What is a niebloid? Ask Question

The term niebloid comes from Eric Niebler's name. In simple words, they are function objects that disable ADL (argument-dependent lookup) from happening so that the overloads in std:: aren't picked up when an algorithm from std::ranges is called.

Here's a tweet (from 2018) and answer from Eric himself suggesting the name. Eric wrote an article in 2014 explaining this concept.

It can best be seen in action in the standard document itself:

25.2.2
The entities defined in the std​::​ranges namespace in this Clause are not found by argument-dependent name lookup (basic.lookup.argdep). When found by unqualified (basic.lookup.unqual) name lookup for the postfix-expression in a function call, they inhibit argument-dependent name lookup.

void foo() {
  using namespace std::ranges;
  std::vector<int> vec{1,2,3};
  find(begin(vec), end(vec), 2);        // #1
}

#1の関数呼び出し式はstd​::​ranges​::​find、 (a) および から返される反復子の型がおよび に関連付けられている可能性がstd​::​findあるにもかかわらず、(b)前者では最初の 2 つのパラメータが同じ型である必要があるため、 よりも特殊化されている ([temp.func.order]) にもかかわらずbegin(vec)、ではなくを呼び出します。end(vec)namespace stdstd​::​findstd​::​ranges​::​find

namespace mystd
{
    class B{};
    class A{};
    template<typename T>
    void swap(T &a, T &b)
    {
        std::cout << "mystd::swap\n";
    }
}

namespace sx
{
    namespace impl {
       //our functor, the niebloid
        struct __swap {
            template<typename R, typename = std::enable_if_t< std::is_same<R, mystd::A>::value >  >
            void operator()(R &a, R &b) const
            {
                std::cout << "in sx::swap()\n";
                // swap(a, b); 
            }
        };
    }
    inline constexpr impl::__swap swap{};
}

int main()
{
    mystd::B a, b;
    swap(a, b); // calls mystd::swap()

    using namespace sx;
    mystd::A c, d;
    swap(c, d); //No ADL!, calls sx::swap!

    return 0;
}

find(begin(vec), end(vec), 2); //unqualified call to find