CODEWITHSUNDEEP
c++design-patternsabstract-classfunction-pointers
Josef
Josef

Reputation: 353

Pure Virtual Method VS. Function Pointer

Recently I've been designing a Thread class library, I've made a Thread abstract class like the following:

class Thread {
public:
    run() { /*start the thread*/ }
    kill() { /*stop the thread*/ }
protected:
    virtual int doOperation(unsigned int, void *) = 0;
};

Real thread classes would inherit this abstract class and implement doOperation method in its own logic, something similar to Strategy Pattern.

The problem is that I'm relying on a C back-end library which defines running the thread in the following function:

int startThread(char* name, (int)(*)(unsigned int, void*), int, int, int, void*);

As you can see; the second parameter is a function pointer to thread's loop (main function), and here is the problem; since I use this C-function to start the thread in the run method, I pass the address of doOperation to the second parameter, and this cannot be done, because of type mismatch.

I've tried to use reinterpret_cast to return a pointer, but I ISO-C++ forbids returning a pointer of un-initialized function member. I don't know how to overcome this conflict, using a static method is the only solution I guess, but it blows up my designing pattern!

Upvotes: 2

Views: 1832

Answers (1)

user51568
user51568

Reputation: 2643

First, be sure to read the link Michael Burr provided, as it contains good information. Then, here is C++ish pseudo-code for it:

int wrapperDoOperation(int v, void *ctx)
{
    Thread *thread = (Thread *)ctx;
    return thread->doOperation(v);
}

class Thread {
public:
    run() {
         startThread("bla", wrapperDoOperation, bla, bla, bla, (void *)this);
    }
    kill() { /*stop the thread*/ }
protected:
    virtual int doOperation(unsigned int) = 0;

friend wrapperDoOperation ......;
};

The idea is that doOperation, being a member function of Thread, doesn't need a void *context, you can just keep whatever you would pass as a context in the object itself. Therefore, you can use the void pointer to pass the actuall this pointer to the doOperation. Notice that the void * details are hidden from the users of your class, which is nice.

Upvotes: 7

Related Questions