此函数（运算符重载）线程安全吗？

Question

Here's the code:

ElementType& operator[] (int key)
{
    //something like boost::mutex::scoped_lock
    MutexLockType lock();

    if(key < 0 || key > m_bound)
        throw std::range_error("access out of bound");

    return m_elements[key];
}

Answer 1

No it is not because you have allowed a reference the element to leak out of the function and away from the safety of the lock.

What's more, thread-safety questions are a little hard to answer without a harder specification of just what flavour of thread-safety you are desirest. At the very least you would need to show us every other access of m_elements and m_bound .

Answer 2

In this exact example you would be even more surprised when you realize that there is no lock at all, just a declaration of a lock() function that returns a LockType.

Not that that it would have helped with a lock anyway.