were just read
More get( ) Functions
In addition to the form shown earlier, the get( ) function is overloaded in several different ways The prototypes for the three most commonly used overloaded forms are shown here: istream &get(char *buf, streamsize num); istream &get(char *buf, streamsize num, char delim); int get( ); The first form reads characters into the array pointed to by buf until either num-1 characters have been read, a newline is found, or the end of the file has been encountered The array pointed to by buf will be null terminated by get( ) If the newline character is encountered in the input stream, it is not extracted Instead, it remains in the stream until the next input operation The second form reads characters into the array pointed to by buf until either num-1 characters have been read, the character specified by delim has been found, or the end of the file has been encountered The array pointed to by buf will be null terminated by get( ) If the delimiter character is encountered in the input stream, it is not extracted Instead, it remains in the stream until the next input operation The third overloaded form of get( ) returns the next character from the stream It returns EOF if the end of the file is encountered This form of get( ) is similar to C's getc( ) function
getline( )
Another function that performs input is getline( ) It is a member of each input stream class Its prototypes are shown here: istream &getline(char *buf, streamsize num); istream &getline(char *buf, streamsize num, char delim);
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