错误：超载距离的调用不明确

Question

我有一些代码（我没有编写，但正在尝试编译）--iostream_combo.cc--，这样做会给我以下错误：

./moses/moses/comboreduct/combo/iostream_combo.cc: In function ‘std::__cxx11::string opencog::combo::l2ph(const string&, const std::vector<std::__cxx11::basic_string<char> >&)’:
./moses/moses/comboreduct/combo/iostream_combo.cc:543:64: error: call of overloaded ‘distance(std::vector<std::__cxx11::basic_string<char> >::const_iterator, __gnu_cxx::__normal_iterator<const std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >&)’ is ambiguous
             arity_t idx = distance(labels.begin(), found_it) + 1;
                                                            ^ In file included from /usr/include/c++/8/bits/stl_algobase.h:66,
             from /usr/include/c++/8/bits/char_traits.h:39,
             from /usr/include/c++/8/ios:40,
             from /usr/include/c++/8/ostream:38,
             from /usr/include/c++/8/iostream:39,
             from ./moses/moses/comboreduct/combo/iostream_combo.h:28,
             from ./moses/moses/comboreduct/combo/iostream_combo.cc:24:
/usr/include/c++/8/bits/stl_iterator_base_funcs.h:138:5: note: candidate: ‘typename std::iterator_traits<_Iterator>::difference_type std::distance(_InputIterator, _InputIterator) [with _InputIterator = __gnu_cxx::__normal_iterator<const std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >; typename std::iterator_traits<_Iterator>::difference_type = long int]’
 distance(_InputIterator __first, _InputIterator __last)
 ^~~~~~~~ In file included from /usr/local/include/boost/range/distance.hpp:18,
             from /usr/local/include/boost/range/functions.hpp:21,
             from /usr/local/include/boost/range/iterator_range_core.hpp:38,
             from /usr/local/include/boost/lexical_cast.hpp:30,
             from ./moses/moses/comboreduct/combo/iostream_combo.h:30,
             from ./moses/moses/comboreduct/combo/iostream_combo.cc:24:
/usr/local/include/boost/iterator/distance.hpp:49:9: note: candidate: ‘constexpr typename boost::iterators::iterator_difference<Iterator>::type boost::iterators::distance_adl_barrier::distance(SinglePassIterator, SinglePassIterator) [with SinglePassIterator = __gnu_cxx::__normal_iterator<const std::__cxx11::basic_string<char>*, std::vector<std::__cxx11::basic_string<char> > >; typename boost::iterators::iterator_difference<Iterator>::type = long int]’
     distance(SinglePassIterator first, SinglePassIterator last)
     ^~~~~~~~

我使用的是 Ubuntu 16.04 x64、Boost 1.68 和 gcc 8.2。因此，重现问题的步骤如下：

1. 在 Ubuntu 16.04 上
2. 安装gcc-8
3. 使用它从源代码构建 boost 1.68
4. Git 克隆 moses repository 并按照那里的说明进行操作：基本上 1) git clone 并构建 cogutil; 2）尝试使moses： cd build、cmake ..、make。

我足够了解 C++（我认为），我可以看到对 std::distance 的调用已超载。我没有看到消除歧义的方法，尽管我猜它必须包括对found_it 的一些重写或一些显式转换而不是auto。

Answer 1

电话是这样的：

    arity_t idx = distance(labels.begin(), found_it) + 1;

这意味着 distance 是通过 ADL 找到的，因此会考虑所有关联的命名空间。如果例如，这可能会证明是有问题的。有两个命名空间为distance 提供同样适用的重载：

Live On Coliru

#include <iterator>
#include <vector>

namespace OyVeh {
    struct X { };

    template <typename It>
    size_t distance(It, It) { return 42; } // just to throw in a wrench
}

int main() {
    std::vector<OyVeh::X> v{3};

    auto f = v.begin();
    auto l = v.end();

    // now f and l have both `::std` and `::OyVeh` as associated namespaces. The following is ambiguous:

    return distance(f, l);
}

大致有两种方法可以解决：

• 从关联的命名空间中删除模棱两可的 distance 声明（如果竞争者是 std::distance 和 boost::distance，这可能是不可能的）
• 编辑调用以消除对 ADL 的依赖（例如，将它们限定为 std::distance(...) 或将它们括起来 (distance)(...)）

显示解决方法：

Live On Coliru

{
    using OyVeh::distance;
    return (distance)(f, l); // parentheses suppress ADL
}

return std::distance(f, l); // also works, obviously
return OyVeh::distance(f, l); // but beware the meaning might change