Multithreading: do I need protect my variable in read-only method?

Question

I have few questions about using lock to protect my shared data structure. I am using C/C++/ObjC/Objc++

For example I have a counter class that used in multi-thread

Answer 1

Yes, unless you can guarantee that changes to the underlying variable counter are atomic, you need the mutex.

Classic example, say counter is a two-byte value that's incremented in (non-atomic) stages:

(a) add 1 to lower byte
    if lower byte is 0:
(b)     add 1 to upper byte

and the initial value is 255.

If another thread comes in anywhere between the lower byte change a and the upper byte change b, it will read 0 rather than the correct 255 (pre-increment) or 256 (post-increment).

In terms of what data types are atomic, the latest C++ standard defines them in the header.

If you don't have C++11 capabilities, then it's down to the implementation what types are atomic.