C++ Primer Plus

Recall that the class str member is just a pointer, so the constructor has to provide the memory for holding a string. You can pass a string pointer to the constructor when you initialize an object:

String boston("Boston");

The constructor must then allocate enough memory to hold the string, and then it must copy the string to that location. Let’s go through the process step-by-step.

First, the function initializes the len member, using the strlen() function to compute the length of the string. Next, it uses new to allocate sufficient space to hold the string, and then it assigns the address of the new memory to the str member. (Recall that strlen() returns the length of a string, not counting the terminating null character, so the constructor adds one to len to allow space for the string, including the null character.)

Next, the constructor uses strcpy() to copy the passed string into the new memory. Then it updates the object count. Finally, to help you monitor what’s going on, the constructor displays the current number of objects and the string stored in the object. This feature will come in handy later when we deliberately lead the Stringbad class into trouble.

To understand this approach, you should realize that the string is not stored in the object. The string is stored separately, in heap memory, and the object merely stores information that tells where to find the string.

Note that you do not use this:

str = s;  // not the way to go

This merely stores the address without making a copy of the string.

The default constructor behaves similarly, except that it provides a default string of "C++".

The destructor contains the example’s most important addition to the handling of classes:

StringBad::~StringBad()               // necessary destructor

{

    cout << "\"" << str << "\" object deleted, ";    // FYI

    --num_strings;                    // required

    cout << num_strings << " left\n"; // FYI

    delete [] str;                    // required

}

The destructor begins by announcing when the destructor gets called. This part is informative but not essential. However, the delete statement is vital. Recall that the str member points to memory allocated with new. When a StringBad object expires, the str pointer expires. But the memory str pointed to remains allocated unless you use delete to free it. Deleting an object frees the memory occupied by the object itself, but it does not automatically free memory pointed to by pointers that were object members. For that, you must use the destructor. By placing the delete statement in the destructor, you ensure that the memory that a constructor allocates with new is freed when the object expires.

Warning

Whenever you use new in a constructor to allocate memory, you should use delete in the corresponding destructor to free that memory. If you use new [] (with brackets), then you should use delete [] (with brackets).

Listing 12.3, which is taken from a program under development at The Daily Vegetable, illustrates when and how the StringBad constructors and destructors work. The program places the object declarations within an inner block because the destructor is called when execution exits the block in which an object is defined. Without the inner block, the destructors would be called after execution exits main(), which would prevent you in some environments from seeing the destructor messages before the execution window closes. Be sure to compile Listing 12.2 along with Listing 12.3.

Listing 12.3. vegnews.cpp

// vegnews.cpp -- using new and delete with classes

// compile with strngbad.cpp

#include

using std::cout;

#include "strngbad.h"

void callme1(StringBad &);  // pass by reference

void callme2(StringBad);    // pass by value

int main()

{

    using std::endl;

    {

        cout << "Starting an inner block.\n";

        StringBad headline1("Celery Stalks at Midnight");

        StringBad headline2("Lettuce Prey");

        StringBad sports("Spinach Leaves Bowl for Dollars");

        cout << "headline1: " << headline1 << endl;

        cout << "headline2: " << headline2 << endl;

        cout << "sports: " << sports << endl;

        callme1(headline1);

        cout << "headline1: " << headline1 << endl;

        callme2(headline2);

        cout << "headline2: " << headline2 << endl;

        cout << "Initialize one object to another:\n";

        StringBad sailor = sports;

        cout << "sailor: " << sailor << endl;

        cout << "Assign one object to another:\n";

        StringBad knot;

        knot = headline1;

        cout << "knot: " << knot << endl;

        cout << "Exiting the block.\n";

    }

    cout << "End of main()\n";

    return 0;

}

void callme1(StringBad & rsb)

{

    cout << "String passed by reference:\n";

    cout << "    \"" << rsb << "\"\n";

}

void callme2(StringBad sb)

{

    cout << "String passed by value:\n";

    cout << "    \"" << sb << "\"\n";

}

Note