Robert Osfield wrote:
2009/3/17 Schmidt, Richard <richard.schm...@eads.com
<mailto:richard.schm...@eads.com>>
http://www.cs.wustl.edu/~schmidt/PDF/DC-Locking.pdf
<http://www.cs.wustl.edu/%7Eschmidt/PDF/DC-Locking.pdf>
Could you explain what the above document is all about...
I just read it an it describes a pattern where you use a mutex to guard
access to the singleton's _instance value, but in such a way that the
mutex is only needed when _instance == NULL, i.e.
class Singleton
{
public:
static Singleton *instance (void)
{
// First check
if (instance_ == 0)
{
// Ensure serialization (guard constructor acquires lock_).
Guard<Mutex> guard (lock_);
// Double check.
if (instance_ == 0)
instance_ = new Singleton;
}
return instance_;
// guard destructor releases lock_.
}
private:
static Mutex lock_;
static Singleton *instance_;
};
This should give you thread-safe access to Singleton->instance() at all
times combined with correct initialization.
Quite neat actually,
Paul
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