154. Shaping C-like unions into memory segments

Let’s consider the C-like union from the following figure (the members of a C union share the same memory location (the member’s largest data type dictates the size of the memory location), so only one of the members has a value at any moment in time):

Figure 7.15: A C-like union

In Figure 7.15, we have a C-like union named product to shape two members, price (double) and sku (int), while only one can have a value at any moment in time. We can shape a memory segment to fit this model as follows (arena is an instance of Arena):

MemorySegment segment = arena.allocate(
  ValueLayout.JAVA_DOUBLE.byteSize(),
  ValueLayout.JAVA_DOUBLE.byteAlignment());

Because double needs 8 bytes and int needs only 4 bytes, we choose ValueLayout.JAVA_DOUBLE to shape the size of the memory segment. This way, the segment can accommodate a double and an int at the same offset.

Next, we can set the price or the sku and...