modifying a private int in a const function issue

Question

My code is below and i am having an error with numberCollisions += collisionNum; The error states, 'numberCollisions' cannot be modified because it is being used through a const object. Is this error just popping up because it is a const function? Thanks for the help.

int HashTable_qp::findPos( const string & x ) const
{
    /* 1*/      int collisionNum = 0;
    /* 2*/      int currentPos = myhash( x, array.size( ) );

    /* 3*/      while( array[ currentPos ].info != EMPTY &&
        array[ currentPos ].element != x )
    {
        /* 4*/          currentPos += 2 * ++collisionNum - 1;  // Compute ith probe
        /* 5*/          if( currentPos >= array.size( ) )
            /* 6*/              currentPos -= array.size( );
    }
    numberCollisions += collisionNum;
    /* 7*/      return currentPos;
}

Answer 1

Yes -- for something like this where you're modifying an internal bookkeeping number that doesn't affect the logical state of the object, you may want to consider defining the variable with the mutable qualifier:

class HashTable_op { 
    mutable int numberCollisions;

};

Answer 2

Yes, exactly because of this reason. Constant method implies you won't change the instance, but you do, since your findPos is supposed to change the instance. So just remove const and move on.