Question on layer/peeling

Michael Barker mikeb01 at
Tue Jan 6 02:14:43 UTC 2015


The SotS talks about the use of 'layer' to create an alternative
implementation of methods when the type of an <any T> is known to be a
reference type.  However, the examples only show the use of the layer
keyword on an interface definition, where as I've encountered at least one
case where the internal implementation needs to differentiate between a
reference-type and value-type based collection.  The example I'm thinking
about is the null-ing out of array elements in a collection (which is
obviously a no-op with a value type, but necessity with reference
types).  Is an interface required in order to define a 'layer' or could it
be done within a concrete class?

E.g. is the following or something similar possible?  If not, how would it
be achieved with current spec?

class ArrayList<any T> {
    T[] values;
    int position;

    void removeLast() {
        if (position <= 0) {


    private void clear(int index) {

    layer<ref T> {
        private void clear(int index) {
            values[index] = null;


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