It's unclear¹ whether static zero-initialization of a static QBasicAtomic actually initializes the object with a zero value, or whether it remains default-constructed (without a value until a following call to std::atomic_init gives it one). Play it safe and use Q_BASIC_ATOMIC_INITIALIZER(0) to dodge any issues the old code may have had. ¹ see ongoing discussion on the Jira ticket Amends 04d6495bf773a6bb0d4fa6980df22d3b81a605b0. Task-number: QTBUG-137465 Pick-to: 6.10 6.9 6.8 6.5 Change-Id: Iee0d75ce10a589390afdd9069b7040a1c9c608e1 Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>