Synchronization coordinates operations across threads when one thread must wait for an event, a condition, or another thread’s task to finish. It avoids repeatedly checking shared flags and supports ordering or observing effects across threads. section 4.0
C++ offers several coordination facilities and broader ways to structure communication between tasks:
- Condition variables, futures, promises, packaged tasks, latches, and barriers help coordinate operations. section 4.5
- Functional-style programming, message passing, and continuations provide ways to organize task dependencies and communication. section 4.5
std::for_each(std::execution::par,v.begin(),v.end(),
[&](int& x){ x=++count; });
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This parallel for_each example updates a shared count, causing a data race and undefined behavior. Making count atomic or protecting it with a mutex restores defined behavior, but may serialize calls and undermine parallelism. section 10.2.3
Parallel-unsequenced algorithms impose a different constraint: their operations must not use synchronization or modify state shared between threads or elements. section 10.2.4
