多线程：写入后首先从另一个线程进行访问...我需要使用volatile吗？

Question

Hope the question is easy to understand. In any case, let's write some code...

struct MyClass 
{
    int a;
    std::vector<char> b;
    ...
}

MyClass object;

Let's say I am running two threads A and B:

1. Thread A creates object and reads or writes some data members except a and b.
2. Thread A passes a pointer to object to a function that will run in thread B.
3. Thread B writes a or adds data to vector b.
4. Thread A reads a and b (ie, access these for the first time).

If the answer is yes, ie, that I need volatile in this case, I have another question: why is it thread safe to use immutable objects where data members are written in one thread and read in many others? It seems very similar to this case :)

Answer 1

why is it thread safe to use immutable objects where data members are written in one thread and read in many others?

Immutable objects are created and then never change after that point. Since multithreaded access to an object is only possible after it is created (what is there to share before then) every thread will always see the same value.

So there is never a case where an immutable object will appear differently on separate threads, since there is no way for an immutable object can change state after it has been shared.