初始化unordered_map时发生错误

Question

我尝试如下初始化unordered_map：

unordered_multimap<struct index,int>index_hash;

，结构定义为：

struct index
{
    int i;
    int j;
    int k;
    index(int a, int b, int c)
    {
        i = a;
        j = b;
        k = c;
    }
};

当我在Visual Studio上编码时，根本没有语法错误，并且在编译后发生了错误：

error: no match for call to ‘(const std::hash<index>) (const index&)’ noexcept(declval<const _Hash&>()(declval<const _Key&>()))>
                                  ^ In file included from /usr/include/c++/5/bits/move.h:57:0,
                  from /usr/include/c++/5/bits/stl_pair.h:59,
                  from /usr/include/c++/5/bits/stl_algobase.h:64,
                  from /usr/include/c++/5/vector:60,
                  from neighbors/../../cpp_utils/cloud/cloud.h:22,
                  from neighbors/neighbors.h:3,
                  from neighbors/neighbors.cpp:2: /usr/include/c++/5/type_traits: In instantiation of ‘struct
 std::__not_<std::__and_<std::__is_fast_hash<std::hash<index> >,
 std::__detail::__is_noexcept_hash<index, std::hash<index> > > >’:
 /usr/include/c++/5/bits/unordered_map.h:853:67:   required from ‘class
 std::unordered_multimap<index, int>’ neighbors/neighbors.cpp:686:38:  
 required from here