C++ can't access field from inherited class - c++

Question

Hello guys a have a problem, that i can't access field tablica[i]->help, in generuj function, its saying that this field is not existing in class Task.
How can i achieve it ?
class Task
{
protected:
string contents;
int id_pyt;
int nr_pyt;
};
class Task4Answ : public Task
{
private:
int help;
public:
Task4Answ(string contents1, int id,int nr,int help1)
{
contents=contents1;
id_pyt=id;
nr_pyt=nr;
help=help1;
}
};
class TaskCollection
{
protected:
Task *collection[60];
public:
friend class Generator;
TaskCollection()
{
collection[0] = new Task4Answ("Ile jest por roku w Polsce? \na) 1 \nb) 2 \nc) 3 \nd) 4",1,0);
collection[1] = new Task4Answ("Kto wygral tegoroczny Roland Garros? \na) Federer \nb) Djokovic \nc) Nadal \nd) Thiem",1,1);
class Generator
{
protected:
Task *tablica[10];
TaskCollection T1;
public:
Generator(){}
void Generuj()
{
if(T1.collection[x]->id_pyt==1)
{
tablica[i]=new Task4Answ("0",0,0);
tablica[i]->contents=T1.collection[x]->contents;
tablica[i]->id_pyt=T1.collection[x]->id_pyt;
tablica[i]->nr_pyt=T1.collection[x]->nr_pyt;
tablica[i]->help=T1.collection[x]->help; //here is the problem
}
}
}
Or maybe there is some other solution of the project im doing now.
Thanks for any help.

Answer 1

The problem is in this line:
tablica[i]=new Task4Answ("0",0,0);
Although you have called the Task4Answ constructor, you are also assigning the memory address returned by new to a Task pointer. Effectively, you have casted the Task4Answ pointer to a Task pointer. On the lines that follow, C++ only sees tablica[i] as a reference to a Task pointer. You need to change:
protected:
Task *tablica[10];
TaskCollection T1;
...to this:
protected:
Task4Answ *tablica[10]; // Task was changed to Task4Answ
TaskCollection T1;
That should allow C++ to see tablica as an array of Task4Answ pointers instead of Task pointers.
Edit: it looks like help is also private. You will have to change help to public or add TaskCollection::TaskCollection() as a friend. Otherwise, C++ will not let you get or set help.
Edit: the OP added that tablica[i] might contain instances of other classes that inherit from Task. In that case, you could do something like this:
void Generuj()
{
if(T1.collection[x]->id_pyt==1)
{
Task4Answ* newTask = new Task4Answ("0",0,0);
newTask->contents=T1.collection[x]->contents;
newTask->id_pyt=T1.collection[x]->id_pyt;
newTask->nr_pyt=T1.collection[x]->nr_pyt;
newTask->help=T1.collection[x]->help; // You will still have to change this from being private.
tablica[i] = newTask;
}
}
}
Later on, in order to access help, you will need to implement some sort of way of checking whether tablica[i] is a Task4Answ and not an instance of some other class that inherits from Task, perhaps by implementing a method in Task named IsTask4Answ that returns false in Task but is overridden to return True in Task4Answ. You can then cast the pointer back to Task4Answ with something like the static_cast operator. In other words:
// Add these functions to the class definitions:
virtual bool Task::IsTask4Answ() const {
return false;
}
bool Task4Answ::IsTask4Answ() const override {
return true;
}
// Later, you can do this:
if(tablica[i].IsTask4Answ()){
Task4Answ* t = static_cast<Task4Answ*>(tablica[i]);
t->help; // Again, you'll have to change this from being private.
}
Although I suggest figuring out a different data structure where you do not need to do any casting, this will allow you to access help.
Do note the virtual keyword in the first function above; it allows the function to be dynamically bound, which means that the code will check whether to call Task::IsTask4Answ() or Task4Answ::IsTask4Answ() at runtime instead of at compile time.