Iterating over an enum (or not)

Hi,

I have the following class:

class ReadIntensity {
public:
  enum Base_Type {
   base_a,base_c,base_g,base_t,base_invalid
  };

  ReadIntensity() : bases(base_count) {
  }

  /// No bounds checking!
  inline double getbase(Base_Type position) const {
    return bases[position];
  }

  /// No bounds checking!
  inline void setbase(Base_Type position,double value) {
    bases[position] = value;
  }

  const static int base_count = 4;
  vector<double> bases;
};

I'm choosing to use an enum here because I feel it represents my
problem well. Sometimes I want to access my bases though name, other
times I'd like to be able to enumerate over the enum. However
iterating over the enum seems messy for example:

#include <vector>
#include <iostream>

using namespace std;

int main() {
  ReadIntensity r;

  r.setbase(ReadIntensity::base_a, 5.5);
  r.setbase(ReadIntensity::base_t, 5.6);
  r.setbase(ReadIntensity::base_g, 5.7);
  r.setbase(ReadIntensity::base_c, 5.8);

  double sum=0;
  ReadIntensity::Base_Type i;
  for(i = ReadIntensity::base_a;i <
ReadIntensity::base_invalid;i=ReadIntensity::Base_Type (i+1)) {
    sum += r.getbase(i);
  }

  cout << sum << endl;
}

Where I'm basically having to hard code in the ends of my enum. Is
there a better way of attacking this problem?
For example would this be a better solution?

#include <vector>
#include <iostream>

using namespace std;

class ReadIntensity {
public:
  static const int base_a = 0;
  static const int base_c = 1;
  static const int base_g = 2;
  static const int base_t = 3;
  static const int base_invalid = 4;
  static const int num_bases = 5;

  ReadIntensity() : bases(num_bases-1) {
  }

  /// No bounds checking!
  inline double getbase(int position) const {
    return bases[position];
  }

  /// No bounds checking!
  inline void setbase(int position,double value) {
    bases[position] = value;
  }

  vector<double> bases;
};

int main() {
  ReadIntensity r;

  r.setbase(ReadIntensity::base_a, 5.5);
  r.setbase(ReadIntensity::base_t, 5.6);
  r.setbase(ReadIntensity::base_g, 5.7);
  r.setbase(ReadIntensity::base_c, 5.8);

  double sum=0;
  int i;
  for(i = 0;i < ReadIntensity::num_bases;i++) {
    sum += r.getbase(i);
  }

  cout << sum << endl;
}

Suggestions and general comments appreciated.