Singleton with multithreads - c++

Question

This question was asked in an interview. The first part was to write the singleton class:
class Singleton
{
static Singleton *singletonInstance;
Singleton() {}
public:
static Singleton* getSingletonInstance()
{
if(singletonInstance == null)
{
singletonInstance = new Singleton();
}
return singletonInstance;
}
};
Then I was asked how to handle this getSingletonInstance() in a multithreaded situation. I wasn't really sure, but I modified as:
class Singleton
{
static Singleton *singletonInstance;
Singleton() {}
static mutex m_;
public:
static Singleton* getSingletonInstance()
{
m_pend();
if(singletonInstance == null)
{
singletonInstance = new Singleton();
}
return singletonInstance;
}
static void releaseSingleton()
{
m_post();
}
};
Then I was told that although a mutex is required, pending and posting a mutex is not efficient as it takes time. And there is a better way to handle to this situation.
Does anybody know a better and more efficient way to handle the singleton class in a multithreaded situation?

Answer 1

In C++11, the following is guaranteed to perform thread-safe initialisation:
static Singleton* getSingletonInstance()
{
static Singleton instance;
return &instance;
}
In C++03, a common approach was to use double-checked locking; checking a flag (or the pointer itself) to see if the object might be uninitialised, and only locking the mutex if it might be. This requires some kind of non-standard way of atomically reading the pointer (or an associated boolean flag); many implementations incorrectly use a plain pointer or bool, with no guarantee that changes on one processor are visible on others. The code might look something like this, although I've almost certainly got something wrong:
static Singleton* getSingletonInstance()
{
if (!atomic_read(singletonInstance)) {
mutex_lock lock(mutex);
if (!atomic_read(singletonInstance)) {
atomic_write(singletonInstance, new Singleton);
}
}
return singletonInstance;
}
This is quite tricky to get right, so I suggest that you don't bother. In C++11, you could use standard atomic and mutex types, if for some reason you want to keep the dynamic allocation of you example.
Note that I'm only talking about synchronised initialisation, not synchronised access to the object (which your version provides by locking the mutex in the accessor, and releasing it later via a separate function). If you need the lock to safely access the object itself, then you obviously can't avoid locking on every access.

Answer 2

As #piokuc suggested, you can also use a once function here. If you have C++11:
#include <mutex>
static void init_singleton() {
singletonInstance = new Singleton;
}
static std::once_flag singleton_flag;
Singleton* getSingletonInstance() {
std::call_once(singleton_flag, init_singleton);
return singletonInstance;
}
And, yes, this will work sensibly if the new Singleton throws an exception.

Answer 3

If you have C++11 you can make singletonInstance an atomic variable, then use a double-checked lock:
if (singletonInstance == NULL) {
lock the mutex
if (singletonInstance == NULL) {
singletonInstance = new Singleton;
}
unlock the mutex
}
return singletonInstance;

Answer 4

You should actually lock the singleton, and not the instance. If the instance requires locking, that should be handled by the caller (or perhaps by the instance itself, depending on what kind of an interface it exposes)
Update sample code:
#include <mutex>
class Singleton
{
static Singleton *singletonInstance;
Singleton() {}
static std::mutex m_;
public:
static Singleton* getSingletonInstance()
{
std::lock_guard<std::mutex> lock(m_);
if(singletonInstance == nullptr)
{
singletonInstance = new Singleton();
}
return singletonInstance;
}
}

Answer 5

If you use POSIX threads you can use pthread_once_t and pthread_key_t stuff, this way you can avoid using mutexes altogether. For example:
template<class T> class ThreadSingleton : private NonCopyable {
public:
ThreadSingleton();
~ThreadSingleton();
static T& instance();
private:
ThreadSingleton( const ThreadSingleton& );
const ThreadSingleton& operator=( const ThreadSingleton& )
static pthread_once_t once_;
static pthread_key_t key_;
static void init(void);
static void cleanUp(void*);
};
And implementation:
template<class T> pthread_once_t ThreadSingleton<T>::once_ = PTHREAD_ONCE_INIT;
template<class T> pthread_key_t ThreadSingleton<T>::key_;
template<class T>
T& ThreadSingleton<T>::instance()
{
pthread_once(&once_,init);
T* value = (T*)pthread_getspecific(key_);
if(!value)
{
value = new T();
pthread_setspecific(key_,value);
}
return *value;
}
template<class T> void ThreadSingleton<T>::cleanUp(void* data)
{
delete (T*)data;
pthread_setspecific(key_,0);
}
template<class T> void ThreadSingleton<T>::init()
{
pthread_key_create(&key_,cleanUp);
}