C++ Primer Plus

// store in a file

ofstream fout("strings.dat", ios_base::out | ios_base::binary);

for_each(vostr.begin(), vostr.end(), Store(fout));

fout.close();

// recover file contents

vector vistr;

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

if (!fin.is_open())

{

cerr << "Could not open file for input.\n";

exit(EXIT_FAILURE);

}

GetStrs(fin, vistr);

cout << "\nHere are the strings read from the file:\n";

for_each(vistr.begin(), vistr.end(), ShowStr);

return 0;

}

Note that the file is opened in binary format and that the intention is that I/O be accomplished with read() and write(). Quite a bit remains to be done:

• Write a void ShowStr(const string &) function that displays a string object followed by a newline character.

• Write a Store functor that writes string information to a file. The Store constructor should specify an ifstream object, and the overloaded operator()(const string &) should indicate the string to write. A workable plan is to first write the string’s size to the file and then write the string contents. For example, if len holds the string size, you could use this:

os.write((char *)&len, sizeof(std::size_t)); // store length

os.write(s.data(), len); // store characters

The data() member returns a pointer to an array that holds the characters in the string. It’s similar to the c_str() member except that the latter appends a null character.

• Write a GetStrs() function that recovers information from the file. It can use read() to obtain the size of a string and then use a loop to read that many characters from the file, appending them to an initially empty temporary string. Because a string’s data is private, you have to use a class method to get data into the string rather than read directly into it.