C++ class members

Question

I came from Java to C++ ...

When I try to do this ...

class Box {
    Table* onTable;
};

class Table {
    Box* boxOnIt;
};

int main() {
    Table table;
    Box box;

    table.boxOnIt = &box;
    box.onTable = &table;

    return 0;
}

the compiler tell me that Table is undefined. If I switch class definitions the compiler tell me that Box is undefined

In java I was able to do something like this with no problem. Is there a solution for this to work? thanks...

Answer 1

You have a circular dependency here and need to forward to declare one of the classes:

// forward declaration
class Box;

class Table
{
    Box* boxOnit;
}  // eo class Table

class Box
{
    Table* onTable
} // eo class Box

Note that, generally speaking, we'd have a separate header file for Box and Table, using forward declarations in both, e.g:

box.h

class Table;

class Box
{
    Table* table;
}; // eo class Box

table.h

class Box;

class Table
{
    Box* box;
};  // eo class Table

Then, include the necessary file in our implementation (.cpp) files:

box.cpp

#include "box.h"
#include "table.h"

table.cpp

#include "box.h"
#include "table.h"

Answer 2

you should use forward declarations. just mention this as your first statement:

class Table;  // Here is the forward declaration

Answer 3

Add this before class Box:

class Table;

Thus you forward declare class Table so that pointer to it can be used in Box.

Answer 4

class Table;

class Box {
    Table* onTable;
};

class Table {
    Box* boxOnIt;
};

int main() {
    Table table;
    Box box;

    table.boxOnIt = &box;
    box.onTable = &table;

    return 0;
}

Answer 5

Use forward declarations so that the class declared first knows about the second one. http://www.eventhelix.com/realtimemantra/headerfileincludepatterns.htm

Answer 6

You should forward declare one of the two classes:

class Table; // forward declare Table so that Box can use it.

class Box {
    Table* onTable;
};

class Table {
    Box* boxOnIt;
};

int main() {
    Table table;
    Box box;

    table.boxOnIt = &box;
    box.onTable = &table;

    return 0;
}

or viceversa:

class Box; // forward declare Box so that Table can use it.

class Table {
    Box* boxOnIt;
};

class Box {
    Table* onTable;
};

int main() {
    Table table;
    Box box;

    table.boxOnIt = &box;
    box.onTable = &table;

    return 0;
}

Answer 7

Add class definition on the top

class Table;

class Box {
    Table* onTable;
};

class Table {
    Box* boxOnIt;
};