C++ Primer Plus

For keyboard entry, the program should use a menu to offer the user the choice of which type of object to create. The menu should use a switch to use new to create an object of the desired type and to assign the object’s address to a pointer in the pc array. Then that object can use the virtual setall() function to elicit the appropriate data from the user:

pc[i]->setall();  // invokes function corresponding to type of object

To save the data to a file, devise a virtual writeall() function for that purpose:

for (i = 0; i < index; i++)

    pc[i]->writeall(fout);// fout ofstream connected to output file

Note

Use text I/O, not binary I/O, for Programming Exercise 6. (Unfortunately, virtual objects include pointers to tables of pointers to virtual functions, and write() copies this information to a file. An object filled by using read() from the file gets weird values for the function pointers, which really messes up the behavior of virtual functions.) Use a newline character to separate each data field from the next; this makes it easier to identify fields on input. Or you could still use binary I/O, but not write objects as a whole. Instead, you could provide class methods that apply the write() and read() functions to each class member individually rather than to the object as a whole. That way, the program could save just the intended data to a file.

The tricky part is recovering the data from the file. The problem is, how can the program know whether the next item to be recovered is an employee object, a manager object, a fink type, or a highfink type? One approach is, when writing the data for an object to a file, precede the data with an integer that indicates the type of object to follow. Then, on file input, the program can read the integer and then use switch to create the appropriate object to receive the data:

enum classkind{Employee, Manager, Fink, Highfink}; // in class header

...

int classtype;

while((fin >> classtype).get(ch)){ // newline separates int from data

    switch(classtype) {

        case Employee  : pc[i] = new employee;

                      : break;

Then you can use the pointer to invoke a virtual getall() function to read the information:

pc[i++]->getall();

7. Here is part of a program that reads keyboard input into a vector of string objects, stores the string contents (not the objects) in a file, and then copies the file contents back into a vector of string objects:

int main()

{

    using namespace std;

    vector vostr;

    string temp;

// acquire strings

    cout << "Enter strings (empty line to quit):\n";

    while (getline(cin,temp) && temp[0] != '\0')

        vostr.push_back(temp);

    cout << "Here is your input.\n";

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

// 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.

18. Visiting with the New C++ Standard

In this chapter you’ll review C++11 features previously covered and then learn about the following:

• Move semantics and rvalue references

• Lambda expressions

• The function wrapper template

• Variadic templates