Re: Help migrating hash_set to c++0x

Paulo da Silva <>
Tue, 21 Dec 2010 17:38:17 +0000
Em 21-12-2010 16:05, Howard Hinnant escreveu:

On Dec 20, 11:25 pm, Paulo da Silva
<> wrote:

Em 21-12-2010 03:37, Paulo da Silva escreveu:

Em 20-12-2010 22:59, Paulo da Silva escreveu:

I just saw (wikipedia for example) that unordered_set should implement
the same behaviour as that of hash_set. Neverthless it is not working in
this code!

#include <memory>
#include <unordered_set>
#include <iostream>

using namespace std;

class Foo
   string s;
   Foo(char const * const sc): s(sc) {}

class eqf
   inline bool operator()(Foo const &s1,Foo const &s2) const
   { return (s1.s==s2.s);

class hf
   inline size_t operator()(Foo const &x) const
   { return hash<char const *>()(x.s.c_str());

// typedef __gnu_cxx::hash_set<Foo,hf,eqf> MSet;
typedef unordered_set<Foo,hf,eqf> MSet;

int main()
{ MSet mc;
   pair<MSet::iterator,bool> r;
   // OK expected and obtained
   cout << "xxxx " << (r.second?"OK":"BAD") << endl;
   // Does not allow duplicates ...
   // BAD (duplicate) expected but OK obtained
   cout << "zzzz " << (r.second?"OK":"BAD") << endl;
   MSet::const_iterator it=mc.find(Foo("xxxx"));
   for (it=mc.begin();it!=mc.end();++it)
           cout << it->s << endl;
   return 0;

Anything wrong? Better way to implement?
Thanks for any comments.

Replacing class hf with this works.
class hf
        inline size_t operator()(Foo const &s) const
        { return hash<string const &>()(s.s);


I still don't understand why the previous example stopped to work!

I suspect it was a change in the behavior of the hash<char const*>
function. In C++0x std::hash<char const*> simply hashes the pointer,
not a null terminated string. Therefore using std::hash<char const*>
combined with your eqf would give you the situation that two Foo's
could compare equal but hash to different buckets. This is a
situation that will essentially corrupt your container. Your fix
(using hash<string const &>) is correct.

That makes sense and explains the whole thing.
Nevertheless it seems a weird behaviour of hash<char const*>.
Thanks Howard.

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