How Can I Safely Call C Class Member Functions from Threads?

Calling Class Member Functions in Threads Safely

In the realm of multithreading, it's common to encounter the need to invoke member functions of a class within the thread's execution. However, this task can present challenges due to the hidden "this" parameter in C class member functions.

For instance, consider the following:

class C {
    void *print(void *) { cout << "Hello"; }
};

Now, let's create a vector of C instances:

vector<C> classes;
pthread_t t1;
classes.push_back(C());
classes.push_back(C());

To create a thread that executes c.print(), you might intuitively write:

pthread_create(&t1, NULL, &c[0].print, NULL);

However, this will result in an error:

cannot convert ‘void* (tree_item::*)(void*)’ to ‘void* (*)(void*)’ 

The issue arises because pthread_create() expects a function pointer of a specific type, whereas c[0].print is a pointer to a member function with a hidden "this" parameter.

The Solution: Using a Static Class Method or a Function Pointer

To overcome this hurdle, you have two options:

Static Class Method

A static class method does not have a "this" parameter and can be called directly without an instance of the class. Here's how you could implement a static class method for the hello function:

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

Then, you can create a thread using the static class method:

pthread_create(&t, NULL, &C::hello, NULL);

Function Pointer

Another option is to use a function pointer that encapsulates the member function and provides the "this" parameter explicitly. This function pointer can then be used to create a thread.

C c;
pthread_create(&t, NULL, &C::hello_helper, &c);

where hello_helper is defined as:

void *hello_helper(void *context) { return ((C *)context)->hello(); }

By using a static class method or a function pointer, you can safely invoke member functions of a class within a thread, avoiding the "this" parameter issue.

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