run threads of class member function in c++

Question

As the title says. The following is my code skeleton.

class CLASS
{
public:
    void A();
private:
    DWORD WINAPI B(LPVOID);
};

void CLASS::A()
{
    DWORD (WINAPI CLASS::*thread)(LPVOID) = &CLASS::B;
    ...
    CreateThread(NULL, 0, thread, &arg, 0, NULL);
    ...
}

the function B needs CLASS's member variables.
But I've got an error code when I compiled this.
It's " can't convert argument 3 from 'DWORD (__stdcall CLASS::* )(LPVOID)' to 'LPTHREAD_START_ROUTINE' " or something like that.
I don't know if it is the same in the english environment.

can anybody help please?

Answer 1

Seriously, use std::thread:

class CLASS
{
public:
    void A();
private:
    void B(your args go here);
};

void CLASS::A()
{
    std::thread t(&CLASS::B, this, your args go here);
    // when done
    t.join();
    // or
    t.detach();
}

Explanation

Your code doesn't compile because CreateThread is a C API, expecting a non-member C function as a callback. In order to call a non-static member function of a C++ class, the caller needs to know about this pointer and how to correctly use it. Since WinAPI clearly doesn't have this and doesn't expect a member function pointer, this cannot possibly work. Your compiler catches this and reports it as a type mismatch error.

This is where the lpParameter of CreateThread comes into play. It allows you to pass this through that parameter. However it doesn't change the fact that CreateThread doesn't know how to call C++ member functions. Therefore it's your responsibility to wrap it in a static function that will do the member function call:

class CLASS
{
public:
    void A();
private:
    DWORD B();
    static DWORD s_B(LPVOID);
};

DWORD CLASS::s_B(LPVOID that) {
    return ((CLASS*)that)->B();
}

void CLASS::A() {
    CreateThread(NULL, 0, s_B, this, 0, NULL);
}

This is what @Nawaz does in their answer, except in a slightly more general way.

Even though this approach works, it clearly has downsides:

• It's verbose.
• It's non portable.
• You need to pass your original args through different means (e.g. through a member of your class).

std::thread already does all that for you.