unordered_map constructor error (equal_to templated function)

Both hashing_func and key_equal_fn should be functor objects (and not functions). In addition, their types must be provided to the unordered_map template, that is, the map should have this type:

unordered_map<string, string, hashing_func, key_equal_fn> 

where hashing_func and key_equal_fn are functor classes:

struct hashing_func { unsigned long operator()(const string& key) const { unsigned long hash = 0; for(size_t i=0; i<key.size(); i++) hash += (71*hash + key[i]) % 5; return hash; } }; struct key_equal_fn { bool operator()(const string& t1, const string& t2) const { return !(t1.compare(t2)); } }; 

Then, mymap is defined in this way:

typedef unordered_map<string, string, hashing_func, key_equal_fn> MapType; MapType::size_type n = 5; MapType mymap(n, hashing_func(), key_equal_fn()); 

Alternatively, hashing_func and/or key_equal_fn can be functions but you have to wrapp them into std::function objects. That is,

unsigned long hashing_func(const string& key) { unsigned long hash = 0; for(size_t i=0; i<key.size(); i++) hash += (71*hash + key[i]) % 5; return hash; } bool key_equal_fn(const string& t1, const string& t2){ return !(t1.compare(t2)); } 

and define mymap in this way

typedef unordered_map<string, string, std::function<unsigned long(const string&)>, std::function<bool(const string&, const string&)>> MapType; MapType::size_type n = 5; MapType mymap(n, hashing_func, key_equal_fn); 

If you wish, you can use lambdas and refrain from writing the two funcions or functor classes:

typedef unordered_map<string, string, std::function<unsigned long(const string&)>, std::function<bool(const string&, const string&)>> MapType; MapType mymap(n, [](const string& key) -> unsigned long { unsigned long hash = 0; for(size_t i=0; i<key.size(); i++) hash += (71*hash + key[i]) % 5; return hash; }, [](const string& t1, const string& t2) { return !(t1.compare(t2)); }); 

Finally, my favorite is an all-lambdas solution

auto hashing_func = [](const string& key) -> unsigned long { unsigned long hash = 0; for(size_t i=0; i<key.size(); i++) hash += (71*hash + key[i]) % 5; return hash; }; auto key_equal_fn = [](const string& t1, const string& t2) { return !(t1.compare(t2)); }; typedef unordered_map<string, string, decltype(hashing_func), decltype(key_equal_fn)> MapType; MapType::size_type n = 5; MapType mymap(n, hashing_func, key_equal_fn);