Singleton - multithreaded issue in C++

Question

I've seen posts about this issue but I'm still trying to figure it out. Is this way alright for implementing a safe singelton? I'm using mutex, static member and return its reference.

#include <mutex>
using namespace std;
mutex mtx;

class MySingleton {
private:
    MySingleton();
public:
    MySingleton& getInstance() {
        mtx.lock();
        static MySingleton instance;
        mtx.unlock();
        return instance;
    }
};

How are you going to create the instance you need to call `getInstance()`? — juanchopanza, Mar 06 '16 at 14:18

πάντα ῥεῖ · Answer 1 · 2016-03-06T15:56:31.753

2

Is this way alright for implementing a safe singelton?

It's overshoot. Just get rid of the mutex and write:

static MySingleton& getInstance() {
    static MySingleton instance;
    return instance;
}

The thread safe creation of instance is guaranteed when the function is called the 1st time.

edited Mar 06 '16 at 15:56

answered Mar 06 '16 at 13:53

πάντα ῥεῖ

1
13
116
190

I'm using visual studio 2010, then I should use the double-locking or the mutex? because using the mutex seems to be more convenient and readable than double-locking. thanks – Maor Cohen Mar 06 '16 at 13:57
@ErezShmiel With visual studio 2010, you don't have c++-11 access. Thus use double locking. – πάντα ῥεῖ Mar 06 '16 at 13:58
ok, so just to be sure: using the mutex as I wrote wouldn't be the good way, right? – Maor Cohen Mar 06 '16 at 14:00
1

@ErezShmiel It would have no effect regarding the `static MySingleton instance;` statement. – πάντα ῥεῖ Mar 06 '16 at 14:01
shouldn't this method be declared `static`? – W.K.S Mar 06 '16 at 15:55
@W.K.S Oops, sure of course. – πάντα ῥεῖ Mar 06 '16 at 15:56

Singleton - multithreaded issue in C++

1 Answers1