如果只有一个线程修改 std::vector，那么同一个线程从 vector 读取时是否需要使用锁？

Question

As I understand a data race can only occur with a std::vector when one or more threads are modifying a vector. If all threads are simply reading that is thread-safe.

Now assume I have 2 threads. One which can only read from the vector, but the other reads and modifies the size of the vector. I have a lock to remain thread-safe to the vector.

When the read-only thread reads from the vector it must use the lock since it does not know what the other thread is doing at the same time.

However, the read and write thread knows that the read-only thread is read-only and thus when it is reading the other thread can only be reading or not. This means that a data-race is impossible in that case and it does not have to use the lock.

The read and write thread must still use the lock when modifying the vector.

Is that right?

Answer 1

You are correct. If your single writer thread is in read mode, then a lock does not need to happen because the other thread is only a reader. Readers can't conflict with each other. The read only thread will still need to acquire the lock on every read, and the writing thread will need to lock when it writes, but it doesn't need to lock to read.