State or no state, that is the question

[Daniel]

Consider a class


template <class Allocator>
struct A_base
{
A_base(const Allocator& allocator)
: allocator_(allocator)
{
}

Allocator allocator_;
};


template <class Allocator>
struct C : public A_base<Allocator>
{
C()
: A_base(Allocator())
{
}
C(const Allocator& allocator)
: A_base{ allocator }
{

}

void f()
{
}
}

Is there a reliable way within function f to determine whether allocator_ is stateful or stateless?

I thought about using std::is_default_constructible<Allocator>, as presumably a stateful allocator would not have a default constructor.

Or testing whether sizeof(C) == sizeof the same data structure except not inheriting from A_base.

Any thoughts?

Thanks,
Daniel

[Christopher J. Pisz]

I'd agree with you that sizeof(C) == sizeof the same data structure
except not inheriting from A_base is pretty much fullproof, but I am no
expert on allocators.

[Öö Tiib]

On Monday, 5 December 2016 06:08:07 UTC+2, Daniel wrote:
> Is there a reliable way within function f to determine whether allocator_ is stateful or stateless?

Take from Boost.TypeTraits 'is_stateless'. Docs of TypeTraits:
http://www.boost.org/doc/libs/1_62_0/libs/type_traits/doc/html/index.html

>
> I thought about using std::is_default_constructible<Allocator>, as presumably a stateful allocator would not have a default constructor.
>
> Or testing whether sizeof(C) == sizeof the same data structure except not inheriting from A_base.
>
> Any thoughts?

Boost docs say that "is_stateless" is implemented like that:

::boost::has_trivial_constructor<T>::value
&& ::boost::has_trivial_copy<T>::value
&& ::boost::has_trivial_destructor<T>::value
&& ::boost::is_class<T>::value
&& ::boost::is_empty<T>::value

[Daniel]

Thanks! And it looks like all the pieces are in <type_traits>.

Daniel

[Daniel]

On Monday, December 5, 2016 at 3:22:23 AM UTC-5, Öö Tiib wrote:
>

> Boost docs say that "is_stateless" is implemented like that:
>
> ::boost::has_trivial_constructor<T>::value
> && ::boost::has_trivial_copy<T>::value
> && ::boost::has_trivial_destructor<T>::value
> && ::boost::is_class<T>::value
> && ::boost::is_empty<T>::value

boost::is_stateless<std::allocator<char>>::value turns out to return false.
boost::has_trivial_constructor<T>::value and boost::has_trivial_copy<T>::value
both evaluate to false (the other parts evaluate to true.)

For my purposes, I'm tentatively using

template <typename T>
struct is_stateless_allocator
: public std::integral_constant<bool,
(std::is_default_constructible<T>::value &&
std::is_empty<T>::value)>
{};

If anyone has any suggestions for other conditions to throw in there, please
let me know.

Thanks,
Daniel