帮助用用户定义的哈希/相等创建unordered_map

C++: Help Creating unordered_map with User-Defined Hash/Equality

本文关键字：创建 unordered map 用户 定义 哈希 帮助      更新时间：2023-10-16

我试图创建一个std::unordered_map，使用用户定义的哈希函数和相等谓词，为集成内置类型的矩阵行。我使用std::bind，因为我需要散列函数和相等函数来处理变量范围。如何使下面的代码按预期的方式编译和工作?我猜我的错误在清单的末尾，在这里我使用std::bind并实例化了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单个位，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，导致错误的结果。此外，我需要哈希函数和等式函数在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。

<>之前 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); } } 之前

错误:

下面是(我认为)在尝试编译时产生的错误中最重要的部分:

/usr/include/c++/4.6/位/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:声明const结构boost::数字:ublas: matrix_row,Boost::numeric::ublas::unbounded_array>>> '

如果你想看全部内容，我已经把它们都放在这里了:

<>之前 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 > > >’

如果你想散列一系列的东西，你需要像hash_combine()这样的东西。我通常从Boost中提取这个函数(令人惊讶的是，它没有包含在标准中!)以下是我如何在std::array s上使用它，我相信您可以将其操作成在矩阵行上工作的东西:

#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;
        }
    };
}

(以上专门化允许您使用std::unordered_set<std::array<int, 10>>等)

如果矩阵行没有相等谓词，您还可以为矩阵行添加std::equal_to专门化。

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