What is thread_local storage?

Question

Accepted Answer

**thread_local** — variable has separate copy per thread.

```cpp
thread_local int counter = 0;

void increment() {
    counter++; // each thread has its own counter
}

std::thread t1(increment);
std::thread t2(increment);
t1.join(); t2.join();
// counter is still 0 in main thread
```

**Use cases:**
- Thread-local caches
- Per-thread error state
- Random number generators
-避免false sharing

**Lifetime:**
- Created when thread starts
- Destroyed when thread exits
- Destructors called in reverse order

**Warning:**
- Don't use with `std::shared_ptr` across threads
- Can cause memory leaks if thread pool is used

C++: What is thread_local storage?