C++ Primer Plus

Some systems, such as Windows, support two file formats: text and binary. If you want to save data in binary form, you should use the binary file format. In C++ you do so by using the ios_base::binary constant in the file mode. If you want to know why you should do this on a Windows system, check the discussion in the following sidebar, “Binary Files and Text Files.”

Binary Files and Text Files

Using a binary file mode causes a program to transfer data from memory to a file, or vice versa, without any hidden translation taking place. Such is not necessarily the case for the default text mode. For example, consider Windows text files. They represent a newline with a two-character combination: carriage return, linefeed. Macintosh text files represent a newline with a carriage return. Unix and Linux files represent a newline with a linefeed. C++, which grew up on Unix, also represents a newline with a linefeed. For portability, a Windows C++ program automatically translates the C++ newline to a carriage return and linefeed when writing to a text mode file; and a Macintosh C++ program translates the newline to a carriage return when writing to a file. When reading a text file, these programs convert the local newline back to the C++ form. The text format can cause problems with binary data because a byte in the middle of a double value could have the same bit pattern as the ASCII code for the newline character. Also there are differences in how end-of-file is detected. So you should use the binary file mode when saving data in binary format. (Unix systems have just one file mode, so on them the binary mode is the same as the text mode.)

To save data in binary form instead of text form, you can use the write() member function. This method, recall, copies a specified number of bytes from memory to a file. This chapter used it earlier to copy text, but it will copy any type of data byte-by-byte with no conversion. For example, if you pass to it the address of a long variable and tell it to copy 4 bytes, it will copy the 4 bytes constituting the long value verbatim to a file and not convert it to text. The only awkwardness is that you have to type cast the address to type pointer-to-char. You can use the same approach to copy an entire planet structure. To get the number of bytes, you use the sizeof operator:

fout.write( (char *) &pl, sizeof pl);

This statement causes the program to go to the address of the pl structure and copy the 36 bytes (the value of the sizeof pl expression) beginning at this address to the file connected to fout.

To recover the information from a file, you use the corresponding read() method with an ifstream object:

ifstream fin("planets.dat", ios_base::in | ios_base::binary);

fin.read((char *) &pl, sizeof pl);

This copies sizeof pl bytes from the file to the pl structure. This same approach can be used with classes that don’t use virtual functions. In that case, just the data members are saved, not the methods. If the class does have virtual methods, then a hidden pointer to a table of pointers to virtual functions is also copied. Because the next time you run the program it might locate the virtual function table at a different location, copying old pointer information into objects from a file can create havoc. (Also see the Note in Programming Exercise 6.)

Tip

The read() and write() member functions complement each other. You use read() to recover data that has been written to a file with write().

Listing 17.19 uses these methods to create and read a binary file. In form, the program is similar to Listing 17.18, but it uses write() and read() instead of the insertion operator and the get() method. It also uses manipulators to format the screen output.

Note

Although the binary file concept is part of ANSI C, some C and C++ implementations do not provide support for the binary file mode. The reason for this oversight is that some systems have only one file type in the first place, so you can use binary operations such as read() and write() with the standard file format. Therefore, if your implementation rejects ios_base::binary as a valid constant, you can just omit it from your program. If your implementation doesn’t support the fixed and right manipulators, you can use cout.setf(ios_base::fixed, ios_base::floatfield) and cout.setf(ios_base::right, ios_base::adjustfield). Also you may have to substitute ios for ios_base. Other compilers, particularly older ones, may have other idiosyncrasies.

Listing 17.19. binary.cpp

// binary.cpp -- binary file I/O

#include // not required by most systems

#include

#include

#include   // for exit()

inline void eatline() { while (std::cin.get() != '\n') continue; }

struct planet

{

    char name[20];      // name of planet

    double population;  // its population

    double g;           // its acceleration of gravity

};

const char * file = "planets.dat";

int main()

{

    using namespace std;

    planet pl;