Inserting objects into hash table (C++)

Question

This is my first time making a hash table. I'm trying to associate strings (the keys) with pointers to objects (the data) of class Strain.

// Simulation.h
#include <ext/hash_map>
using namespace __gnu_cxx;

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

...
hash_map< const char *, Strain *, hash< const char * >, struct eqstr > liveStrainTable;

In the Simulation.cpp file, I attempt to initialize the table:

string MRCA;
for ( int b = 0; b < SEQ_LENGTH; b++ ) {
  int randBase = rgen.uniform(0,NUM_BASES); 
  MRCA.push_back( BASES[ randBase ] );
}
Strain * firstStrainPtr;
firstStrainPtr = new Strain( idCtr, MRCA, NUM_STEPS );
liveStrainTable[ MRCA ]= firstStrainPtr;

I get an error message that reads "no match for ‘operator[]’ in ‘((Simulation*)this)->Simulation::liveStrainTable[MRCA]’." I've also tried using "liveStrainTable.insert(...)" in different ways, to no avail.

Would really love some help on this. I'm having a difficult time understanding the syntax appropriate for SGI hash_map, and the SGI reference barely clarifies anything for me. Thanks.

Answer 1

Try liveStrainTable[ MRCA.c_str() ]= firstStrainPtr;. It expects const char * as type of key value, but MRCA has type string.

Another way is to change liveStrainTable to:

hash_map< string, Strain *, hash<string>, eqstr > liveStrainTable;

Answer 2

The hash_map is defined with const char * as the key type and you are using an std::string as the key when accessing. These are 2 different types, the template did not build an operator for the second type, so this is an error. Use std::string for the hashmap definition or use MRCA.c_str()

Answer 3

Right now, you have a type mis-match. You're passing MRCA (a string) where a char const * is expected. You can either use c_str() to get a char const * from the string, or (far better) change the definition of your hash table to take a string as its key type.

Answer 4

Others answered your direct question, but may I suggest using unordered_map instead - it's coming with the next version of the STL and is supported by all major compilers.

Answer 5

Thank you for catching this! It's still not working, unfortunately... I now have

struct eqstr {
  bool operator()(string s1, string s2) const {
    return ( s1.compare(s2) == 0 );
  }
};

  hash_map< string, Strain *, hash< string >, eqstr > liveStrainTable;

and

  liveStrainTable[ MRCA ]= firstStrainPtr;

but now I'm getting a "no match for call to '(const __gnu_cxx::hash) (const std::basic_string, std::allocator >&)’"

I feel like there might be something fundamental about strings and chars that I'm missing.

Answer 6

hash_map is not part of STL. There's no implementation provided for hash, or in other words, the hash_map can't hash strings by default. You need your own hash function. T

Try:

typedef struct {
  size_t operator()( const string& str ) const {
     return __gnu_cxx::__stl_hash_string( str.c_str() );
  }
} strhash;

hash_map< string, Strain *, strhash, eqstr > liveStrainTable;