I am trying to use a custom class as key for an `unordered_map`, like the following:

    #include &lt;iostream&gt;
    #include &lt;algorithm&gt;
    #include &lt;unordered_map&gt;

    using namespace std;

    class node;
    class Solution;

    class Node {
	public:
		int a;
		int b; 
		int c;
		Node(){}
		Node(vector&lt;int&gt; v) {
			sort(v.begin(), v.end());
			a = v[0];		
			b = v[1];		
			c = v[2];		
		}

		bool operator==(Node i) {
			if ( i.a==this-&gt;a &amp;&amp; i.b==this-&gt;b &amp;&amp;i.c==this-&gt;c ) {
				return true;
			} else {
				return false;
			}
		}
    };

    int main() {
    	unordered_map&lt;Node, int&gt; m;    

    	vector&lt;int&gt; v;
    	v.push_back(3);
    	v.push_back(8);
    	v.push_back(9);
    	Node n(v);

    	m[n] = 0;

    	return 0;
    }

However, g++ gives me the following error:

    In file included from /usr/include/c++/4.6/string:50:0,
                     from /usr/include/c++/4.6/bits/locale_classes.h:42,
                     from /usr/include/c++/4.6/bits/ios_base.h:43,
                     from /usr/include/c++/4.6/ios:43,
                     from /usr/include/c++/4.6/ostream:40,
                     from /usr/include/c++/4.6/iostream:40,
                     from 3sum.cpp:4:
    /usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::equal_to&lt;_Tp&gt;::operator()(const _Tp&amp;, const _Tp&amp;) const [with _Tp = Node]’:
    /usr/include/c++/4.6/bits/hashtable_policy.h:768:48:   instantiated from ‘bool std::__detail::_Hash_code_base&lt;_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false&gt;::_M_compare(const _Key&amp;, std::__detail::_Hash_code_base&lt;_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false&gt;::_Hash_code_type, std::__detail::_Hash_node&lt;_Value, false&gt;*) const [with _Key = Node, _Value = std::pair&lt;const Node, int&gt;, _ExtractKey = std::_Select1st&lt;std::pair&lt;const Node, int&gt; &gt;, _Equal = std::equal_to&lt;Node&gt;, _H1 = std::hash&lt;Node&gt;, _H2 = std::__detail::_Mod_range_hashing, std::__detail::_Hash_code_base&lt;_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false&gt;::_Hash_code_type = long unsigned int]’
    /usr/include/c++/4.6/bits/hashtable.h:897:2:   instantiated from ‘std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Node* std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_M_find_node(std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Node*, const key_type&amp;, typename std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Hash_code_type) const [with _Key = Node, _Value = std::pair&lt;const Node, int&gt;, _Allocator = std::allocator&lt;std::pair&lt;const Node, int&gt; &gt;, _ExtractKey = std::_Select1st&lt;std::pair&lt;const Node, int&gt; &gt;, _Equal = std::equal_to&lt;Node&gt;, _H1 = std::hash&lt;Node&gt;, _H2 = std::__detail::_Mod_range_hashing, _Hash = std::__detail::_Default_ranged_hash, _RehashPolicy = std::__detail::_Prime_rehash_policy, bool __cache_hash_code = false, bool __constant_iterators = false, bool __unique_keys = true, std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Node = std::__detail::_Hash_node&lt;std::pair&lt;const Node, int&gt;, false&gt;, std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::key_type = Node, typename std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Hash_code_type = long unsigned int]’
    /usr/include/c++/4.6/bits/hashtable_policy.h:546:53:   instantiated from ‘std::__detail::_Map_base&lt;_Key, _Pair, std::_Select1st&lt;_Pair&gt;, true, _Hashtable&gt;::mapped_type&amp; std::__detail::_Map_base&lt;_Key, _Pair, std::_Select1st&lt;_Pair&gt;, true, _Hashtable&gt;::operator[](const _Key&amp;) [with _Key = Node, _Pair = std::pair&lt;const Node, int&gt;, _Hashtable = std::_Hashtable&lt;Node, std::pair&lt;const Node, int&gt;, std::allocator&lt;std::pair&lt;const Node, int&gt; &gt;, std::_Select1st&lt;std::pair&lt;const Node, int&gt; &gt;, std::equal_to&lt;Node&gt;, std::hash&lt;Node&gt;, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true&gt;, std::__detail::_Map_base&lt;_Key, _Pair, std::_Select1st&lt;_Pair&gt;, true, _Hashtable&gt;::mapped_type = int]’
    3sum.cpp:149:5:   instantiated from here
    /usr/include/c++/4.6/bits/stl_function.h:209:23: error: passing ‘const Node’ as ‘this’ argument of ‘bool Node::operator==(Node)’ discards qualifiers [-fpermissive]
    make: *** [threeSum] Error 1
        
I guess, I need the tell C++ how to hash class `Node`, however, I am not quite sure how to do it. How can I accomplish this tasks?

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C++ unordered_map using a custom class type as the key