Further Assignment Operator Overloading

Before looking at the new listings for the String class example, let’s consider another matter. Suppose you want to copy an ordinary string to a String object. For example, suppose you use getline() to read a string and you want to place it in a String object. The class methods already allow you to do the following:

String name;
char temp[40];
cin.getline(temp, 40);
name = temp;  // use constructor to convert type

However, this might not be a satisfactory solution if you have to do it often. To see why, let’s review how the final statement works:

1. The program uses the String(const char *) constructor to construct a temporary String object containing a copy of the string stored in temp. Remember from Chapter 11, “Working with Classes,” that a constructor with a single argument serves as a conversion function.

2. In Listing 12.6, later in this chapter, the program uses the String & String::operator=(const String &) function to copy information from the temporary object to the name object.

3. The program calls the ~String() destructor to delete the temporary object.

The simplest way to make the process more efficient is to overload the assignment operator so that it works directly with ordinary strings. This removes the extra steps of creating and destroying a temporary object. Here’s one possible implementation:

String & String::operator=(const char * s)
{
    delete [] str;
    len = std::strlen(s);
    str = new char[len + 1];
    std::strcpy(str, s);
    return *this;
}

As usual, you must deallocate memory formerly managed by str and allocate enough memory for the new string.

Listing 12.4 shows the revised class declaration. In addition to the changes already mentioned, it defines the constant CINLIM, which is used in implementing operator>>().

Listing 12.4. string1.h

// string1.h -- fixed and augmented string class definition

#ifndef STRING1_H_
#define STRING1_H_
#include <iostream>
using std::ostream;
using std::istream;

class String
{
private:
    char * str;             // pointer to string
    int len;                // length of string
    static int num_strings; // number of objects
    static const int CINLIM = 80;  // cin input limit
public:
// constructors and other methods
    String(const char * s); // constructor
    String();               // default constructor
    String(const String &); // copy constructor
    ~String();              // destructor
    int length () const { return len; }
// overloaded operator methods
    String & operator=(const String &);
    String & operator=(const char *);
    char & operator[](int i);
    const char & operator[](int i) const;
// overloaded operator friends
    friend bool operator<(const String &st, const String &st2);
    friend bool operator>(const String &st1, const String &st2);
    friend bool operator==(const String &st, const String &st2);
    friend ostream & operator<<(ostream & os, const String & st);
    friend istream & operator>>(istream & is, String & st);
// static function
    static int HowMany();
};
#endif

Listing 12.5 presents the revised method definitions.

Listing 12.5. string1.cpp

// string1.cpp -- String class methods
#include <cstring>                 // string.h for some
#include "string1.h"               // includes <iostream>
using std::cin;
using std::cout;

// initializing static class member

int String::num_strings = 0;

// static method
int String::HowMany()
{
    return num_strings;
}

// class methods
String::String(const char * s)     // construct String from C string
{
    len = std::strlen(s);          // set size
    str = new char[len + 1];       // allot storage
    std::strcpy(str, s);           // initialize pointer
    num_strings++;                 // set object count
}

String::String()                   // default constructor
{
    len = 4;
    str = new char[1];
    str[0] = '';                 // default string
    num_strings++;
}

String::String(const String & st)
{
    num_strings++;             // handle static member update
    len = st.len;              // same length
    str = new char [len + 1];  // allot space
    std::strcpy(str, st.str);  // copy string to new location
}

String::~String()                     // necessary destructor
{
    --num_strings;                    // required
    delete [] str;                    // required
}

// overloaded operator methods

    // assign a String to a String
String & String::operator=(const String & st)
{
    if (this == &st)
        return *this;
    delete [] str;
    len = st.len;
    str = new char[len + 1];
    std::strcpy(str, st.str);
    return *this;
}

    // assign a C string to a String
String & String::operator=(const char * s)
{
    delete [] str;
    len = std::strlen(s);
    str = new char[len + 1];
    std::strcpy(str, s);
    return *this;
}

    // read-write char access for non-const String
char & String::operator[](int i)
{
    return str[i];
}

    // read-only char access for const String
const char & String::operator[](int i) const
{
    return str[i];
}

// overloaded operator friends

bool operator<(const String &st1, const String &st2)
{
    return (std::strcmp(st1.str, st2.str) < 0);
}

bool operator>(const String &st1, const String &st2)
{
    return st2 < st1;
}

bool operator==(const String &st1, const String &st2)
{
    return (std::strcmp(st1.str, st2.str) == 0);
}

    // simple String output
ostream & operator<<(ostream & os, const String & st)
{
    os << st.str;
    return os;
}

    // quick and dirty String input
istream & operator>>(istream & is, String & st)
{
    char temp[String::CINLIM];
    is.get(temp, String::CINLIM);
    if (is)
        st = temp;
    while (is && is.get() != ' ')
        continue;
    return is;
}

The overloaded >> operator provides a simple way to read a line of keyboard input into a String object. It assumes an input line of String::CINLIM or fewer characters and discards any characters beyond that limit. Keep in mind that the value of an istream object in an if condition evaluates to false if input fails for some reason, such as encountering an end-of-file condition, or in the case of get(char *, int), reading an empty line.

Listing 12.6 exercises the String class with a short program that lets you enter a few strings. The program has the user enter sayings, puts the strings into String objects, displays them, and reports which string is the shortest and which comes first alphabetically.

Listing 12.6. sayings1.cpp

// sayings1.cpp -- using expanded String class
// compile with string1.cpp
#include <iostream>
#include "string1.h"
const int ArSize = 10;
const int MaxLen =81;
int main()
{
    using std::cout;
    using std::cin;
    using std::endl;
    String name;
    cout <<"Hi, what's your name? >> ";
    cin >> name;

    cout << name << ", please enter up to " << ArSize
         << " short sayings <empty line to quit>: ";
    String sayings[ArSize];     // array of objects
    char temp[MaxLen];          // temporary string storage
    int i;
    for (i = 0; i < ArSize; i++)
    {
        cout << i+1 << ": ";
        cin.get(temp, MaxLen);
        while (cin && cin.get() != ' ')
            continue;
        if (!cin || temp[0] == '')    // empty line?
            break;              // i not incremented
        else
            sayings[i] = temp;  // overloaded assignment
    }
    int total = i;              // total # of lines read

    if ( total > 0)
    {
        cout << "Here are your sayings: ";
        for (i = 0; i < total; i++)
            cout << sayings[i][0] << ": " << sayings[i] << endl;

        int shortest = 0;
        int first = 0;
        for (i = 1; i < total; i++)
        {
            if (sayings[i].length() < sayings[shortest].length())
                shortest = i;
            if (sayings[i] < sayings[first])
                first = i;
        }
        cout << "Shortest saying: " << sayings[shortest] << endl;;
        cout << "First alphabetically: " << sayings[first] << endl;
        cout << "This program used "<< String::HowMany()
             << " String objects. Bye. ";
    }
    else
        cout << "No input! Bye. ";
     return 0;
}

The program in Listing 12.6 asks the user to enter up to 10 sayings. Each saying is read into a temporary character array and then copied to a String object. If the user enters a blank line, a break statement terminates the input loop. After echoing the input, the program uses the length() and operator<() member functions to locate the shortest string and the alphabetically earliest string. The program also uses the subscript operator ([]) to preface each saying with its initial character. Here’s a sample run:

Hi, what's your name?
>> Misty Gutz
Misty Gutz, please enter up to 10 short sayings <empty line to quit>:
1: a fool and his money are soon parted
2: penny wise, pound foolish
3: the love of money is the root of much evil
4: out of sight, out of mind
5: absence makes the heart grow fonder
6: absinthe makes the hart grow fonder
7:
Here are your sayings:
a: a fool and his money are soon parted
p: penny wise, pound foolish
t: the love of money is the root of much evil
o: out of sight, out of mind
a: absence makes the heart grow fonder
a: absinthe makes the hart grow fonder
Shortest saying:
penny wise, pound foolish
First alphabetically:
a fool and his money are soon parted
This program used 11 String objects. Bye.

Things to Remember When Using new in Constructors

By now you’ve noticed that you must take special care when using new to initialize pointer members of an object. In particular, you should do the following:

• If you use new to initialize a pointer member in a constructor, you should use delete in the destructor.

• The uses of new and delete should be compatible. You should pair new with delete and new [] with delete [].

• If there are multiple constructors, all should use new the same way—either all with brackets or all without brackets. There’s only one destructor, so all constructors have to be compatible with that destructor. However, it is permissible to initialize a pointer with new in one constructor and with the null pointer (0, or, with C++11, nullptr) in another constructor because it’s okay to apply the delete operation (with or without brackets) to the null pointer.

• You should define a copy constructor that initializes one object to another by doing deep copying. Typically, the constructor should emulate the following example:

String::String(const String & st)
{
     num_strings++;            // handle static member update if necessary
     len = st.len;             // same length as copied string
     str = new char [len + 1]; // allot space
     std::strcpy(str, st.str); // copy string to new location
}

In particular, the copy constructor should allocate space to hold the copied data, and it should copy the data, not just the address of the data. Also it should update any static class members whose value would be affected by the process.

• You should define an assignment operator that copies one object to another by doing deep copying. Typically, the class method should emulate the following example:

String & String::operator=(const String & st)
{
     if (this == &st)            // object assigned to itself
          return *this;          // all done
     delete [] str;              // free old string
     len = st.len;
     str = new char [len + 1];   // get space for new string
     std::strcpy(str, st.str);   // copy the string
     return *this;               // return reference to invoking object
}

In particular, the method should check for self-assignment; it should free memory formerly pointed to by the member pointer; it should copy the data, not just the address of the data; and it should return a reference to the invoking object.