boost::uuids::random_generator and uniqueness with multiple threads

Question

When I generate the random number with single thread, no duplicate in 4M uuids generated but if I generate with two threads each 1M, I see roughly 16-20 duplicates. What could

Answer 1

This is a common error when using RAII locks: you forgot to give your lock a name in the line

      boost::mutex::scoped_lock(m_mRandomGen);

so it didn't lock anything at all. Change it to

      boost::mutex::scoped_lock lk(m_mRandonGen); // note the typo in mutex name

EDIT: what really happened: There was no compiler error despite the typo in the mutex name because the declaration

type(name);

is the same as

type name;

if the name has not been declared before. In other words, you've default-constructed a new scoped_lock called m_mRandomGen, not associated with a mutex.