CPP Concurrency & Multithreading 5

Question 1

What is the purpose of `std::barrier` introduced in C++20?

Accepted Answer

To synchronize a fixed number of threads at a checkpoint before any can proceed past it

Answer

`std::barrier` synchronizes a set of threads at a reusable synchronization point; all participating threads must arrive before any can continue, and it resets for the next phase.

Question 2

What distinguishes `std::latch` from `std::barrier` in C++20?

Accepted Answer

`std::latch` is a single-use countdown that threads can wait on; `std::barrier` is reusable across multiple phases

Answer

`std::latch` is a one-shot downward counter — once it reaches zero it stays open; `std::barrier` resets after each synchronization phase, making it suitable for iterative parallel work.

Question 3

What is a semaphore, and which C++20 class provides one?

Accepted Answer

A counting synchronization primitive; provided by `std::counting_semaphore`

Answer

A semaphore is a counting synchronization primitive; C++20 provides `std::counting_semaphore<N>` (and `std::binary_semaphore` as a specialization with max count 1).

Question 4

What does `std::memory_order_relaxed` guarantee?

Accepted Answer

Only atomicity of the operation itself with no synchronization or ordering constraints

Answer

`memory_order_relaxed` provides only atomicity — the operation is indivisible — but places no ordering constraints relative to other memory operations in the same or other threads.

Question 5

In a producer-consumer pattern using `std::atomic<bool>`, why might `memory_order_release` on the store and `memory_order_acquire` on the load be preferred over `memory_order_seq_cst`?

Accepted Answer

Release-acquire is sufficient to establish happens-before and avoids the global ordering overhead of seq_cst

Answer

A release store paired with an acquire load establishes the necessary happens-before relationship at lower cost than `seq_cst`, which requires a global total order across all threads.

(CPP) C++ Programming Professional Certificate Practice Test

(CPP) C++ Programming Professional Certificate Practice Test

CPP Concurrency & Multithreading 5