C++ Primer Plus

// sayings2.cpp -- using pointers to objects

// compile with string1.cpp

#include

#include       // (or stdlib.h) for rand(), srand()

#include         // (or time.h) for time()

#include "string1.h"

const int ArSize = 10;

const int MaxLen = 81;

int main()

{

    using namespace std;

    String name;

    cout <<"Hi, what's your name?\n>> ";

    cin >> name;

    cout << name << ", please enter up to " << ArSize

         << " short sayings :\n";

    String sayings[ArSize];

    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() != '\n')

            continue;

        if (!cin || temp[0] == '\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:\n";

        for (i = 0; i < total; i++)

            cout << sayings[i] << "\n";

    // use pointers to keep track of shortest, first strings

        String * shortest = &sayings[0]; // initialize to first object

        String * first = &sayings[0];

        for (i = 1; i < total; i++)

        {

            if (sayings[i].length() < shortest->length())

                shortest = &sayings[i];

            if (sayings[i] < *first)     // compare values

                first = &sayings[i];     // assign address

        }

        cout << "Shortest saying:\n" << * shortest << endl;

        cout << "First alphabetically:\n" << * first << endl;

        srand(time(0));

        int choice = rand() % total; // pick index at random

    // use new to create, initialize new String object

        String * favorite = new String(sayings[choice]);

        cout << "My favorite saying:\n" << *favorite << endl;

        delete favorite;

    }

    else

        cout << "Not much to say, eh?\n";

    cout << "Bye.\n";

    return 0;

}

Object Initialization with new

In general, if Class_name is a class and if value is of type Type_name, the statement

Class_name * pclass = new Class_name(value);

invokes this constructor:

Class_name(Type_name);

There may be trivial conversions, such as to this:

Class_name(const Type_name &);

Also the usual conversions invoked by prototype matching, such as from int to double, takes place as long as there is no ambiguity. An initialization in the following form invokes the default constructor:

Class_name * ptr = new Class_name;

Here’s a sample run of the program in Listing 12.7:

Hi, what's your name?

>> Kirt Rood

Kirt Rood, please enter up to 10 short sayings :

1: a friend in need is a friend indeed

2: neither a borrower nor a lender be

3: a stitch in time saves nine

4: a niche in time saves stine

5: it takes a crook to catch a crook

6: cold hands, warm heart

7:

Here are your sayings:

a friend in need is a friend indeed

neither a borrower nor a lender be

a stitch in time saves nine

a niche in time saves stine

it takes a crook to catch a crook

cold hands, warm heart

Shortest saying:

cold hands, warm heart

First alphabetically:

a friend in need is a friend indeed

My favorite saying:

a stitch in time saves nine

Bye

Because the program selects the favorite saying randomly, different runs of the program will show different choices, even for identical input.

Looking Again at new and delete

Note that the program generated from Listings 12.4, 12.5, and 12.7 uses new and delete on two levels. First, it uses new to allocate storage space for the name strings for each object that is created. This happens in the constructor functions, so the destructor function uses delete to free that memory. Because each string is an array of characters, the destructor uses delete with brackets. Thus, memory used to store the string contents is freed automatically when an object is destroyed. Second, the code in Listing 12.7 uses new to allocate an entire object:

String * favorite = new String(sayings[choice]);

This allocates space not for the string to be stored but for the object—that is, for the str pointer that holds the address of the string and for the len member. (It does not allocate space for the num_strings member because it is a static member that is stored separately from the objects.) Creating the object, in turn, calls the constructor, which allocates space for storing the string and assigns the string’s address to str. The program then uses delete to delete this object when it is finished with it. The object is a single object, so the program uses delete without brackets. Again, this frees only the space used to hold the str pointer and the len member. It doesn’t free the memory used to hold the string str points to, but the destructor takes care of that final task (see Figure 12.4).

Figure 12.4. Calling destructors.

Again, destructors are called in the following situations (refer to Figure 12.4):