C++ Inheritance problem

Question

I hope I got the relevant code in here. I have some problem when I want to fetch the menu option that I've added into to menu_1. I have this function on_select(int) that I use to fetch one sub-menu's options, which I do by using the display() function. But when I compile it will say that there are no function named display() in menu_option() class, which is the Base class, but what I want to is to access the display() function which is located in the sub_menu() class.

I have tried multiple thing to get the relevant object from the array without any success, so I'm here now asking for help with this one.

I have this following main()

#include <iostream>

using namespace std;

    #include "menu.h"

    int main()
    {


        sub_menu* main_menu = new sub_menu("Warehouse Store Menu");

        sub_menu* menu_1 = new sub_menu("Menu1");

        main_menu->add_option(new sub_menu("Menu2"));
        main_menu->add_option(menu_1);

        product_menu->add_option(new add_item("sub_item1"));
        product_menu->add_option(new add_item("sub_item2"));
        product_menu->add_option(new add_item("sub_item3"));  

        main_menu->display();
        main_menu->on_select(1);

        delete main_menu;

        return 0;
    }

header file

#include <iomanip>
#include <iostream>
#include <string>
using namespace std;

const int MAX_SIZE = 9;

class menu_option
{
public:
    menu_option(string const& n) : title(n) {};
    virtual ~menu_option();

    virtual void on_select(int) = 0;
    string get_title() { return title; }
protected:
    string title;
};


/* ------------------------------------- */
class sub_menu : public menu_option
{

public:
    sub_menu(string const& n)
        : menu_option(n) { no_item = 0; }

    ~sub_menu() { delete[] list; };

    void on_select(int);

    void add_option(menu_option*);
    void display();

private:
    menu_option* list[MAX_SIZE];   //container for options in the sub_menu
    int no_item;

};

implement file

void sub_menu::on_select(int i)
{
    cout << (list[i])->get_title() << endl;
    cout << (list[i])->display() << endl; //<------ Doesn't work
}

void sub_menu::add_option(menu_option* item)
{
    list[no_item] = item;
    no_item++;
}

void sub_menu::display()
{
    cout << ">> " << get_title() << " <<"<< endl;
    for( int i = 0; i < no_item; i++ )
    {
       cout << setw(2) << i << ": " << (list[i])->get_title() << endl;
    }
}

Please cut your code down to the smallest set of lines that demonstrates your problem. Nobody wants to read 2 pages of dense text to discover if they want to solve your problem or not. — Alex Brown
– Alex Brown, Commented Nov 23, 2010 at 19:53
ah sorry, I didn't mean to post a huge set of code. It just that I don't know whether I should post the relevant code, or post the code which produce the error which in this case would be the on_select(int) function. Please do explain for me, so I don't do same error next time — starcorn
– starcorn, Commented Nov 23, 2010 at 20:07

Edward Strange · Accepted Answer · 2010-11-23 19:55:35Z

1

You can do what you want to do, but it's bad. You have to cast down to sub_menu when you call display() in on_select(). Of course it's not going to work the way you have it, and the compiler is telling you exactly why.

The other option, which is probably better (though without a clear understanding of the problem space may not be the best) would be to add display() as a virtual function to the menu_option class.

answered Nov 23, 2010 at 19:55

Edward Strange

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1 Comment

starcorn Over a year ago

Could you tell me more or refer to some page why it's bad to cast?

fbrereto · Accepted Answer · 2010-11-23 20:02:43Z

1

To solve your immediate problem you'll want to use dynamic_cast to turn a menu_option* into a sub_menu*, like so:

sub_menu* submenu(dynamic_cast<sub_menu*>(list[i]));

Note that if the cast fails (i.e., the menu_option pointed to by list[i] is not a sub_menu after all) the value of the submenu pointer will be NULL, so make sure you check that it is a valid pointer before using it in subsequent code.

edited Nov 23, 2010 at 20:02

answered Nov 23, 2010 at 19:53

fbrereto

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5 Comments

Edward Strange Over a year ago

Do NOT use reinterpret_cast!! If you are to cast down, use a static_cast. If you want to verify that the cast is correct, use dynamic_cast. -1

fbrereto Over a year ago

@Noah: Ack! That's the one I meant, sorry. Corrected.

Edward Strange Over a year ago

-1 redacted, but I'd still be happier with the answer if it mentioned that this almost certainly isn't a /good/ fix.

starcorn Over a year ago

thanks you and Noah had the same solution, but I picked Noah's as solution, and gave you +1 for useful answer. Anyway thanks for the help

fbrereto Over a year ago

@Noah: That's why I prefixed the answer with "To solve your immediate problem...", but yes, being more explicit about a proper solution would have been better.

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