Question

I’m using a generic hashtable class however when I declare string in my main it gives the following error. I assumed the problem caused on “typename” part however when I changed into “class” the problem didn’t solved.

My main:

HashTable<string> H;

My header:

#ifndef QUADRATIC_PROBING_H
#define QUADRATIC_PROBING_H

#include <vector>
#include <string>
using namespace std;

int nextPrime( int n );

// QuadraticProbing Hash table class
//
// CONSTRUCTION: an approximate initial size or default of 101
//
// ******************PUBLIC OPERATIONS*********************
// bool insert( x )       --> Insert x
// bool remove( x )       --> Remove x
// bool contains( x )     --> Return true if x is present
// void makeEmpty( )      --> Remove all items
// int hash( string str ) --> Global method to hash strings

template <typename HashedObj>
class HashTable
{
  public:
    explicit HashTable( int size = 101 ) : array( nextPrime( size ) )
      { makeEmpty( ); }

    bool contains( const HashedObj & x ) const
    {
        return isActive( findPos( x ) );
    }

    void makeEmpty( )
    {
        currentSize = 0;
        for( size_t i = 0; i < array.size( ); i++ )
            array[ i ].info = EMPTY;
    }

    bool insert( const HashedObj & x )
    {
            // Insert x as active
        size_t currentPos = findPos( x );
        if( isActive( currentPos ) )
            return false;

        array[ currentPos ] = HashEntry( x, ACTIVE );

            // Rehash; see Section 5.5
        if( ++currentSize > array.size( ) / 2 )
            rehash( );

        return true;
    }

    bool remove( const HashedObj & x )
    {
        int currentPos = findPos( x );
        if( !isActive( currentPos ) )
            return false;

        array[ currentPos ].info = DELETED;
        return true;
    }

    enum EntryType { ACTIVE, EMPTY, DELETED };

  private:
    struct HashEntry
    {
        HashedObj element;
        EntryType info;

        HashEntry( const HashedObj & e = HashedObj( ), EntryType i = EMPTY )
          : element( e ), info( i ) { }
    };

    vector<HashEntry> array;
    int currentSize;

    bool isActive( int currentPos ) const
      { return array[ currentPos ].info == ACTIVE; }

    int findPos( const HashedObj & x ) const
    {
        int offset = 1;
        size_t currentPos = myhash( x );

          // Assuming table is half-empty, and table length is prime,
          // this loop terminates
        while( array[ currentPos ].info != EMPTY &&
                array[ currentPos ].element != x )
        {
            currentPos += offset;  // Compute ith probe
            offset += 2;
            if( currentPos >= array.size( ) )
                currentPos -= array.size( );
        }

        return currentPos;
    }

    void rehash( )
    {
        vector<HashEntry> oldArray = array;

            // Create new double-sized, empty table
        array.resize( nextPrime( 2 * oldArray.size( ) ) );
        for( size_t j = 0; j < array.size( ); j++ )
            array[ j ].info = EMPTY;

            // Copy table over
        currentSize = 0;
        for( size_t i = 0; i < oldArray.size( ); i++ )
            if( oldArray[ i ].info == ACTIVE )
                insert( oldArray[ i ].element );
    }
    int myhash( const HashedObj & x ) const
    {
        int hashVal = hash( x );

        hashVal %= array.size( );
        if( hashVal < 0 )
            hashVal += array.size( );

        return hashVal;
    }
};

int hash( const string & key );
int hash( int key );

#endif

And I’m getting the following errors:

    d:\users\suuser\documents\visual studio 2010\projects\cs300-hw3\cs300-hw3\quadraticprobing.h(50): warning C4018: '>' : signed/unsigned mismatch
1>          d:\users\suuser\documents\visual studio 2010\projects\cs300-hw3\cs300-hw3\quadraticprobing.h(41) : while compiling class template member function 'bool HashTable<HashedObj>::insert(const HashedObj &)'
1>          with
1>          [
1>              HashedObj=std::string
1>          ]
1>          d:\users\suuser\documents\visual studio 2010\projects\cs300-hw3\cs300-hw3\compressor.cpp(130) : see reference to class template instantiation 'HashTable<HashedObj>' being compiled
1>          with
1>          [
1>              HashedObj=std::string
1>          ]
1>d:\users\suuser\documents\visual studio 2010\projects\cs300-hw3\cs300-hw3\quadraticprobing.h(120): error C2872: 'hash' : ambiguous symbol
1>          could be 'hash'
1>          or       'c:\program files\microsoft visual studio 10.0\vc\include\xfunctional(763) : std::hash'
1>          d:\users\suuser\documents\visual studio 2010\projects\cs300-hw3\cs300-hw3\quadraticprobing.h(119) : while compiling class template member function 'int HashTable<HashedObj>::myhash(const HashedObj &) const'
1>          with
1>          [
1>              HashedObj=std::string
1>          ]

Answer 1

The error is telling you that you have a clash between your hash(const std::string&) function and the std::hash overload with the same argument. This is one of the reasons you should avoid using namespace std.

The usual advice: Never use using namespace std; in headers.

My personal advice: Never use using namespace std, anywhere.