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我正在尝试使用用户定义的哈希函数和相等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函子来处理变量范围。我将如何获得以下代码以按预期编译和工作？我猜我的错误是在列表的底部，我使用 std::bind 并实例化 std::unordered_map。

一些澄清：

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 会组合整个整数，从而导致错误的结果。此外，我需要散列和相等函子在 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);
        }
}

错误：

这是（我认为）尝试编译时产生的错误中最重要的部分：

/usr/include/c++/4.6/bits/stl_pair.h:92:11: 错误：'std::pair<_T1, _T2>::first' 类型不完整 /usr/include/boost/numeric/ublas/fwd .hpp:73:11: 错误: 'const struct boost::numeric::ublas::matrix_row, boost::numeric::ublas::unbounded_array > > >'的声明</p>

如果您想查看全部内容，我已将其全部倾倒在这里：


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 > > >’

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1 回答 1

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如果要对一系列事物进行哈希处理，则需要类似hash_combine(). 我通常从 Boost 中提升这个功能（令人惊讶的是，它没有包含在标准中！）。以下是我在std::arrays 上使用它的方法，我相信你可以将它操纵成一些东西来处理矩阵行：

#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;
        }
    };
}

（上述专业化允许您使用std::unordered_set<std::array<int, 10>>等）

如果矩阵行不带有等式谓词，您还可std::equal_to以为矩阵行添加一个特化。

于 2011-11-30T23:46:08.137 回答