static thread function access non-static class member in C++

Question

Class Test{
    int value;
    static void* thread_func(void* args){
        value++;
    }
    void newthread(){
        pthread_create(&thread_func,...);
    }
}

I'm trying to create a thread in Class Test. Therefore compiler forces me to make thread_func static. However I cannot access the non-static member "value" anymore. It says:

invalid use of member 'Class::value' in static member function

Is there a way around it?

Answer 1

However I cannot access the non-static member "value" anymore.

That is because static function in your class doesn't have (and cannot have ) this pointer. All you need to pass the pointer to your Test object to pthread_create() function as fourth argument, and then do this:

static void* thread_func(void* args)
{
      Test *test = static_cast<Test*>(args);
      test->value++;
      //write return statement properly
}

---

However, if you're doing too many things in thread_func() that require access to Test class members at many places, then I would suggest this design:

//this design simplifies the syntax to access the class members!
class Test
{
    //code omitted for brevity

    static void* thread_func(void* args)
    {
          Test *test = static_cast<Test*>(args);
          test->run(); //call the member function!
          //write return statement properly
    }
    void run() //define a member function to run the thread!
    { 
          value++;//now you can do this, because it is same as 'this->value++;
          //you do have 'this' pointer here, as usual; 
          //so access other members like 'value++'.
    }
    //code omitted for brevity
  }

---

Better design : define a reusable class!

Even better would be to define a reusable class with pure virtual function run() to be implemented by the derived classes. Here is how it should be designed:

//runnable is reusable class. All thread classes must derive from it! 
class runnable
{
public:
    virtual ~runnable() {}
    static void run_thread(void *args)
    {
        runnable *prunnable = static_cast<runnable*>(args);
        prunnable->run();
    }
protected:
    virtual void run() = 0; //derived class must implement this!
};

class Test : public runnable //derived from runnable!
{
public:
    void newthread()
    {
        //note &runnable::run_thread
        pthread_create(&runnable::run_thread,..., this);
    }
protected:
    void run() //implementing the virtual function!
    {
        value++; // your thread function!
    }
}

Looks better?