Re: [boost] [outcome] Exception safety guarantees

Subject: Re: [boost] [outcome] Exception safety guarantees
From: Peter Dimov (lists_at_[hidden])
Date: 2017-05-27 22:45:57

Andrzej Krzemienski wrote:
> But tell me this. Consider the example with class Man above:
>
> ```
> struct Man { std::string fist_name, last_name; };
> Man m1 = {"April", "Jones"};
> Man m2 = {"Theresa", "May"};
>
> try {
>   m2 = m1; // suppose it throws
> }
> catch(...)  {
> }
> ```
>
> Object m2 after recovering from stack unwinding may be in the state 
> {"April", "May"}, which is a "valid state". Would you call it a valid 
> state?

It depends on the invariant of Man. Assuming no invariant, yes, it's valid. 
It doesn't crash if I access it.

> Same with outcome<T>: if I assign to it and it fails:
>
> o1 = o2; // assume basic guarantee
>
> provided I get transactional guarantee, I know what its value is. But if I 
> get "basic guarantee" as you describe it (valid but unspecfied state), 
> what good does it make that I can safely call has_value() if the object 
> contains a different value than o1 or o2 had initially?

What good it makes is that you can call has_value() on it without crashing.

Without this guarantee, you could never safely call has_value() on a foreign 
object without checking valueless() first.