C++ Primer Plus

2. Asks which record you want to modify.

3. Modifies that record.

4. Shows the revised file.

A more ambitious program would use a menu and a loop to let you select from the list of actions indefinitely, but this version performs each action just once. This simplified approach allows you to examine several aspects of read/write files without getting bogged down in matters of program design.

Caution

This program assumes that the planets.dat file already exists and was created by the binary.cpp program in Listing 17.19.

The first question to answer is what file mode to use. In order to read the file, you need the ios_base::in mode. For binary I/O, you need the ios_base::binary mode. (Again, on some nonstandard systems you can omit—indeed, you may have to omit—this mode.) In order to write to the file, you need the ios_base::out or the ios_base::app mode. However, the append mode allows a program to add data to the end of the file only. The rest of the file is read-only; that is, you can read the original data but not modify it—so, to be able to modify the data, you have to use ios_base::out. As Table 17.8 indicates, using the in and out modes simultaneously provides a read/write mode, so you just have to add the binary element. As mentioned earlier, you use the | operator to combine modes. Thus, you need the following statement to set up business:

finout.open(file,ios_base::in | ios_base::out | ios_base::binary);

Next, you need a way to move through a file. The fstream class inherits two methods for this: seekg() moves the input pointer to a given file location, and seekp() moves the output pointer to a given file location. (Actually, because the fstream class uses buffers for intermediate storage of data, the pointers point to locations in the buffers, not in the actual file.) You can also use seekg() with an ifstream object and seekp() with an ofstream object. Here are the seekg() prototypes:

basic_istream& seekg(off_type, ios_base::seekdir);

basic_istream& seekg(pos_type);

As you can see, they are templates. This chapter uses a template specialization for the char type. For the char specialization, the two prototypes are equivalent to the following:

istream & seekg(streamoff, ios_base::seekdir);

istream & seekg(streampos);

The first prototype represents locating a file position measured, in bytes, as an offset from a file location specified by the second argument. The second prototype represents locating a file position measured, in bytes, from the beginning of a file.

Type Escalation

When C++ was young, life was simpler for the seekg() methods. The streamoff and streampos types were typedefs for some standard integer type, such as long. However, the quest for creating a portable standard had to deal with the realization that an integer argument might not provide enough information for some file systems, so streamoff and streampos were allowed to be structure or class types, as long as they allowed some basic operations, such as using an integer value as an initialization value. Next, the old istream class was replaced with the basic_istream template, and streampos and streamoff were replaced with the template-based types pos_type and off_type. However, streampos and streamoff continue to exist as char specializations of pos_type and off_type. Similarly, you can use the wstreampos and wstreamoff types if you use seekg() with a wistream object.

Let’s take a look at the arguments to the first prototype of seekg(). Values of the streamoff type are used to measure offsets, in bytes, from a particular location in a file. The streamoff argument represents the file position, in bytes, measured as an offset from one of three locations. (The type may be defined as an integer type or as a class.) The seek_dir argument is another integer type that is defined, along with three possible values, in the ios_base class. The constant ios_base::beg means measure the offset from the beginning of the file. The constant ios_base::cur means measure the offset from the current position. The constant ios_base::end means measure the offset from the end of the file. Here are some sample calls, assuming that fin is an ifstream object:

fin.seekg(30, ios_base::beg);    // 30 bytes beyond the beginning

fin.seekg(-1, ios_base::cur);    // back up one byte

fin.seekg(0, ios_base::end);     // go to the end of the file