C++ Primer Plus

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

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                  // string.h for some

#include "string1.h"               // includes

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] = '\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);

}