Why is ADL not working with Boost.Range?

问题

Considering:

#include <cassert>
#include <boost/range/irange.hpp>
#include <boost/range/algorithm.hpp>

int main() {
    auto range = boost::irange(1, 4);
    assert(boost::find(range, 4) == end(range));
}

Live Clang demo Live GCC demo

this gives:

main.cpp:8:37: error: use of undeclared identifier 'end'

Considering that if you write using boost::end; it works just fine, which implies that boost::end is visible:

Why is ADL not working and finding boost::end in the expression end(range)? And if it's intentional, what's the rationale behind it?

To be clear, the expected result would be similar to what happens in this example using std::find_if and unqualified end(vec).

回答1:

In boost/range/end.hpp they explicitly block ADL by putting end in a range_adl_barrier namespace, then using namespace range_adl_barrier; to bring it into the boost namespace.

As end is not actually from ::boost, but rather from ::boost::range_adl_barrier, it is not found by ADL.

Their reasoning is described in boost/range/begin.hpp:

// Use a ADL namespace barrier to avoid ambiguity with other unqualified
// calls. This is particularly important with C++0x encouraging
// unqualified calls to begin/end.

no examples are given of where this causes a problem, so I can only theorize what they are talking about.

Here is an example I have invented of how ADL can cause ambiguity:

namespace foo {
  template<class T>
  void begin(T const&) {}
}

namespace bar {
  template<class T>
  void begin(T const&) {}

  struct bar_type {};
}

int main() {
  using foo::begin;
  begin( bar::bar_type{} );
}

live example. Both foo::begin and bar::begin are equally valid functions to call for the begin( bar::bar_type{} ) in that context.

This could be what they are talking about. Their boost::begin and std::begin might be equally valid in a context where you have using std::begin on a type from boost. By putting it in a sub-namespace of boost, std::begin gets called (and works on ranges, naturally).

If the begin in the namespace boost had been less generic, it would be preferred, but that isn't how they wrote it.

回答2:

Historical background

The underlying reason is discussed in this closed Boost ticket

With the following code, compiler will complain that no begin/end is found for "range_2" which is integer range. I guess that integer range is missing ADL compatibility ?

#include <vector>

#include <boost/range/iterator_range.hpp>
#include <boost/range/irange.hpp>

int main() {
    std::vector<int> v;

    auto range_1 = boost::make_iterator_range(v);
    auto range_2 = boost::irange(0, 1); 

    begin(range_1); // found by ADL
      end(range_1); // found by ADL
    begin(range_2); // not found by ADL
      end(range_2); // not found by ADL

    return 0;
}

boost::begin() and boost::end() are not meant to be found by ADL. In fact, Boost.Range specifically takes precautions to prevent boost::begin() and boost::end() from being found by ADL, by declaring them in the namespace boost::range_adl_barrier and then exporting them into the namespace boost from there. (This technique is called an "ADL barrier").

In the case of your range_1, the reason unqualified begin() and end() calls work is because ADL looks not only at the namespace a template was declared in, but the namespaces the template arguments were declared in as well. In this case, the type of range_1 is boost::iterator_range<std::vector<int>::iterator>. The template argument is in namespace std (on most implementations), so ADL finds std::begin() and std::end() (which, unlike boost::begin() and boost::end(), do not use an ADL barrier to prevent being found by ADL).

To get your code to compile, simply add "using boost::begin;" and "using boost::end;", or explicitly qualify your begin()/end() calls with "boost::".

Extended code example illustrating the dangers of ADL

The danger of ADL from unqualified calls to begin and end is two-fold:

the set of associated namespaces can be much larger than one expects. E.g. in begin(x), if x has (possibly defaulted!) template parameters, or hidden base classes in its implementation, the associated namespaces of the template parameters and of its base classes are also considered by ADL. Each of those associated namespace can lead to many overloads of begin and end being pulled in during argument dependent lookup.
unconstrained templates cannot be distinguished during overload resolution. E.g. in namespace std, the begin and end function templates are not separately overloaded for each container, or otherwise constrained on the signature of the container being supplied. When another namespace (such as boost) also supplies similarly unconstrained function templates, overload resolution will consider both an equal match, and an error occurs.

The following code samples illustrate the above points.

A small container library

The first ingredient is to have a container class template, nicely wrapped in its own namespace, with an iterator that derives from std::iterator, and with generic and unconstrained function templates begin and end.

#include <iostream>
#include <iterator>

namespace C {

template<class T, int N>
struct Container
{
    T data[N];
    using value_type = T;

    struct Iterator : public std::iterator<std::forward_iterator_tag, T>
    {
        T* value;
        Iterator(T* v) : value{v} {}
        operator T*() { return value; }
        auto& operator++() { ++value; return *this; }
    };

    auto begin() { return Iterator{data}; }
    auto end() { return Iterator{data+N}; }
};

template<class Cont>
auto begin(Cont& c) -> decltype(c.begin()) { return c.begin(); }

template<class Cont>
auto end(Cont& c) -> decltype(c.end()) { return c.end(); }

}   // C

A small range library

The second ingredient is to have a range library, also wrapped in its own namespace, with another set of unconstrained function templates begin and end.

namespace R {

template<class It>
struct IteratorRange
{
    It first, second;

    auto begin() { return first; }
    auto end() { return second; }
};

template<class It>
auto make_range(It first, It last)
    -> IteratorRange<It>
{
    return { first, last };    
}

template<class Rng>
auto begin(Rng& rng) -> decltype(rng.begin()) { return rng.begin(); }

template<class Rng>
auto end(Rng& rng) -> decltype(rng.end()) { return rng.end(); }

} // R

Overload resolution ambiguity through ADL

Trouble begins when one tries to make an iterator range into a container, while iterating with unqualified begin and end:

int main() 
{
    C::Container<int, 4> arr = {{ 1, 2, 3, 4 }};
    auto rng = R::make_range(arr.begin(), arr.end());
    for (auto it = begin(rng), e = end(rng); it != e; ++it)
        std::cout << *it;
}

Live Example

Argument-dependent name lookup on rng will find 3 overloads for both begin and end: from namespace R (because rng lives there), from namespace C (because the rng template parameter Container<int, 4>::Iterator lives there), and from namespace std (because the iterator is derived from std::iterator). Overload resolution will then consider all 3 overloads an equal match and this results in a hard error.

Boost solves this by putting boost::begin and boost::end in an inner namespace and pulling them into the enclosing boost namespace by using directives. An alternative, and IMO more direct way, would be to ADL-protect the types (not the functions), so in this case, the Container and IteratorRange class templates.

Live Example With ADL barriers

Protecting your own code may not be enough

Funny enough, ADL-protecting Container and IteratorRange would -in this particular case- be enough to let the above code run without error because std::begin and std::end would be called because std::iterator is not ADL-protected. This is very surprising and fragile. E.g. if the implementation of C::Container::Iterator no longer derives from std::iterator, the code would stop compiling. It is therefore preferable to use qualified calls R::begin and R::end on any range from namespace R in order to be protected from such underhanded name-hijacking.

Note also that the range-for used to have the above semantics (doing ADL with at least std as an associated namespace). This was discussed in N3257 which led to semantic changes in range-for. The current range-for first looks for member functions begin and end, so that std::begin and std::end will not be considered, regardless of ADL-barriers and inheritance from std::iterator.

int main() 
{
    C::Container<int, 4> arr = {{ 1, 2, 3, 4 }};
    auto rng = R::make_range(arr.begin(), arr.end());
    for (auto e : rng)
        std::cout << e;
}

Live Example