Could somebody override this callBack()? - class

Question

Usually, callBack() gotta be overridden in Child class.
but it doesn't. When the thread calls callBack(), it runs original method.
Is there any way to right this?
I compiled it with "g++ -o file source.cpp -lpthread"
I'm sure it is not about a compiler.
#include <iostream>
#include <unistd.h>
#include <pthread.h>
using namespace std;
class Parent
{
public:
virtual void callBack()
{
cout << "Original callBack() reported this: " << this << endl;
}
private:
pthread_t th = 0;
static void *th_func(void *arg)
{
Parent *p = (Parent*)arg;
cout << "*th_func() reported *arg: " << arg << endl;
p->callBack();
}
public:
Parent()
{
if(pthread_create(&th, NULL, th_func, (void*)this) < 0)
cerr << "thread not born." << endl;
else
cout << "thread has born." << endl;
}
~Parent()
{
if(th!=0)
pthread_join(th, NULL);
cout << "joined. Parent leaving." << endl;
}
};
class Child : public Parent
{
public:
void callBack()
{
cout << "child overridden." << endl;
}
Child() : Parent(){}
};
int main()
{
Child *ch = new Child();
delete ch;
return 0;
}

Answer 1

The problem with your code is that you are calling the thread function from inside the parent constructor. At that point, the Child object is not constructed yet (look up object initialisation order in C++), thus the only virtual function that it can call is the parent's.
From a C++ point of view, it's doing the right thing :).
In order to get your code to work, you have to separate the thread creation from the object creation, otherwise you'll never be able to call a function in a derived class.
Here's some more info from the C++ FAQ. And here's what Scott Meyers has to say about this topic.