Using Friend Class

/*
I'm trying to develope a fried class (c2) capable of accessing
class cl for the purpose of gathering data. I want to pass a
cl pointer to c2::runShow. runShow would subsequently pass the
pointer to other c2 member functions. The code compiles without
error or warnings. It reads the first object of the array of
objects, but when c2::prn increments i, the following error
message is displayed:

Loaded 'C:\WINDOWS\SYSTEM\KERNEL32.DLL', no matching symbolic
information found. First-chance exception in testCPP.exe:
0xC0000005: Access Violation.

I know it can't locate the subsequent objects of the array of
objects and therefore has an access violation. I can't figure
out why.

If you could point out my blounder, I'd be happy.

Thanks
*/

#include <iostream>

class cl {
int a, b, c;
static int cntr;
public:
cl( int j, int k, int l )
{ a = j; b = k; c = l; cntr++;
std::cout << "Object " << cntr << '\n'; }
~cl() { std::cout << "Object " << cntr-- << " destroyed.\n"; }
int get_a () { return a; }
int get_b () { return b; }
int get_c () { return c; }

friend class c2;

};


class c2 {
public:
void prn( cl **obj )
{
std::cout << "Using class cl access function\n\n";
for( int i = 0; i < 3; i++ ) {
std::cout << obj[i]->get_a() << ", ";
std::cout << obj[i]->get_b() << ", ";
std::cout << obj[i]->get_c() << ",\t";
} // end for

std::cout << "\n\n";
} // end prn

void runShow( cl *ob )
{
prn( &ob );
} // end show
}; // end c2

int cl::cntr = 0; // initialize counter


int main() {

cl ob[3] = { cl(1, 4, 7), // create array of objects
cl(2, 5, 8),
cl(3, 6, 9) }; // initializer
c2 m; // create friend object

m.runShow( ob );

return 0;
} // end

Below are the changes I made.

Code:

---

#include <iostream>

class cl
{
        int a, b, c;
        static int cntr;
public:
        cl( int j, int k, int l )
        {
                a = j; b = k; c = l; cntr++;
                std::cout << "Object " << cntr << '\n';
        }
       
        ~cl() { std::cout << "Object " << cntr-- << " destroyed.\n"; }

        int get_a () { return a; }

        int get_b () { return b; }

        int get_c () { return c; }

friend class c2;

};


class c2
{
public:

        void prn( cl **obj )
        {
                std::cout << "Using class cl access function\n\n";
                for( int i = 0; i < 3; i++ ) {
                        std::cout << obj[i]->get_a() << ", ";
                        std::cout << obj[i]->get_b() << ", ";
                        std::cout << obj[i]->get_c() << ",\t";
                } // end for

        std::cout << "\n\n";
        } // end prn

        void runShow( cl **ob ) //added * so it would take pointer to pointers
        {
                prn( ob ); //remove & reference
        } // end show
}; // end c2

int cl::cntr = 0; // initialize counter


int main() {

cl* ob[3] = { //added * for array of pointers of cl
        &cl(1, 4, 7), // added reference to take address of
        &cl(2, 5, 8),
        &cl(3, 6, 9)
                        }; // initializer

        c2 m; // create friend object

        m.runShow( ob );

        return 0;
} // end

---

Thank you for your help.

Could you explain why the array of pointers to objects works better than the array of objects?

Also why does it immediatly destroy the object after it is created?

Object 1 created
Object 1 destroyed.
Object 1 created
Object 1 destroyed.
Object 1 created
Object 1 destroyed.
Using class cl access function

1, 4, 7, 2, 5, 8, 3, 6, 9,


Press any key to continue

It still gives me the correct data and it can be manipulated. Any ideas?

I don't know why when I was here last time it didn't use my name it just called me Unregistered, anyway.

If you change main to below it will give the objects their desired life expectancies.

The reason the objects were destroy before the creation of the next object is because they were just temporary and had no name to associate with allocated space so they were deleted.

&cl(1, 4, 7),// note there is no name just address of a cl.
&cl(2, 5, 8),
&cl(3, 6, 9)

The reason the objects could still be displayed is that they were allocated on the stack and while they had been destroyed the location they were at had not been reallocated for something else so the data was availible. I'm glad you caught that, I didn't see it. Using invalid memory is bad ju-ju and should not be done unless you enjoy the trap handler throwing exceptions at you.

Code:

---


int main()
{
        cl* ob[3];

        cl a(1, 4, 7);
        cl b(2, 5, 8);
        cl c(3, 6, 9);

        ob[0] = &a;
        ob[1] = &b;
        ob[2] = &c;

        c2 m; // create friend object

        m.runShow( ob );

        return 0;
} // end

---