[libcxx-dev] Providing custom template specialization for std::optional?

[Владимир Гончаров via libcxx-dev]

> Where does the standard say that?

[namespace.std].3 <http://eel.is/c++draft/namespace.std#3>: The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.

> On 9 Mar 2019, at 04:49, Eric Fiselier <eric at efcs.ca> wrote:
> 
> 
> 
> On Thu, Mar 7, 2019 at 10:18 AM Владимир Гончаров <dev.zelta at gmail.com <mailto:dev.zelta at gmail.com>> wrote:
>> Can you explain what your specialization does differently? And why it's needed?
> 
> 
> I have a class which is a C++ interface for an object defined in a C library. Internally, it is a pointer which can never be null (if C interface returns null, the class constructor throws).
> I want to use this fact to optimise optional's storage. That would allow shrinking optional's size to eight bytes. Since we store a lot of optionals of this class, this change would reduce memory usage by about five percent (I've done some testing).
> I really don't want to invent a new optional specifically for this case. I also don't want to allow storing null pointers inside of the original class because this would require an assert before almost any usage of the class.
> 
>> I think the standard is being too permissive here.
> 
> 
> Maybe it is. But again, if I can provide a more efficient specialization which would utilise some knowledge about the wrapped type, why shouldn't I?
> 
>> `<variant>` is a good example.
> 
> 
> By the way, providing custom specializations for variadic templates is forbidden =)
> 
> Where does the standard say that?
>  
> 
> Kind regards, Vladimir Goncharov
> 
>> On 7 Mar 2019, at 08:39, Eric Fiselier <eric at efcs.ca <mailto:eric at efcs.ca>> wrote:
>> 
>> 
>> 
>> On Wed, Mar 6, 2019 at 5:41 PM Владимир Гончаров via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
>> Hello!
>> 
>> I'm implementing a specialization for std::optional for my program-defined type.
>> 
>> Can you explain what your specialization does differently? And why it's needed?
>>  
>> As I understand [namespace.std], nothing forbids me from doing that.
>> 
>> I agree that you've interpreted the standard correctly. But I think the standard is being too permissive here.
>> Implementing a complex component may necessitate knowledge of the internals. `<variant>` is a good example.
>> 
>> I would like to know what other library maintainers think.
>>  
>> /Eric
> 

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