call derived method from base class

Hi All,

I'm having troubles achieving something, I need to call an overriden method from the base class as follow :

Code:

---

class Base{
        public:
                void start(){
                        new thread(boost::bind(&Base::run, this));
                }
               
                virtual void go(){
                        // do something
                }
 
}
 
 
class Derived : public Base {
        public:
                void go(){
                        //do something different
                }
}
 
 
int main(int argc, char** argv){
        Derived* der = new Derived();
        der->start();
 
        return 0;
}

---

I thought this way the binding is done at runtime, and the method of the base class is called but this is not what I'm getting !!

Just to discard the boost::bind, I tried this as well :

Code:

---

class Base{
        public:
                void start(){
                        new thread(boost::ref(*this));
                }
               
                virtual void go(){
                        //do something.
                }
               
                void operator()(){
                        this->go();
                }
 
}
 
 
class Derived : public Base {
        public:
                void go(){
                        //do something different
                }
}
 
 
int main(int argc, char** argv){
        Derived* der = new Derived();
        der->start();
 
        return 0;
}

---

OS : windows
COMPILER : gcc

am I doing something wrong ?

thanks in advance.

BR

I see no definition of Base::run?

sorry I updated the post, it's actually Base::go()

ok, now it's driving me crazy, if I do :

Code:

---

class Base{
        public:
                void start(){
                        new thread(boost::bind(&Base::go, this));
                }
               
                virtual void go(){
                        // do something
                }

                string print(){
                      return "something";
                }
 
}
 
 
class Derived : public Base {
        public:
                void go(){
                        //do something different
                }
}
 
 
int main(int argc, char** argv){
        Derived* der = new Derived();
        der->start();
        std::cout << der->print();
 
        return 0;
}

---

it actually works, but if I remove the std::cout << der->print(); from the main, it will not !!!

any guru that can explain this ? is this a bug in gcc ?

When you write "&Base::run", you are making a direct call to Base::run, bypassing the entire virtual mechanism, and calling BAse::run regardless of the type of this. Use the "non virtual interface calls a virtual implementation" design:

Code:

---

class Base{
    public:
        void start(){
            new thread(boost::bind(&Base::run, this));
        }

        void run(){
            //run is called directly
            this->go(); //now go is called virtually
        }

        string print(){
          return "something";
        }
       
    private: //yes private
        virtual void go(){
            // do something
        }
 
}
 
 
class Derived : public Base {
    private: //yes private
        void go(){
            //do something different
        }
}
 
 
int main(int argc, char** argv){
    Derived* der = new Derived();
    der->start();
        std::cout << der->print();
 
    return 0;
}

---

[/code]

Hi monarch_dodra,

actually it's working, even with the first version I posted, the problem comes from using boost::thread or boost::bind, if I remove it and replace it with a simple this->run, it'll work.

now what I really want to konw, is why does it work normally when I add the call to print in the main ? do you have an idea ?

thanks again

Because in one situation the virtual function call mechanism is suppressed, and in the other it is not. Simple.

Hello Lindley,

how is that happening, is there a rule ?

it doesn't make sense, since the class definition is the same in both situations, the difference is how I'm using the class (if I add a call to the print member method, it will work, if I remove the call, it simply won't !!)

If you call the method via a function pointer or class method pointer, you lose polymorphism. That's just the way it works.

The way around this is to instead call another function which in turn calls the specified method "normally", so that the virtual function call dispatch can work. This is what monarch_dodra was demonstrating above.

Quote:

---

Originally Posted by Lindley

If you call the method via a function pointer or class method pointer, you lose polymorphism. That's just the way it works.

---

that's not true; member function pointers preserve polymorphism. For example,

Code:

---

struct Base
{
        virtual void run() { std::cout << "1" << std::endl; }
};

struct Derived: Base
{
        virtual void run() { std::cout << "2" << std::endl; }
};

int main()
{
        typedef void(Base::*FunctionType)();

        FunctionType ptr = &Base::run;
        Base b;
        Derived d;

        (b.*ptr)(); // prints 1
        (d.*ptr)(); // prints 2
}

---

that said, the OP has provided only bogus non compiling code, so it's difficult to say more ...

hi superbonzo,

the code from post 4 compiles fine, just include boost::thread and boost::bind,

my question is : what is happening when I include der->print() in the main that makes it work (even with boost::thread) !!

ps : the problem comes from boost::thread, if I remove it the virtual mechanism works just fine

BR

Quote:

---

Originally Posted by let_me_in

the code from post 4 compiles fine, just include boost::thread and boost::bind,

---

Well, quite aside from the main question, there are two problems with post #4's code which should be fixed:

1) You use new without a corresponding delete. This is never acceptable even in the most trivial of test programs....especially since the relevant functionality could be tested without using new at all, by just putting "der" on the stack.

2) You do not wait for the thread to exit before reaching the end of main(), so you are not guaranteed that it will ever run before the program finishes.

Hi Lindley,

in my real code, I've those two problems fixed, I just ommited them to shorten the code a little bit.

Quote:

---

Originally Posted by let_me_in

in my real code, I've those two problems fixed, I just ommited them to shorten the code a little bit.

---

and how are we supposed to see what happens in the code you've not shown us ? post the minimal complete correct code that reproduce the problem, along with your observed and expected program behavior ...

Hello all,

here is a code that reproduces the error :

Code:

---

#include <boost/thread.hpp>
#include <boost/bind.hpp>
#include <iostream>

class Base1{
    public:

        virtual ~Base1(){myThread->join();}
        void start(){
            myThread = new boost::thread(boost::bind(&Base1::go, this));
        }

        virtual void go(){
            std::cout<<"Base"<<std::endl;
        }


        boost::thread* myThread;



};


class Derived1 : public Base1 {
    public:
        void go(){
            std::cout<<"Derived"<<std::endl;
        }
};


int main(int argc, char** argv){
    Derived1* der = new Derived1();
    der->start();

    delete der;
    //der->myThread->join();

    return 0;
}

---

now I know where the problem is, it's in the destructor, if you remove the "delete der;" from the main and replace it with "der->myThread->join();" it will work.

hope someone can tell me why

BR

I've not run your code on my machine, but the reason should be that the Base dtor is invoked after the Derived1 dtor. Therefore, the call to go() does not invoke Derived1::go. The same thing happens in the opposit direction when you try to invoke a virtual function in the ctor of the base. Indeed, try declaring Base1::go as pure virtual: you should see a pure virtual call exception in that case.

EDIT: FYI, note that if you changed the code to something like

Code:

---

int main(int argc, char** argv){
    Derived1* der = new Derived1();
    der->start();

    std::cin.get();

    delete der;

    return 0;
}

---

it will appear to "work" again, because it's highly probable that go() gets invoked before der is deleted.

ok thanks all for your help,

and thanks superbonzo, it's just as you said