Subject: Re: [boost] [variant2] Andrzej's review -- design
From: Steven Watanabe (watanabesj_at_[hidden])
Date: 2019-04-06 16:09:43
On 4/6/19 3:37 AM, Rainer Deyke via Boost wrote:
> On 06.04.19 02:46, Emil Dotchevski via Boost wrote:
>> I do not think that it is a bug to access an object that is in a valid
>> state. In fact, the reason why the state is defined as valid is so
>> that you
>> can safely work with that object, even after a failure was reported.
> One thing that bothers me about this view is that it is inconsistent
> with how the language treats uninitialized variables.Â Every bit pattern
> of a variable of type 'char' is a valid value.Â However, reading from an
> uninitialized variable of type 'char' is undefined behavior.Â On a
> physical level, a variable of type char will always have a valid value,
> but on the conceptual level, it can also have a special "uninitialized"
> state from which it is legal to assign a new value but not to read the
> current value.
> I find that using this conceptual uninitialized state actually makes it
> /easier/ to reason about the correctness of my code.Â Any code that
> depends on the value of an uninitialized variable for its behavior is
> automatically incorrect.Â Any code that reads an uninitialized variable,
> even if the value read does not affect the observable behavior of the
> code, is automatically incorrect.Â Just declaring an uninitialized
> variable is a red warning light that I need to be careful to assign a
> value to that variable before reading from it.Â So long as I follow
> these rules, I never have to worry about my code unexpectedly breaking
> due to an uninitialized variable having an unexpected value.
> To me, the "valid but unspecified" state of an object after an exception
> is thrown from a function with the basic exception guarantee is
> conceptually very similar to the state of an uninitialized variable.Â If
> I catch such an exception while the object is still in scope, that's a
> red warning light that I need to either assign a new state to the object
> or allow the object to go out of scope.Â Anything else risks the same
> sort of errors as are caused by reading from an uninitialized variable.
That's completely different. The scope
in which a variable is uninitialized should
always be known statically. Also, uninitialized
variables are never really a good thing. It's
just that it's sometimes more convenient than
initializing them properly at the point of definition,
and we can get away with it for trivial types.
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