const member function

[rumbu via Digitalmars-d-learn]

Often I'm using the following code pattern:

class S
{
    private SomeType cache;

    public SomeType SomeProp() @property
    {
       if (cache is null)
         cache = SomeExpensiveOperation();
       return cache;
    }
}

Is there any way to mark SomeProp() as const? Because I want to 
call somewhere const(S).SomeProp, which for the outside world is 
must be "const", returning just a value, even that internaly it 
modifies the internal S structure.