Re: [boost] [variant] Opinion on boost::safe_get<> and default boost::get<> behavior

Subject: Re: [boost] [variant] Opinion on boost::safe_get<> and default boost::get<> behavior
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2014-12-09 13:26:00

Le 09/12/14 18:39, Peter Dimov a écrit :
> Antony Polukhin wrote:
> ...
>> boost::variant<int, string> v(100);
>>
>> boost::safe_get<bool>(&v); // compile time error
>> boost::safe_get<bool>(v);   // compile time error
>> boost::unsafe_get<bool>(&v); // returns NULL instead of compile time 
>> error
>> boost::unsafe_get<bool>(v);   // throws exception on runtime instead 
>> of compile time error
>>
>>
>> I'm in doubt, what the default behavior of boost::get must be:
>> * safe_get behavior is much closer to Standard Library behavior (just 
>> like std::get for tuples) and allows to avoid errors in user code
>> * unsafe_get behavior is same as behavior of old boost::get and won't 
>> break user's code if boost::get is used in some generic contexs
>>
>> What's your opinion?
>
> My opinion is that these functions are misnamed. unsafe_get is not 
> unsafe. I'd say that they are strict and relaxed, respectively, not 
> safe and unsafe.
>
> Regarding the default, there's only one way to find out. Namely, you 
> switch boost::get to be strict, then see if there are any complaints. 
> (Having the default be relaxed doesn't seem to make much sense.)
>
> There's nothing wrong (or unsafe) with the old behavior - it 
> consistently answers the question "does v contain a T" - so the 
> decision can't be made based on principles alone and must be informed 
> by practice. That is, do the benefits of the strict get - which are 
> pretty much only catching your typo when you say get<T>(v) instead of 
> get<T>(w) - outweigh the loss of functionality.
>
>
I agree with peter strict/relaxed covers better the
Even if changing the default behavior to strict_get could break some 
user code, this will result on a compile error, which wouldn't be a 
silent break. If you provide both strict/relaxed versions, the user 
could always change its defaulted get<bool>(v) to relaxed_get(bool)(v).

Best,
Vicente

P.S. I don't think there will be too much uses of relaxed_get, but who 
know what C++ programmers use ;-)