Where exactly can an error occur without 'volatile' in Double-Checked Locking?

Without using the volatile keyword in the Double-Checked Locking (DCL) pattern, there can be issues with visibility and instruction reordering.

The main problem is that an object may be partially constructed, and a reference to it is written to a variable, but other threads may see this reference before the object is fully initialized.

For example, in JVM, the following sequence is possible:

1. Memory is allocated for the object.
2. A reference to the object is written to a variable (e.g., singletonInstance).
3. The object's constructor executes.

If another thread sees the reference to the object (step 2), but the object is not yet initialized (step 3 not completed), it will get an incorrect object.

Using volatile guarantees that the reference assignment and initialization are visible to other threads only after the object is fully initialized, preventing such errors.