C++ Boost Unordered_set Compile Error: Illegal Static Data Member and is_always_equal

The error message 'Illegal static data member and is_always_equal' typically occurs when using the boost::unordered_set class in C++ with a non-copyable or non-movable type.

The boost::unordered_set class requires its element type to be either copyable or movable. If the element type is not copyable or movable, the is_always_equal trait must be specialized to boost::true_type for that type.

Here's an example that demonstrates the error and how to fix it:

#include <boost/unordered_set.hpp>

class NonCopyableType {
public:
    NonCopyableType() = default;
    NonCopyableType(const NonCopyableType&) = delete; // Non-copyable
    NonCopyableType& operator=(const NonCopyableType&) = delete; // Non-copyable
};

struct MyHash {
    std::size_t operator()(const NonCopyableType& obj) const {
        // Custom hash function for NonCopyableType
        // ...
    }
};

struct MyEqual {
    bool operator()(const NonCopyableType& lhs, const NonCopyableType& rhs) const {
        // Custom equality operator for NonCopyableType
        // ...
    }
};

// Specialize is_always_equal trait for NonCopyableType
namespace boost {
    template<>
    struct is_always_equal<NonCopyableType> : boost::true_type {};
}

int main() {
    boost::unordered_set<NonCopyableType, MyHash, MyEqual> mySet;
    // ...
    return 0;
}

In this example, NonCopyableType is a non-copyable class, so it cannot be used directly as an element type in boost::unordered_set. To make it work, we specialize the is_always_equal trait for NonCopyableType to boost::true_type, indicating that instances of NonCopyableType are always equal. Additionally, we provide custom hash and equality operators (MyHash and MyEqual) for NonCopyableType.

By doing this, the boost::unordered_set class can handle the non-copyable type correctly, and the compile error should be resolved.