unordered_map :: find不适用于STL类型

Question

for C++ basic types, it works

unordered_map<string, int> m;
unordered_map<string, int>::iterator itr;
string s;
itr = m.find(s);

It compiles with no problem.

However, when I try

unordered_map<set<string>, int> m;
unordered_map<set<string>, int>::iterator itr;
set<string> s;

itr = m.find(s);

The compiler complains something that I can't read. Does anyone know what is going on? Do I have to use exception here?

g++ output:

/tmp/ccdwrdVP.o: In function `std::__detail::_Hash_code_base<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::pair<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const, int>, std::_Select1st<std::pair<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const, int> >, std::equal_to<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >, std::hash<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, false>::_M_hash_code(std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&) const':
test_unordermap_find.cpp:(.text._ZNKSt8__detail15_Hash_code_baseISt3setISsSt4lessISsESaISsEESt4pairIKS5_iESt10_Select1stIS8_ESt8equal_toIS5_ESt4hashIS5_ENS_18_Mod_range_hashingENS_20_Default_ranged_hashELb0EE12_M_hash_codeERS7_[std::__detail::_Hash_code_base<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >, std::pair<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const, int>, std::_Select1st<std::pair<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const, int> >, std::equal_to<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >, std::hash<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, false>::_M_hash_code(std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&) const]+0x37): undefined reference to `std::hash<std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > > >::operator()(std::set<std::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::less<std::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > >) const'
collect2: ld returned 1 exit status

Answer 1

std::unordered_map is implemented by means of a hash table, so it needs a hash function that can be applied to its key type. The default hash function is the std::hash template:

template < class Key,
           class T,
           class Hash = std::hash<Key>,
           class Pred = std::equal_to<Key>,
           class Alloc = std::allocator< std::pair<const Key,T> >
           > class unordered_map;

However, std::hash is not defined for std::set . Therefore you need to either provide a definition of std::hash<set<string>> , or else specify your own hash function using the third template parameter of unordered_map .