Re: [boost] Boost.Outcome review - First questions

Subject: Re: [boost] Boost.Outcome review - First questions
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-05-22 19:12:11

>> The fact optional implemented .value_or() with a return by value is a
>> mistake. Expected need not repeat that mistake.
> 
> I might have missed something, but if .value_or() returns a reference, won't:
> 
> auto &&s = std::optional<std::string>{}.value_or("temporary"s);
> std::cout << s << std::endl;
> 
> triggers undefined behavior? 
> 
> '"temporary"s' is destroyed right after the declaration of 's', hence 's' is
> a dangling reference.
> 
> By requiring .value_or() to return by value, this should just work.

Does not the bind to a reference keep the std::string temporary object
alive?

https://godbolt.org/g/ZcLpe5 suggests that it does.

But you might be onto something. Some other combination with value_or()
might well produce a reference to a deleted temporary. That would
explain the choice of value returning semantics for value_or().

Niall

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