Const sucks

[Bruce Adams]

Walter Bright Wrote:

> Russell Lewis wrote:
> > Walter Bright wrote:
> >> What we are trying to achieve:
> >>
> >> a) utility for functional programming
> >> b) better modularity by self-documenting interfaces better
> >> c) be able to treat reference types as if they were value types (i.e. 
> >> strings should behave to the user like value types, even though they 
> >> are references)
> > 
> > Is there a way to express (for a pointer passed as a function parameter) 
> > "the callee is allowed to modify this object through the pointer, but 
> > the caller ensures that no other code will be modifying it at the same 
> > time"?   Sort of a "you have exclusive write access" modifier?  Is that 
> > a common enough case to even support?
> 
> That's sort of what the often proposed 'unique' type qualifier does. The 
> problem is, there's just no reasonable way to implement it in a 
> *checkable* manner. And if it ain't checkable, it's about as useful as 
> painting a black stripe across your lap when driving instead of wearing 
> a seatbelt.

I see a least two ways of interpreting it at run-time.
Acquire a lock before entering the function (or at least the parts where the value is modified) or in an STM atomic transaction roll-back and try again if the value is modified.
Being run-time only I suspect a function or template and lazy evaluation might suffice.

At compile time you could use it as a hint for 'linting' race conditions out of your code. That is a useful instrumentation to have.
(cyclist wearing a flourescent band as opposed to a black stripe)

These are just suggestions I wouldn't advocate anything without more thought. As I said on the next branch over this is off topic for a discussion about const.

Regards,

Bruce.