Currently an object consists of an object-header and an array of instance variables. The object-header includes various flags, a reference count and a pointer to the class. The size of an object header is 12 bytes. Each instance variable consumes an additional 4 bytes. For example a point object has `x' and `y' instance variables and thus consumes 12 + 2 * 4 = 20 bytes.
The method `class<-instance_size'
returns the size of an instance of this class in bytes. Note that the
costs of supporting objects is not considered in this value. For example
a box object has instance size:
1 ?- get(class(box), instance_size, S). S = 72
But a box has an <-area instance variable
consuming an additional 28 bytes.