static method with polymorphism in c++

Question

I have a weird issue using polymorphism. I have a base class that implements a static method. This method must be static for various reasons. The base class also has a pure virtual method run() that gets implemented by all the extended classes. I need to be able to call run() from the static class.

The problem, of course, is that the static class doesn't have a this pointer. This method can be passed in a void * parameter. I have been trying to come up with a clever way to pass the run method into it, but nothing has worked so far. have also tried passing this into it. The problem with this is that I would then have to instantiate it, which requires knowledge of the extended class. This defeats the whole purpose of the polymorphism.

Any ideas on how to go about this?

I think you mean "static method" last in the first paragraph? — unwind, Oct 09 '09 at 14:44
Ori, you object to passing `this` to the static method because "I would then have to instantiate it," but you want to call the virtual `run` method. You can't call a virtual method without an instance, so you would have to instantiate the class anyway. The static method doesn't need to know *which* class was instantiated; it's just receiving a base-class pointer, as Chris's answer demonstrates. — Rob Kennedy, Oct 09 '09 at 15:13

score 10 · Accepted Answer · answered Oct 09 '09 at 14:46

10

Don't pass it as a void* pointer, pass it as a pointer (or reference) to the base class:

class BaseClass
{
public:
  static void something(BaseClass* self) { self->foo(); }
  virtual void foo() = 0;  
};

answered Oct 09 '09 at 14:46

ChrisW

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sbi · Answer 2 · 2011-03-29T15:17:52.800

5

This usually happens when you have to squirrel your C++ objects through a C API. A classic example is a thread class.

Here's the standard idiom for doing this:

/** calls thread_func in a new thread passing it user_data as argument */
thrd_hdl_t c_api_thread_start(int (*thread_func)(void*), void* user_data);

/** abstract thread base class
* override my_thread::run to do work in another thread
*/
class my_thread {
  public:
    my_thread() hdl_(c_api_thread_start(my_thread::thread_runner,this)) {}
    // ...

  private:
    virtual int run() = 0; // we don't want this to be called from others

    thrd_hdl_t hdl_; // whatever the C threading API uses as a thread handle

    static int thread_runner(void* user_data)
    {
      my_thread* that = static_cast<my_thread*>(user_data);
      try {
        return that->run();
      } catch(...) {
        return oh_my_an_unknown_error;
      }
    }
};

Would that help?

edited Mar 29 '11 at 15:17

answered Oct 09 '09 at 15:06

sbi

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nice snippet, but isn't it better to use `static_cast` to cast from `void*` to another pointer? – Eli Bendersky Mar 29 '11 at 07:09
@Eli: Oops. Can you do that? I didn't know. If you're sure, I'll certainly change it. (Feel free make the change yourself.) – sbi Mar 29 '11 at 09:25
you certainly *can* do that (since `void*` is statically cast-able to any pointer). But whether you want to is really a matter of good practice. See http://stackoverflow.com/questions/310451/should-i-use-static-cast-or-reinterpret-cast-when-casting-a-void-to-whatever Since it's your answer I'll let you decide :) – Eli Bendersky Mar 29 '11 at 13:42
@Eli: Thanks, that's an interesting discussion. FWIW, I changed my answer. Thank you for pointing out this issue! – sbi Mar 29 '11 at 15:17

score 3 · Answer 3 · answered Oct 09 '09 at 14:46

3

Why not pass a reference to the object in rather than the method e.g.

static void func( BaseObject& o)
{
     o.run();
}

answered Oct 09 '09 at 14:46

mmmmmm

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score 1 · Answer 4 · answered Oct 09 '09 at 14:51

1

IMO, your best bet is to get rid of the static method. Find a way around that and you're golden.

answered Oct 09 '09 at 14:51

Randolpho

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static method with polymorphism in c++

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