Inheritance - Problem calling methods

I have a class RAW_FILE which is the base class, and a class DIRECTORY which is derived from RAW_FILE.
The code below shows my problem, and I would like to know if there is a way to solve it without just creating a virtual method (printToScreen) in the base class.

In this code, I cannot assign the address held in temp_dir->next (RAW_FILE* next) to temp_dir (DIRECTORY* temp_dir).
Note also that first_subdir is declared as 'DIRECTORY* first_subdir;'

Code:

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unsigned int DIRECTORY::printToScreen(bool print_directories = false) {
...
        DIRECTORY *temp_dir = first_subdir;
        while(temp_dir != NULL) {
            counter = counter + temp_dir->printToScreen(print_directories);
            temp_dir = temp_dir->next;
            // 'next' is a RAW_FILE pointer, so this causes a compile error when trying to
            // assign it to temp_dir (DIRECTORY POINTER)
        }
}

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But if, I make temp_dir a RAW_FILE pointer, I cannot call the method printToScreen, because the method doesn't exist in RAW_FILE objects.

Code:

---

unsigned int DIRECTORY::printToScreen(bool print_directories = false) {
...
        RAW_FILE *temp_dir = first_subdir;
        while(temp_dir != NULL) {
            counter = counter + temp_dir->printToScreen(print_directories);
            //printToScreen is a method in the DIRECTORY class, so can't be called from
            //the base class RAW_FILE
            temp_dir = temp_dir->next;
        }
}

---

Code:

---

class RAW_FILE {
protected:
    char* path;
public:
    RAW_FILE *previous, *next;
};

class DIRECTORY: public RAW_FILE {
private:
    DIRECTORY *first_subdir, *last_subdir;
    RAW_FILE *first_file, *last_file;
public:
    unsigned int printToScreen(bool);
};

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If I'm understanding your problem correctly, you want this:

Code:

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temp_dir = dynamic_cast<DIRECTORY*>(temp_dir->next);

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That will downcast RAW_FILE* to DIRECTORY* as long as temp_dir->next actually points to a DIRECTORY. If it doesn't, this will give you a NULL pointer.

Hmm... thanks for the suggestion, but I tried this:

Code:

---

DIRECTORY *temp_dir = first_subdir;
while(temp_dir != NULL) {
    counter = counter + temp_dir->printToScreen(print_directories);
    temp_dir = dynamic_cast<DIRECTORY*>(temp_dir->next);
}

---

and got a compile error:
cannot dynamic_cast `((RAW_FILE*)temp_dir)->RAW_FILE::next' (of type `class RAW_FILE*') to type `class DIRECTORY*' (source type is not polymorphic)

Just to reiterate - RAW_FILE is the base class, DIRECTORY is the derived class.
printToScreen is a method in DIRECTORY, but doesn't exist in RAW_FILE.

temp_dir->next will always point to DIRECTORY objects at the point this code is run: BUT
temp_dir->next has to stay as type RAW_FILE* because in other parts of the code it points to RAW_FILE objects

You aren't using private or protected inheritance, are you?

Could you post the relevant parts of the class definitions?

Quote:

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Originally Posted by AverageSoftware

You aren't using private or protected inheritance, are you?

Could you post the relevant parts of the class definitions?

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I've put the class definitions in the original post.

I did some experiments, and it looks like dynamic_cast will not work if the parent class doesn't contain any virtual functions.

If you write a class that you know will be inherited from, it is good practice to provide a virtual destructor, even if it does nothing. This ensures proper deletion of objects if you delete them from a base class pointer.

So if you add a virtual destructor to RAW_FILE, the dynamic_cast should work, or at least it does on GCC 4.0.