Is it necessary to lock an array that is *only written to* from one thread and *only read from* another?

Question

I have two threads running. They share an array. One of the threads adds new elements to the array (and removes them) and the other uses this array (read operations only).

Answer 1

If you use e.g. vector for your array (so that it can dynamically grow), then reallocation may occur during the writes, you lose.

If you use data entries larger than is always written and read atomically (virtually any complex data type), you lose.

If the compiler / optimizer decides to keep certain things in registers (such as the counter holding the number of valid entries in the array) during some operations, you lose.

Or even if the compiler / optimizer decides to switch order of execution for your array element assignments and counter increments/decrements, you lose.

So you certianly do need some sort of synchronization. What is the best way to do so (for example it may be worth while to lock only parts of the array), depends on your specifics (how often and in what pattern do the threads access the array).