Question

I’m trying to create a thread in c++ with this code:

pthread_t mythread;
void* f (void*) = MyClass::myfunction;
pthread_create(&mythread, NULL, &f, NULL);

It’s not working. Any idea what’s wrong ?

myfunction is of type:

void* MyClass::myfunction(void* argv);

The error returned is:

error: declaration of ‘void* Class::f(void*)’ has ‘extern’ and is initialized

error: invalid pure specifier (only ‘= 0’ is allowed) before ‘::’ token

error: function ‘void* Class::f(void*)’ is initialized like a variable

Answer 1

You’re declaring f as a function rather than a function pointer. It should be:

void* (*f) (void*) = &MyClass::myfunction;
      ^^^^

pthread_create(&mythread, NULL, f, NULL);
                                ^ no & since it's already a pointer

This will also only work if myfunction is static, since you can’t convert a pointer-to-member-function to a pointer-to-function.

If you do need to the thread to execute a non-static member function on a particular object, then one approach is to write a static wrapper taking the object as an argument:

class MyClass {
public:
    void start_thread() {
        // Pass "this" to the thread we're creating
        pthread_create(&mythread, NULL, &MyClass::thread_entry, this);
    }
private:
    static void * thread_entry(void * object) {
        // Call the member function on the object passed to the thread
        return static_cast<MyClass*>(object)->thread();
    }
    void * thread() {
        // do the interesting stuff, with access to the member variables
    }
};

Of course, these days there’s a standard thread library which removes the need for this dance:

std::thread thread(&MyClass::thread, this);