🚀 go-pugleaf

In article <1993Jan1.172044.9659@ucc.su.OZ.AU>, maxtal@extro.ucc.su.OZ.AU (John MAX Skaller) writes:
|> Further to a previous post describing modifications to the type
|> system, I want to try to explain the underlying concept with some
|> further examples.
|> 
|> The basic idea is that 'int' and 'T' are object types,
|> whereas 'int&' or 'const int&' are not object types.
|> I would say they are 'accessors' to an object of type 'int'.
 
The semantics of "const" in types is confused because it is also
unsed with a different meaning in the English descriptions of
C++.  For several years I have been using "immutable" for this
sense of "const".  That adjective hasn't yet caught on but, 
I live in hope.

The way I understand it, when an object is created (constructed)
it can be "marked" as mutable or immutable. ("Marked" in quotes
because no runtime marking is implied).  This mark cannot be
changed.  The type system is designed so that (barring the
use of unsafe casts) the following rule is assured.

	If an lvalue has non-const type,
	then it refers to a mutable object.

Thus |int&| and |const int&| differ precisely because they
refer to objects rather than values.  The values that these
objects can hold are the same, but the information we have
about the setting of the mutability flag is different.

Discussions of "const" in the type system become slippery because
when talking about values mutability is irrelevant and so "int"
and "const int" are in some sense the same.

  -- Jerry Schwarz