std :: set插入程序不符合自定义比较器。 （可能的编译器错误？）

Question

I'm finding that std::inserter on an std::map<T, C> does not always respect the custom comparator, but rather will sometimes fall back to the default operator< / std::less . Does anyone have any experience with why this may be? I feel like it could be a compiler bug, given the sporadic nature. In particular, I have a function like so:

template <typename T, typename C>
std::set<T, C> operator|(const std::set<T, C> &lhs, const std::set<T, C> &rhs)
{
  std::set<T, C> out;
  std::set_union(lhs.begin(), lhs.end(),
                 rhs.begin(), rhs.end(),
                 inserter(out, out.end()));
  return out;
}

But this does not always use my custom C comparator, and sometimes (I know, I too wish I could find a pattern) will just use std::less instead. If I replace it with a maunal construction of the std::insert_iterator<std::set<T, C>> , everything works correctly:

template <typename T, typename C>
std::set<T, C> operator|(const std::set<T, C> &lhs, const std::set<T, C> &rhs)
{
  std::set<T, C> out;
  std::set_union(lhs.begin(), lhs.end(),
                 rhs.begin(), rhs.end(),
                 std::insert_iterator<std::set<T, C>>(out, out.end()));
  return out;
}

I feel like this could be a compiler bug (gcc version 7.3.0) because the reference available on http://en.cppreference.com/w/cpp/iterator/inserter describes inserter to simply delegate to insert_iterator :

template< class Container >
std::insert_iterator<Container> inserter( Container& c, typename Container::iterator i )
{
    return std::insert_iterator<Container>(c, i);
}

Furthermore, the official C++ standard, section 24.5.2.5 (accessed via http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3690.pdf , page 859) puts the definition of inserter as returning an insert iterator of the same type as is passed in the first argument:

template <class Container>
      insert_iterator<Container> inserter(Container& x, typename Container::iterator i);

Which clearly isn't happening here, as the returned insert_iterator is for a class of type std::set<T, C = std::less<T>> , not std::set<T, C> .

Is there an explanation for this besides compiler bug?

Answer 1

"possible compiler bug" is to C++ what "It might be Lupus" is to House MD.

That being said, following the Documentation , you appear to be using version 1) of the function, which clearly states:

1) Elements are compared using operator< and the ranges must be sorted with respect to the same.

What you should be doing, is pass your comparator to std::set_union , as per version 3) of the function.

So the real mystery is: why is your comparator used at all sometimes. However, since you do not respect the preconditions of the function:

the ranges must be sorted with respect to the same.

Then there is no expected behavior defined, so the behavior is not "incorrect".