为什么Type Traits在命名空间（std :: enable_if）内部不起作用-C ++ 11

Question

If the following code is placed inside a namespace then its not working, but without namespace its working fine...

template <typename T,typename _F>
inline typename std::enable_if< is_iterable<T>::value >::type
for_all_ele(T&& arr,_F&& fn)
{
    for(auto &x:arr)
        for_all_ele(x,fn);

}

template <typename T,typename _F>
inline typename std::enable_if< not is_iterable<T>::value >::type
for_all_ele(T&& x,_F&& fn)
{
    fn(x);
}

is_iterable is defined over it...

The full code is here : http://ideone.com/goJ0Zf

Till now all I can find is the template specialization of 'for_all_ele' for scalar data is not being detected inside namespace...

Answer 1

Your recursion works when it is found via Koenig Lookup (aka ADL, aka Argument Dependent Lookup) and not when it is not found that way.

This call:

{
  for(auto &x:arr)
    for_all_ele(x,fn);
}

does not see the later declared for_all_ele , unless one of x or fn finds it via ADL. As it happens, when it isn't in a namespace, the lambda you pass as fn is in the same (root) namespace as your for_all_ele function is. So it is found.

When for_all_ele is in a namespace, it doesn't work, because the lambda is in a different namespace.

There are three approaches.

First, you can forward-declare it.

Second, you can create a wrapping function. It then instantiates a toy type ( struct toy_tag{}; ) and passes it together with the other arguments to the for_all_ele_impl , which pass said toy_tag when they recurse.

Third, you can simply move the recursive case after the terminating case: https://ideone.com/17h75Q

template <class T, class F>
inline typename std::enable_if< is_iterable<T>::value >::type
for_all_ele(T&& arr,F&& f)
{
  for(auto&&x:arr)
    for_all_ele(decltype(x)(x),f);
}

template <class T, class F>
inline typename std::enable_if<
  !is_iterable<T>::value
>::type
for_all_ele(T&& t,F&& f) {
  std::forward<F>(f)(std::forward<T>(t));
}

I renamed your _F to F , as the name _F is reserved by the standard in all contexts. I also perfect forwarded the function object, because I'm like that. And I did some perfect forwarding your code was missing.