构造执行派生类的成员函数的线程

Question

I have two classes Base and Derived inheriting from each other. In Base I want to create a thread executing the member function Handle of the class ( TThread is MT library of ROOT). I want to override this handle function in Derived , but my program always executes the function from the base class rather than the one from the derived class. How can I change it so that the overridden Handle is executed instead?

Here is the code:

#include "TThread.h"
#include <iostream>

using namespace std;

class Base
{
public:
    Base()
    {
        thread = new TThread("BaseClass", (void(*)(void*))&Handle,(void*)this);
        thread->Run();
    }
private:
    TThread *thread;
    static void* Handle(void *arg)
    {
        cout<<"AAAA"<<endl;
    }
};

class Derived : public Base
{
public:
    Derived() : Base(){}
private:    
    static void* Handle(void *arg)
    {
        cout<<"BBBB"<<endl;
    }
};


int main()
{   
    Derived *b = new Derived();

    return 0;
}

Answer 1

You are trying to achieve polymorphism with on a non- virtual function.

The reference to Handle in your base class constructor gets resolved at compile time to always point to Base::Handle , no matter what the concrete type of the object at runtime will be. This can be fixed by changing Handle from a static to a virtual function.

The other problem is that you are trying to create the thread from the base class constructor. The derived object has not been fully constructed at this point, so you cannot polymorphically dispatch to Derived::Handle , even if you change it to a virtual function. A quick solution for this would be to move the thread construction to a Base::startThread() method and call that after the constructor has returned.

Answer 2

Make Handle virtual as @ComicSansMS says , and introduce a static member function to handle the virtual dispatch correctly:

#include "TThread.h"
#include <iostream>

using namespace std;

class Base
{
public:
    Base() : thread() {}
    ~Base() { wait(); }

    void wait() {
        if (thread)
        {
            thread->Join();
            delete thread;
            thread = NULL;
        }
    }

    void start()
    {
        thread = new TThread("BaseClass", &Dispatch, this);
        thread->Run();
    }

private:
    TThread *thread;

    virtual void Handle()
    {
        cout<<"AAAA"<<endl;
    }

    static void* Dispatch(void *arg)
    {
        static_cast<Base*>(arg)->Handle();
        return NULL;
    }
};

class Derived : public Base
{
public:
    Derived() { start(); }
    ~Derived() { wait(); }
private:
    virtual void Handle()
    {
        cout<<"BBBB"<<endl;
    }
};


int main()
{   
    Derived b;
}