boost::unordered_set of char16_t strings

Why does following

#include <string>
#include <boost/unordered_set.hpp>

int main()
{    
    typedef boost::unordered_set<std::string> unordered_set;
    unordered_set animals;

    animals.emplace("cat");
    animals.emplace("shark");
    animals.emplace("spider");
    return 0;
}

work and following results in too many compilation errors.

#include <string>
#include <boost/unordered_set.hpp>

int main()
{    
    typedef boost::unordered_set<std::u16string> unordered_set;
    unordered_set animals;

    animals.emplace("cat");
    animals.emplace("shark");
    animals.emplace("spider");
    return 0;
}

Also, what's the solution for this ? Do I need to write my own hash_function and operator== in function objects as mentioned here ?

The operator== is not a concern, because it is already defined in the standard library. However, the hash function has to be adapted from the std::hash specialization for std::u16string provided by the standard library, which will work for the std::unordered_* containers, but not Boost's ones.

One solution might be to define the hashing function in the following way:

std::size_t hash_value(std::u16string const &s) {
    return std::hash<std::u16string>{}(s);
}

This wrapper will get you an already written logic wrapped to work nicely with Boost.

Lastly, let me remind you of the availability of the equivalent std::unordered_set container in the C++11 standard library, in case you didn't know about it.