Question

I have the following C++ code:

#include <iostream>
#include <google/dense_hash_map>
#include <string.h>

using google::dense_hash_map;      // namespace where class lives by default
using std::cout;
using std::endl;
using std::tr1::hash;  // or __gnu_cxx::hash, or maybe tr1::hash, depending on your OS

struct eqstr
{
  bool operator()(const char* s1, const char* s2) const
  {
    return (s1 == s2) || (s1 && s2 && strcmp(s1, s2) == 0);
  }
};

int main(void){
  dense_hash_map<const int, const char*, hash<const int>, eqstr> months;

  months.set_empty_key(0);
  months[1234] = "1234";

  cout << "1234:" << months[1234] << endl;
}

As you can see, I’m implementing a hash table using Google’s sparsehash library.

I am using integers as the key and strings as the value.

But I keep getting the following compilation error that I can’t get to the bottom of:

make all Building file: ../src/Main.cpp Invoking: GCC C++ Compiler
g++ -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF”src/Main.d”
-MT”src/Main.d” -o”src/Main.o” “../src/Main.cpp” In file included from
/usr/local/include/google/dense_hash_map:104:0,
from ../src/Main.cpp:2:
/usr/local/include/google/sparsehash/densehashtable.h: In member
function ‘bool google::dense_hashtable::KeyInfo::equals(const key_type&,
const key_type&) const [with Value = std::pair, Key = int, HashFcn = std::tr1::hash, ExtractKey =
google::dense_hash_map,
eqstr>::SelectKey, SetKey = google::dense_hash_map, eqstr>::SetKey, EqualKey = eqstr, Alloc =
google::libc_allocator_with_realloc

, key_type = int]’:
/usr/local/include/google/sparsehash/densehashtable.h:1306:32:
instantiated from ‘bool google::dense_hashtable::equals(const key_type&, const
key_type&) const [with Value = std::pair, Key
= int, HashFcn = std::tr1::hash, ExtractKey =
google::dense_hash_map,
eqstr>::SelectKey, SetKey = google::dense_hash_map, eqstr>::SetKey, EqualKey = eqstr, Alloc =
google::libc_allocator_with_realloc
, key_type = int]’
/usr/local/include/google/sparsehash/densehashtable.h:514:5:
instantiated from ‘void google::dense_hashtable::set_empty_key(google::dense_hashtable::const_reference) [with Value =
std::pair, Key = int, HashFcn =
std::tr1::hash, ExtractKey = google::dense_hash_map, eqstr>::SelectKey, SetKey =
google::dense_hash_map,
eqstr>::SetKey, EqualKey = eqstr, Alloc =
google::libc_allocator_with_realloc
, google::dense_hashtable::const_reference = const std::pair&]’ /usr/local/include/google/dense_hash_map:298:5:
instantiated from ‘void google::dense_hash_map::set_empty_key(google::dense_hash_map::key_type&) [with Key = int, T = const
char*, HashFcn = std::tr1::hash, EqualKey = eqstr, Alloc =
google::libc_allocator_with_realloc
, google::dense_hash_map::key_type
= int]’ ../src/Main.cpp:21:25: instantiated from here
/usr/local/include/google/sparsehash/densehashtable.h:1293:39: error:
invalid conversion from ‘google::dense_hashtable, int, std::tr1::hash, google::dense_hash_map, eqstr,
google::libc_allocator_with_realloc

::SelectKey, eqstr,
google::libc_allocator_with_realloc
, google::dense_hash_map,
eqstr, google::libc_allocator_with_realloc > >::SetKey, eqstr,
google::libc_allocator_with_realloc
, eqstr, google::libc_allocator_with_realloc > >::key_type’ to ‘const char*’
/usr/local/include/google/sparsehash/densehashtable.h:1293:39: error:
initializing argument 1 of ‘bool eqstr::operator()(const char*, const
char*) const’
/usr/local/include/google/sparsehash/densehashtable.h:1293:39: error:
invalid conversion from ‘google::dense_hashtable, int, std::tr1::hash, google::dense_hash_map, eqstr,
google::libc_allocator_with_realloc
::SelectKey, eqstr,
google::libc_allocator_with_realloc
, google::dense_hash_map,
eqstr, google::libc_allocator_with_realloc > >::SetKey, eqstr,
google::libc_allocator_with_realloc
, eqstr, google::libc_allocator_with_realloc > >::key_type’ to ‘const char*’
/usr/local/include/google/sparsehash/densehashtable.h:1293:39: error:
initializing argument 2 of ‘bool eqstr::operator()(const char*, const
char*) const’ make: * [src/Main.o] Error 1

It seems very verbose and I can’t make any sense of it.

I should add that when I use strings as the key and integers as the value, it works fine:

dense_hash_map<const char*, int, hash<const char*>, eqstr> months;
...
months["february"] = 2;   //works fine

Anyone got any ideas?

Many thanks in advance,

---

Edit: got it working now!

#include <iostream>
#include <google/dense_hash_map>
#include <string.h>

using google::dense_hash_map;      // namespace where class lives by default
using std::cout;
using std::endl;
using std::tr1::hash;  // or __gnu_cxx::hash, or maybe tr1::hash, depending on your OS

struct eqstr
{
  bool operator()(int s1, int s2) const
  {
    return (s1 == s2); //|| (s1 && s2 && strcmp(s1, s2) == 0);
  }
};

int main(void){
  dense_hash_map<int, const char*, hash<int>, eqstr> months;

  months.set_empty_key(0);
  months[1234] = "1234";

  cout << "1234:" << months[1234] << endl;
}

Totally forgot about editing the eqstr struct to accomodate the new data types… bangs head off desk

Answer 1

As you pointed out by yourself, it works if you use const char* as key. The hashmap indeed requires a hashing function to hash the keys to buckets and a comparison function to establish a strict weak ordering within the buckets – all this for the key type, the value type is just stored! So to make it work, define a comparison function for int (I don’t know whether const int is fine with google::dense_hash_map, I think it should be int).