How to Create Threads for C Class Member Functions Invoked from a Vector?

Creating Threads for Class Member Functions

Problem:

How do you create a thread for a class member function when the function is invoked from a vector of class instances?

Example Code and Error:

Consider the following code:

class c {
    void *print(void *) { std::cout << "Hello"; }
};

std::vector<c> classes;
pthread_t t1;

classes.push_back(c());
classes.push_back(c());

// Attempt to create a thread for c.print()
pthread_create(&t1, NULL, &c[0].print, NULL);

// Error: "cannot convert 'void* (tree_item::*)(void*)' to 'void* (*)(void*)'"

Explanation:

The error occurs because C class member functions have an implicit this parameter, which is passed internally. However, pthread_create() does not handle this hidden parameter, causing a type mismatch when casting the member function to a function pointer.

Solution:

There are two approaches to this issue:

1. Use a static class method:

This method does not have a this parameter, as it is associated with the class itself, not an instance. Like so:

class C
{
public:
    static void *hello(void *)
    {
        std::cout << "Hello, world!" << std::endl;
        return 0;
    }

    static void *hello_helper(void *context)
    {
        return ((C *)context)->hello();
    }
};
...
C c;
pthread_t t;
pthread_create(&t, NULL, &C::hello_helper, &c);

2. Use a plain ordinary function:

This method uses a function outside the class definition, which can access the class and its members like so:

// Outside the class
void c_print_wrapper(c *c_instance)
{
    c_instance->print();
}

...
c c1, c2;
pthread_t t1;
classes.push_back(c1);
classes.push_back(c2);

// Create the thread for c.print() using wrapper function
pthread_create(&t1, NULL, (void *(*)(void *))c_print_wrapper, &classes[0]);

The above is the detailed content of How to Create Threads for C Class Member Functions Invoked from a Vector?. For more information, please follow other related articles on the PHP Chinese website!