[Issue 2027] New: No way to declare only the reference 'const' or 'invariant' for reference types.

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=2027

           Summary: No way to declare only the reference 'const' or
                    'invariant' for reference types.
           Product: D
           Version: 2.012
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: htvennik at zonnet.nl


While using D I often want a reference to a class object to be invariant, while
the object itself should be mutable. Or, in my current case, I want a delegate
variable to be invariant (i.e. the reference may not change), but I do not want
that only a invariant function may be assigned to it on initialization...

This is obviously an undesirable limitation of the D 2.0 type system. Probably
there should be some storage class for these cases.

The following piece of code demonstates the problem:

class A {
    private:
        invariant Object o_;

    this(Object o) {
        o_ = o;        // error, cannot implicitly convert to invariant
    }
}

Because o_ is of type invariant(Object), it cannot be assigned a value of type
Object, which is perfecly correct. But I do not intend o_ to be of type
invariant(Object), I only want o_ to reference the same instance of Object,
throughout the lifetime of the instance of class A, and thus forbid
re-assignment...


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