将成员函数的指针转换为C函数的指针是一种好习惯吗

Question

This code compiles (with g++ (GCC) 6.3.1) and appears to "work". But I'm VERY skeptical as to whether it constitutes good practice .

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>


class ThreadObj
{
  public:
    void *memberFunction(void*);
    void startThread(void);
  static int w;
};

int ThreadObj::w = 13;

void* ThreadObj::memberFunction(void*)
{
  w = 17;
  return 0;
}

void  ThreadObj::startThread(void)
{
  void * (*p)(void *) = *((void * (*)(void *))&ThreadObj::memberFunction);
  pthread_t    tid;

  if (pthread_create(&tid, 0, p, this) == 0)
    pthread_detach(tid);
}

int main(int argc, char *argv[] )
{
  ThreadObj thr;
  thr.startThread();

  return 0;
}

Casting a pointer to a member function as a pointer to a C function and passing this as data seems to rely (TOO MUCH) upon the mercies of how the compiler chooses to interpret/compile this.

This has been covered before but not definitively answered.

How can this example be broken?

Answer 1

The old way (with recent syntax) to go were:

class ThreadObj
{
public:
    void memberFunction() { w = 17;}
    void startThread() {
        pthread_t    tid;

        if (pthread_create(&tid, 0, &Func, this) == 0) {
            pthread_detach(tid);
        }
    }
    static void* Func(void* p) {
        auto* that = static_cast<ThreadObj*>(p);
        that->memberFunction();
        return nullptr;
    }
private:
    int w = 13;
};

But in c++11, std::thread has a better interface.

Answer 2

Not really. It's undefined behavior. It is rather cute though. Whether it works or not is kind of up in the air. My guess is that it generally would.

The author of the code is taking advantage of the fact that this is generally passed in as the first parameter to the implementation of member functions.

Answer 3

Using pointer to methods of a classe is very usefull and, for me, is a good practice because it allows you implements another kind of patterns, for example: trigger a event over severas class that has the same prototype of method.

Answering your question: It is a matter of designe and how it will be usefull for your goal.

I share you a code where i create tow thread over diferents methods of the same class, i hope you it will help you.

https://github.com/jorgemedra/C-11-Threads/blob/master/README.md