CODEWITHSUNDEEP
c++unordered-map
Euklides
Euklides

Reputation: 584

insert custom class into unordered_map c++

I have a problem which probably has a simple solution but I have looked around for a while and still no success.

I have a simple class as follows:

class Node{
public:
int value;
bool visited;
Node(int argvalue) {value = argvalue;visited = false; }
bool operator==(const Node& n) {return (value == n.value);}
};

class MyHash {
size_t operator()(const Node& x) const { return std::hash<int>()(x.value); }
};

Now when I try to insert I get error and can not understand why? Did I implement my hash-function incorrectly or is the equal operator == not sufficient?

unordered_map<Node, int, MyHash> my_items;
my_items.insert(Node(33), 894);

Upvotes: 2

Views: 9878

Answers (2)

juanchopanza
juanchopanza

Reputation: 227418

None of the two-parameter insert methods matches insert(key_type, mapped_type), which is what you are attempting.

The map holds std::pair<const Key, T>, so you need to insert a pair, either explicitly:

my_items.insert(std::make_pair(Node(33), 894));

or using brace initialization:

my_items.insert({Node(33), 894});

You could use the std::unordered_map::emplace member function, which allows you to pass the constructor arguments of a pair:

my_items.emplace(Node(33), 894);

Other things:

  • bool Node::operator== should be const (a comparison should not change the objects being compared)
  • size_t MyHash::operator()(...) const should be public.

Upvotes: 10

Yuushi
Yuushi

Reputation: 26040

You have a couple of errors. Firstly, operator() is private in MyHash since it is a class and default access for classes is private. Either mark it as public or change it to struct MyHash.

Secondly, you cannot insert values like that. It will need to be something like my_items.insert(std::make_pair(Node(33), 894));

Upvotes: 0

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