C++ Primer Plus

Sometimes new C++ users become confused about when to use the dot operator and when to use the arrow operator to specify a structure member. The rule is simple: If the structure identifier is the name of a structure, use the dot operator. If the identifier is a pointer to the structure, use the arrow operator.

A second, uglier approach to accessing structure members is to realize that if ps is a pointer to a structure, then *ps represents the pointed-to value—the structure itself. Then, because *ps is a structure, (*ps).price is the price member of the structure. C++’s operator precedence rules require that you use parentheses in this construction.

Listing 4.21 uses new to create an unnamed structure and demonstrates both pointer notations for accessing structure members.

Listing 4.21. newstrct.cpp

// newstrct.cpp -- using new with a structure

#include

struct inflatable   // structure definition

{

    char name[20];

    float volume;

    double price;

};

int main()

{

    using namespace std;

    inflatable * ps = new inflatable; // allot memory for structure

    cout << "Enter name of inflatable item: ";

    cin.get(ps->name, 20);            // method 1 for member access

    cout << "Enter volume in cubic feet: ";

    cin >> (*ps).volume;              // method 2 for member access

    cout << "Enter price: $";

    cin >> ps->price;

    cout << "Name: " << (*ps).name << endl;              // method 2

    cout << "Volume: " << ps->volume << " cubic feet\n"; // method 1

    cout << "Price: $" << ps->price << endl;             // method 1

    delete ps;                        // free memory used by structure

    return 0;

}

Here is a sample run of the program in Listing 4.21:

Enter name of inflatable item: Fabulous Frodo

Enter volume in cubic feet: 1.4

Enter price: $27.99

Name: Fabulous Frodo

Volume: 1.4 cubic feet

Price: $27.99

An Example of Using new and delete

Let’s look at an example that uses new and delete to manage storing string input from the keyboard. Listing 4.22 defines a function getname() that returns a pointer to an input string. This function reads the input into a large temporary array and then uses new [] with an appropriate size to create a chunk of memory sized to fit to the input string. Then the function returns the pointer to the block. This approach could conserve a lot of memory for programs that read in a large number of strings. (In real life, where many of us live, it would be easier to use the string class, which has the use of new and delete built in to its design.)

Suppose your program has to read 1,000 strings and that the largest string might be 79 characters long, but most of the strings are much shorter than that. If you used char arrays to hold the strings, you’d need 1,000 arrays of 80 characters each. That’s 80,000 bytes, and much of that block of memory would wind up being unused. Alternatively, you could create an array of 1,000 pointers to char and then use new to allocate only the amount of memory needed for each string. That could save tens of thousands of bytes. Instead of having to use a large array for every string, you fit the memory to the input. Even better, you could also use new to find space to store only as many pointers as needed. Well, that’s a little too ambitious for right now. Even using an array of 1,000 pointers is a little too ambitious for right now, but Listing 4.22 illustrates some of the technique. Also just to illustrate how delete works, the program uses it to free memory for reuse.

Listing 4.22. delete.cpp

// delete.cpp -- using the delete operator

#include

#include       // or string.h

using namespace std;

char * getname(void);   // function prototype

int main()

{

    char * name;        // create pointer but no storage

    name = getname();   // assign address of string to name

    cout << name << " at " << (int *) name << "\n";

    delete [] name;     // memory freed

    name = getname();   // reuse freed memory

    cout << name << " at " << (int *) name << "\n";

    delete [] name;     // memory freed again

    return 0;

}

char * getname()        // return pointer to new string

{

    char temp[80];      // temporary storage

    cout << "Enter last name: ";

    cin >> temp;

    char * pn = new char[strlen(temp) + 1];

    strcpy(pn, temp);   // copy string into smaller space

    return pn;          // temp lost when function ends

}

Here is a sample run of the program in Listing 4.22:

Enter last name: Fredeldumpkin

Fredeldumpkin at 0x004326b8

Enter last name: Pook

Pook at 0x004301c8

Program Notes