C++:使用用户定义的哈希/相等性帮助创建 unordered_map

Question

我正在尝试使用用户定义的散列函数和等式谓词为整数内置类型的矩阵行创建一个 std::unordered_map。我使用 std::bind，因为我需要散列和相等仿函数来处理可变范围。我将如何让以下代码按预期编译和工作？我猜我的错误是在 list 的底部，我在那里使用了 std::bind 并实例化了 std::unordered_map。

一些说明:

我不能使用 boost::hash_combine，因为我关心存储在矩阵行中的整数中的各个位，所以除非我创建自己的迭代器，否则 boost::hash_combine 会合并整个整数，从而导致错误的结果。此外，我需要哈希和相等仿函数在 1 到 ~200,000 的范围内工作，因此在模板参数中指定范围不是一个合理的选择。

template <class T,class F,class A>
struct row_hash_right : std::function<size_t(
        ublas::matrix_row<ublas::matrix<T,F,A>>,
        unsigned, unsigned)>
{
        typedef ublas::matrix_row<ublas::matrix<T,F,A>> row_type;

        // I want to use std::bind to bind the last two arguments.
        size_t operator()(row_type& a, unsigned s, unsigned e)
        {
                // Implementation of hash function.
        }
};

template <class T,class F,class A>
struct row_equal_right : std::function<bool(
        ublas::matrix_row<ublas::matrix<T,F,A>>,
        ublas::matrix_row<ublas::matrix<T,F,A>>,
        unsigned, unsigned)>
{
        typedef ublas::matrix_row<ublas::matrix<T,F,A>> row_type;

        bool operator()(row_type& a, row_type& b, unsigned s, unsigned e)
        {
                // Implementation of equality predicate.
        }
};

// Inside a function.
for (unsigned i = 0; i < len; ++i) {
        for (unsigned j = i + 1; j < len; ++j) {
                auto x = std::bind(r_hash, _1, i, j);
                auto y = std::bind(r_equal, _1, _2, i, j);
                // ERROR:
                std::unordered_map<row_type, unsigned,
                        decltype(x), decltype(y)> m(256, x, y);
        }
}

The error:

Here is (what I think) the most important part of the error produced upon attempted compilation:

/usr/include/c++/4.6/bits/stl_pair.h:92:11: error: ‘std::pair<_T1, _T2>::first’ has incomplete type /usr/include/boost/numeric/ublas/fwd.hpp:73:11: error: declaration of ‘const struct boost::numeric::ublas::matrix_row, boost::numeric::ublas::unbounded_array > > >’

If you want the see the whole thing, I've dumped it all here:

In file included from /usr/include/c++/4.6/bits/stl_algobase.h:65:0,
                 from /usr/include/c++/4.6/bits/char_traits.h:41,
                 from /usr/include/c++/4.6/ios:41,
                 from /usr/include/c++/4.6/ostream:40,
                 from /usr/include/c++/4.6/iostream:40,
                 from src/test/read_test.cpp:1:
/usr/include/c++/4.6/bits/stl_pair.h: In instantiation of ‘std::pair, boost::numeric::ublas::unbounded_array > > >, unsigned int>’:
/usr/include/c++/4.6/bits/stl_function.h:486:12:   instantiated from ‘std::_Select1st, boost::numeric::ublas::unbounded_array > > >, unsigned int> >’
/usr/include/c++/4.6/bits/hashtable_policy.h:789:20:   instantiated from ‘std::__detail::_Hash_code_base, boost::numeric::ublas::unbounded_array > > >, std::pair, boost::numeric::ublas::unbounded_array > > >, unsigned int>, std::_Select1st, boost::numeric::ublas::unbounded_array > > >, unsigned int> >, std::_Bind, boost::numeric::ublas::unbounded_array > >(std::_Placeholder, std::_Placeholder, unsigned int, unsigned int)>, std::_Bind, boost::numeric::ublas::unbounded_array > >(std::_Placeholder, unsigned int, unsigned int)>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, false>’
/usr/include/c++/4.6/bits/hashtable.h:105:11:   instantiated from ‘std::_Hashtable, boost::numeric::ublas::unbounded_array > > >, std::pair, boost::numeric::ublas::unbounded_array > > >, unsigned int>, std::allocator, boost::numeric::ublas::unbounded_array > > >, unsigned int> >, std::_Select1st, boost::numeric::ublas::unbounded_array > > >, unsigned int> >, std::_Bind, boost::numeric::ublas::unbounded_array > >(std::_Placeholder, std::_Placeholder, unsigned int, unsigned int)>, std::_Bind, boost::numeric::ublas::unbounded_array > >(std::_Placeholder, unsigned int, unsigned int)>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true>’
/usr/include/c++/4.6/bits/unordered_map.h:44:11:   instantiated from ‘std::__unordered_map, boost::numeric::ublas::unbounded_array > > >, unsigned int, std::_Bind, boost::numeric::ublas::unbounded_array > >(std::_Placeholder, unsigned int, unsigned int)>, std::_Bind, boost::numeric::ublas::unbounded_array > >(std::_Placeholder, std::_Placeholder, unsigned int, unsigned int)>, std::allocator, boost::numeric::ublas::unbounded_array > > >, unsigned int> >, false>’
/usr/include/c++/4.6/bits/unordered_map.h:256:11:   instantiated from ‘std::unordered_map, boost::numeric::ublas::unbounded_array > > >, unsigned int, std::_Bind, boost::numeric::ublas::unbounded_array > >(std::_Placeholder, unsigned int, unsigned int)>, std::_Bind, boost::numeric::ublas::unbounded_array > >(std::_Placeholder, std::_Placeholder, unsigned int, unsigned int)>, std::allocator, boost::numeric::ublas::unbounded_array > > >, unsigned int> > >’
./sal/alg/ehh.hpp:144:31:   instantiated from ‘sal::ehh_results sal::compute_ehh(boost::numeric::ublas::matrix&, unsigned int) [with FloatType = double, T = unsigned int, F = boost::numeric::ublas::basic_row_major, A = boost::numeric::ublas::unbounded_array >]’
src/test/read_test.cpp:11:51:   instantiated from here
/usr/include/c++/4.6/bits/stl_pair.h:92:11: error: ‘std::pair::first’ has incomplete type
/usr/include/boost/numeric/ublas/fwd.hpp:73:11: error: declaration of ‘const struct boost::numeric::ublas::matrix_row, boost::numeric::ublas::unbounded_array > > >’





Answer 1

如果你想散列一系列的东西，你需要类似 hash_combine() 的东西.我通常从 Boost 中提取这个功能(令人惊讶的是，它没有包含在标准中!)。这是我在 std::array 上使用它的方式s，我相信您可以将其操作成可以处理矩阵行的东西:

#include <array>

template <class T>
inline void hash_combine(std::size_t & seed, const T & v)
{
    std::hash<T> hasher;
    seed ^= hasher(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
}

namespace std
{
    template<typename T, size_t N> struct hash<array<T, N>>
    {
        inline size_t operator()(const array<T, N> & a) const
        {
            size_t seed = 0;
            for (size_t i = 0; i != N; ++i)
            {
                ::hash_combine(seed, a[i]);
            }
            return seed;
        }
    };
}

(以上特化允许你使用std::unordered_set<std::array<int, 10>>等)

如果矩阵行没有等式谓词，您还可以添加 std::equal_to 的特化对于矩阵行。