Re: [boost] [variant2] Andrzej's review -- design

Subject: Re: [boost] [variant2] Andrzej's review -- design
From: Rainer Deyke (rainerd_at_[hidden])
Date: 2019-04-06 09:37:17

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.  Every bit pattern 
of a variable of type 'char' is a valid value.  However, reading from an 
uninitialized variable of type 'char' is undefined behavior.  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.  Any code that 
depends on the value of an uninitialized variable for its behavior is 
automatically incorrect.  Any code that reads an uninitialized variable, 
even if the value read does not affect the observable behavior of the 
code, is automatically incorrect.  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.  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.  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.  Anything else risks the same 
sort of errors as are caused by reading from an uninitialized variable.

-- 
Rainer Deyke (rainerd_at_[hidden])