No hash function in C++ STL (std::unordered_set) for std::list?

Question

I am trying to create a unordered_set of list of string .

But I am stuck with this error:

/usr/include/c++/4.8/bits/hashtable_policy.h:1103:22: error: no match for call to '(const std::hash<std::list<std::basic_string<char> > >) (const std::list<std::basic_string<char> >&)'

I suspect, this is because STL doesn't have a hash function for list of string .

Creating a set of the same type works, but I am worried about efficiency issues (insertion and look up times).

Is there any workaround for this? I don't want to implement a hash function for lists! But may be someone can suggest me some alternative ideas.

Answer 1

class MyClass {
private:
    struct Hash : public std::unary_function< std::list<std::string> const& ls, 
                                                                       size_t> {
        std::size_t operator()(std::list<std::string> const& ls) const;
    };
typedef boost::unordered_set< std::list<std::string>, Hash > MyClassSet;
MyClassSet set_;
};

std::size_t MyClass::Hash::operator()(std::list<std::string> const& ls) const {
    std::size_t seed = 0;
    std::list<std::string>::iterator it = ls.begin();
    while ( it != ls.end()) {
      boost::hash_combine(seed, *it);
      ++it;
    }
    return seed;
}