C++ Primer Plus

Listing 7.14. topfive.cpp

// topfive.cpp -- handling an array of string objects

#include

#include

using namespace std;

const int SIZE = 5;

void display(const string sa[], int n);

int main()

{

    string list[SIZE];     // an array holding 5 string object

    cout << "Enter your " << SIZE << " favorite astronomical sights:\n";

    for (int i = 0; i < SIZE; i++)

    {

        cout << i + 1 << ": ";

        getline(cin,list[i]);

    }

    cout << "Your list:\n";

    display(list, SIZE);

    return 0;

}

void display(const string sa[], int n)

{

    for (int i = 0; i < n; i++)

        cout << i + 1 << ": " << sa[i] << endl;

}

Here’s a sample run of the program in Listing 7.14:

Enter your 5 favorite astronomical sights:

1: Orion Nebula

2: M13

3: Saturn

4: Jupiter

5: Moon

Your list:

1: Orion Nebula

2: M13

3: Saturn

4: Jupiter

5: Moon

The main point to note in this example is that, aside from the getline() function, this program treats string just as it would treat any of the built-in types, such as int. If you want an array of string, you just use the usual array-declaration format:

string list[SIZE];     // an array holding 5 string object

Each element of the list array, then, is a string object and can be used as such:

getline(cin,list[i]);

Similarly, the formal argument sa is a pointer to a string object, so sa[i] is a string object and can be used accordingly:

cout << i + 1 << ": " << sa[i] << endl;

Functions and array Objects

Class objects in C++ are based on structures, so some of the same programming considerations that apply to structures also apply to classes. For example, you can pass an object by value to a function, in which case the function acts on a copy of the original object. Alternatively, you can pass a pointer to an object, which allows the function to act on the original object. Let’s look at an example using the C++11 array template class.

Suppose we have an array object intended to hold expense figures for each of the four seasons of the year:

std::array expenses;

(Recall that using the array class requires the array header file and that the name array is part of the std namespace.) If we want a function to display the contents of expenses, we can pass expenses by value:

show(expenses);

But if we want a function that modifies the expenses object, we need to pass the address of the object to the function:

fill(&expenses);

(The next chapter discusses an alternative approach, using references.) This is the same approach that Listing 7.13 used for structures.

How can we declare these two functions? The type of expenses is array, so that’s what must appear in the prototypes:

void show(std::array da);   // da an object

void fill(std::array * pa); // pa a pointer to an object

These considerations form the core of the sample program. The program adds a few more features. First, it replaces 4 with a symbolic constant:

const int Seasons = 4;

Second, it adds a const array object containing four string objects representing the four seasons:

const std::array Snames =

    {"Spring", "Summer", "Fall", "Winter"};

Note that the array template is not limited to holding the basic data types; it can use class types too. Listing 7.15 presents the program in full.

Listing 7.15. arrobj.cpp

//arrobj.cpp -- functions with array objects (C++11)

#include

#include

#include

// constant data

const int Seasons = 4;

const std::array Snames =

    {"Spring", "Summer", "Fall", "Winter"};

// function to modify array object

void fill(std::array * pa);

// function that uses array object without modifying it

void show(std::array da);

int main()

{

    std::array expenses;

    fill(&expenses);

    show(expenses);

    return 0;

}

void fill(std::array * pa)

{

    using namespace std;

    for (int i = 0; i < Seasons; i++)

    {

        cout << "Enter " << Snames[i] << " expenses: ";

        cin >> (*pa)[i];

    }

}

void show(std::array da)

{

    using namespace std;

    double total = 0.0;

    cout << "\nEXPENSES\n";

    for (int i = 0; i < Seasons; i++)

    {

        cout << Snames[i] << ": $" << da[i] << endl;

        total += da[i];

    }

    cout << "Total Expenses: $" << total << endl;

}

Here’s a sample run:

Enter Spring expenses: 212

Enter Summer expenses: 256

Enter Fall expenses: 208

Enter Winter expenses: 244

EXPENSES

Spring: $212

Summer: $256

Fall: $208

Winter: $244

Total: $920

Program Notes

Because the const array object Snames is declared above all the functions, it can be used in any of the following function definitions. Like the const Seasons, Snames is shared by the whole source code file. The program doesn’t have a using directive, so array and string have to be used with the str:: qualifier. To keep the program short and focused on how functions can use objects, the fill() function doesn’t check for valid input.