How to use unordered_set::contains() with a custom class?

I’m using std::unordered_set for the first time with a custom type, and I can’t figure out what I’m missing to make contains() compile for my set.

I basically have a class which looks something like this:

class MyClass
{
// Ctor/Dtor
public:
    MyClass(const std::string& name /* + other params in my real use case */) : m_name(name) {}
    ~MyClass() = default;

// Members
private:
    std::string m_name;

// Operators
public:
    // Equal operators
    bool operator==(const MyClass &other) const
    {
        return m_name == other.m_name;
    }

    bool operator==(const std::string& other) const
    {
        return m_name == other;
    }

    bool operator==(const char* other) const
    {
        return m_name == other;
    }

// Functors
public:
    // Hash functor
    struct Hash
    {
        std::size_t operator()(const MyClass &class_) const
        {
            return std::hash<std::string>()(class_.m_name);
        }

        std::size_t operator()(const std::string &name) const
        {
            return std::hash<std::string>()(name);
        }

        std::size_t operator()(const char* name) const 
        {
            return std::hash<std::string>()(name);
        }
    };

    // Equal functor
    struct Equal
    {
        bool operator()(const MyClass &lhs, const MyClass &rhs) const
        {
            return lhs.m_name == rhs.m_name;
        }

        bool operator()(const MyClass &lhs, const std::string &rhs) const
        {
            return lhs.m_name == rhs;
        }

        bool operator()(const std::string &lhs, const MyClass &rhs) const
        {
            return lhs == rhs.m_name;
        }

        bool operator()(const MyClass &lhs, const char* rhs) const
        {
            return lhs.m_name == rhs;
        }

        bool operator()(const char* lhs, const MyClass &rhs) const
        {
            return lhs == rhs.m_name;
        }
    };

};

Using this class, I want to create a std::unordered_set and check if the element who’s name is key1 exists. To do so, I’d like to use the following code:

int main()
{
    // I tried both, but none of them seem to work
    std::unordered_set<MyClass, MyClass::Hash, MyClass::Equal> set;
    // std::unordered_set<MyClass, MyClass::Hash> set;

    // Add sample elements to the set
    set.emplace("key1");
    set.emplace("key2");

    // Check if the set contains "key1"
    if (set.contains("key1")) // Compile error on this line. Why does this not work?
    {
        std::wcout << L"set contains key1." << std::endl;
    } else {
        std::wcout << L"set doesn't contain key1." << std::endl;
    }

    return 0;
}

But I keep getting a compiler error on the call to contains() saying:

error: no matching function for call to ‘std::unordered_set<MyClass, MyClass::Hash, MyClass::Equal>::contains(const char [5])’ […]

I can’t figure out why, and cannot find any resource about it.

I guess I’m missing something, since the following code does work without issue using std::string:

int main()
{
    std::unordered_set<std::string> set;
   
    // Add sample elements to the set
    set.emplace("key1");
    set.emplace("key2");

    // Check if the set contains "key1"
    if (set.contains("key1")) // Why does this not work?
    {
        std::wcout << L"set contains key1." << std::endl;
    } else {
        std::wcout << L"set doesn't contain key1." << std::endl;
    }

    return 0;
}

>Solution :

Your issue is that set.contains("key1") is passing in a parameter that is not a MyClass object, which only works when both Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. This means you need to change your Hash and Equals structs to the following:

struct Hash
{
    using is_transparent = std::true_type;
    
    std::size_t operator()(const MyClass &class_) const
    {
        return std::hash<std::string>()(class_.m_name);
    }

    std::size_t operator()(const std::string &name) const
    {
        return std::hash<std::string>()(name);
    }

    std::size_t operator()(const char* name) const 
    {
        return std::hash<std::string>()(name);
    }
};

// Equal functor
struct Equal
{
    using is_transparent = std::true_type;
    
    bool operator()(const MyClass &lhs, const MyClass &rhs) const
    {
        return lhs.m_name == rhs.m_name;
    }

    bool operator()(const MyClass &lhs, const std::string &rhs) const
    {
        return lhs.m_name == rhs;
    }

    bool operator()(const std::string &lhs, const MyClass &rhs) const
    {
        return lhs == rhs.m_name;
    }

    bool operator()(const MyClass &lhs, const char* rhs) const
    {
        return lhs.m_name == rhs;
    }

    bool operator()(const char* lhs, const MyClass &rhs) const
    {
        return lhs == rhs.m_name;
    }
};

That will now inform std::unordered_set that it can use your hasher and comparator with objects other than the key type.