How to understand double checked locking?

The simplest problem scenario is in the case where the intialization of some_resource doesn't depend on resource_ptr . In that case, the compiler is free to assign a value to resource_ptr before it fully constructs some_resource .

In this case (depending on the implementation of . Reset and!) there may be a problem when Thread 1 gets part-way through initializing resource_ptr and then gets paused/switched. Thread 2 then comes along, performs the first check, sees that the pointer is not null, and skips the lock/fully-initialized check.

It then uses the partially-initialized object (probably resulting in bad things happening). Thread 1 then comes back and finishes initializing, but it's too late.

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