Array of different data types

Question

I need to store an array of newspapers and booklets And then in a loop, print out how much paper it takes to print each product, but when I call the price calculation function, the function is called from the main class and not from the child ones, how can I call the function from the child classes in an array with different objects?

#include <iostream>
using namespace std;

class PrintedProduct
{
public:
    string name;
    int pages;
    int paperCost()
    {
        return 1;
    }
    PrintedProduct() {}
    PrintedProduct(string name, int pages)
    {
        this->name = name;
        this->pages = pages;
    }
};

class NewsPaper : public PrintedProduct
{
public:
    int pageSize;
    int quantity;
    int period;
    NewsPaper(string name, int pages, int pageSize, int quantity, int period) : PrintedProduct(name, pages)
    {
        this->pageSize = pageSize;
        this->quantity = quantity;
        this->period = period;
    }
    int paperCost()
    {
        return pages * pageSize * quantity * period;
    }
};

class Booklet : public PrintedProduct
{
public:
    int pageSize;
    int quantity;
    Booklet(string name, int pages, int pageSize, int quantity)
        : PrintedProduct(name, pages)
    {
        this->pageSize = pageSize;
        this->quantity = quantity;
    }
    int paperCost()
    {
        return pages * pageSize * quantity;
    }
};

void print(PrintedProduct a)
{
    cout << a.paperCost() << endl;
};

int main()
{
    // Write C++ code here
    size_t size;
    cout << "Please, enter a size of the array: " << endl;
    cin >> size;
    PrintedProduct *array = new PrintedProduct[size];

    cout
        << "Please, enter "
        << size
        << " Printed products: ";
    for (size_t i = 0; i < size; i++)
    {
        cout << "Please enter: 1. Newspaper or 2. Booklet";
        int type;
        cin >> type;
        if (type == 1)
        {
            cout << "Enter name:" << endl;
            string name;
            cin >> name;

            cout << "Enter page count" << endl;
            int pages;
            cin >> pages;

            cout << "Enter page size " << endl;
            int pageSize;
            cin >> pageSize;

            cout << "Enter quantity" << endl;
            int quantity;
            cin >> quantity;

            cout << "Enter period" << endl;
            int period;
            cin >> period;

            array[i] = NewsPaper(name, pages, pageSize, quantity, period);
        }
        else
        {
            cout << "Enter name:";
            string name;
            cin >> name;

            cout << "Enter page count";
            int pages;
            cin >> pages;

            cout << "Enter page size";
            int pageSize;
            cin >> pageSize;

            cout << "Enter quantity";
            int quantity;
            cin >> quantity;

            array[i] = Booklet(name, pages, pageSize, quantity);
        }
    }

    for (size_t j = 0; j < size; j++)
    {
        print(array[j]);
    }
    delete[] array;

    std::system("PAUSE");
    return EXIT_SUCCESS;
}

Read up on `virtual` functions in whatever resource you use to learn C++. You will need to change some other things as well, since your array can only hold objects of type `PrintedProduct` - everything else is sliced — UnholySheep, May 03 '22 at 15:24
It's not possible in C++ to have polymorphic arrays (or other containers). Use an array (or vector) of `PrintedProduct` **pointers** instead. Also, the `paperCost` member function should be declared `virtual` in the base class. — Wolf, May 03 '22 at 15:42

eerorika · Accepted Answer · 2022-05-03T16:09:37.960

I need to store an array of newspapers and booklets

This isn't possible. Arrays can only contain objects of one type. In case of new PrintedProduct[size] that array contains objects of type PrintedProduct. The elements of the array are not instances of classes derived from PrintedProduct.

The solution to this is to use indirection. You can create an array of pointers to PrintedProduct. Those pointers may point to base sub object of derived classes. In following example, I will be using std::vector and std::unique_ptr in order to avoid owning bare pointers:

std::vector<std::unique_ptr<PrintedProduct>> array(size);
...
array[i] = std::make_unique<NewsPaper>(
    name, pages, pageSize, quantity, period);

how can I call the function from the child classes

You can call a member function from a derived class by overriding a virtual function. Example:

class PrintedProduct
{
public:
    virtual int paperCost() = 0;
// ...

class NewsPaper : public PrintedProduct
{
public:
    int paperCost() override
    {
        return pages * pageSize * quantity * period;
    }
// ...
a.paperCost(); // virtual dispatch

"Arrays can only contain objects of one type." What about using arrays of union or std::variant<>? — Gabriel Staples, May 03 '22 at 16:13
@GabrielStaples Those arrays contain objects of the union type, or objects of the variant type. They are just another form of indirection; albeit one where the storage is in-place. — eerorika, May 03 '22 at 16:17

Array of different data types

1 Answers1