Hash Object problem in C++

Question

In some C++ program I have two classes - Abc and Xyz. The Abc class contains an integer member Foo, like in the following listing:

class Abc
{
 public:
 int Foo;
 Abc(int f)
 {
  Foo = f;
 }
};
class Xyz{};

I'm using a map<Abc, Xyz> but the performance is extremly poor. That's why I'd like to use a hash_map<Abc, Xyz> (Dev-Cpp extension libraries) and some tricky hashing. Let's assume the following AbcHash unary function object is going to act well as a hash generator:

class AbcHash
{
 public:
 size_t operator() (const Abc &a)
 {
  return a.Foo % 123;
 } 
};

For consistency with STL I've also implemented an AbcCmp comparator:

class AbcCmp
{
 public:
 bool operator() (const Abc &a1, const Abc &a2)
 {
  return (a1.Foo == a2.Foo);
 }
};

I had tested both Abc and Xyz classes, when I wanted to use the hash_map at last. That's my main:

int main(int argc, char *argv[])
{
 hash_map myMap;

 Abc a(10);
 a.Foo = 23;

 Xyz x();

 myMap[a] = x; // this line fails the compilation
}

The last line causes some strange compilation errors:

Compiler: Default compiler
Building Makefile: "C:\user\hdr\hashfunc\Makefile.win"
Executing  make...
make.exe -f "C:\user\hdr\hashfunc\Makefile.win" all
g++.exe -c main.cpp -o main.o -I"C:/Dev-Cpp/lib/gcc/mingw32/3.4.2/include"  -I"C:/Dev-Cpp/include/c++/3.4.2/backward"  -I"C:/Dev-Cpp/include/c++/3.4.2/mingw32"  -I"C:/Dev-Cpp/include/c++/3.4.2"  -I"C:/Dev-Cpp/include"   

In file included from C:/Dev-Cpp/include/c++/3.4.2/backward/hash_map.h:59,
                 from main.cpp:2:
C:/Dev-Cpp/include/c++/3.4.2/backward/backward_warning.h:32:2: warning: #warning This file includes at least one deprecated or antiquated header. Please consider using one of the 32 headers found in section 17.4.1.2 of the C++ standard. Examples include substituting the  header for the  header for C++ includes, or  instead of the deprecated header . To disable this warning use -Wno-deprecated.
main.cpp: In function `int main(int, char**)':
main.cpp:45: error: no match for 'operator=' in '(&myMap)->__gnu_cxx::hash_map::operator[] [with _Key = Abc, _Tp = Xyz, _HashFcn = AbcHash, _EqualKey = AbcCmp, _Alloc = std::allocator](((const Abc&)((const Abc*)(&a)))) = x'
main.cpp:14: note: candidates are: Xyz& Xyz::operator=(const Xyz&)

C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h: In member function `size_t __gnu_cxx::hashtable::_M_bkt_num_key(const _Key&, size_t) const [with _Val = std::pair, _Key = Abc, _HashFcn = AbcHash, _ExtractKey = std::_Select1st >, _EqualKey = AbcCmp, _Alloc = std::allocator]':
C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h:523:   instantiated from `size_t __gnu_cxx::hashtable::_M_bkt_num(const _Val&, size_t) const [with _Val = std::pair, _Key = Abc, _HashFcn = AbcHash, _ExtractKey = std::_Select1st >, _EqualKey = AbcCmp, _Alloc = std::allocator]'
C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h:887:   instantiated from `void __gnu_cxx::hashtable::resize(size_t) [with _Val = std::pair, _Key = Abc, _HashFcn = AbcHash, _ExtractKey = std::_Select1st >, _EqualKey = AbcCmp, _Alloc = std::allocator]'
C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h:701:   instantiated from `typename __gnu_cxx::hashtable::reference __gnu_cxx::hashtable::find_or_insert(const _Val&) [with _Val = std::pair, _Key = Abc, _HashFcn = AbcHash, _ExtractKey = std::_Select1st >, _EqualKey = AbcCmp, _Alloc = std::allocator]'
C:/Dev-Cpp/include/c++/3.4.2/ext/hash_map:181:   instantiated from `_Tp& __gnu_cxx::hash_map::operator[](const typename __gnu_cxx::hashtable, _Key, _HashFcn, std::_Select1st >, _EqualKey, _Alloc>::key_type&) [with _Key = Abc, _Tp = Xyz, _HashFcn = AbcHash, _EqualKey = AbcCmp, _Alloc = std::allocator]'
main.cpp:45:   instantiated from here
C:/Dev-Cpp/include/c++/3.4.2/ext/hashtable.h:518: error: passing `const AbcHash' as `this' argument of `size_t AbcHash::operator()(const Abc&)' discards qualifiers

make.exe: *** [main.o] Error 1

Execution terminated

I have completely no idea what can be wrong here. How can it be fixed?

Please post your actual code; your declaration of your hash_map doesn't include the templatized parameters, for instance. Also, you should be using unordered_map from tr1 instead of hash_map. Lastly, you declare a(10) then reassign foo right after. Why not declare a as Abc a(23) in a single step? — Joe
Does your class Abc implement the less than operator? If it didn't that might well be the reason for the performance issues you saw. — Timo Geusch

Unknown Unknown · Accepted Answer · 2009-12-19T13:53:35

My first approach would be to identify why map's performance is poor. Maps are actually pretty good all round performers - they may wel outperform hash tables in some circumstances. Changing containers without knowing the problem is not a good idea.

Then I would junk DevC++. This antique is extremely buggy and is no longer under active development - I'd be especially dubious about using any libraries that came with it. I'd switch to a nightly build of Code::Blocks, the Twilight Dragon build of GCC, and (if I still wanted a hash) to either the TR1 or the Boost hashtable implementations.

Hash Object problem in C++

2 Answers