const_iterator与const的iterator的区别

const_iterator与const的iterator的区别:
通过简单的代码区别,更能说明问题,希望你能明白
  vector<int> ivec;
  vector<int>::const_iterator citer1 = ivec.begin();
  const vector<int>::iterator citer2 = ivec.begin();
  *citer1 = 1; //error
  *citer2 = 1; //right
  ++citer1; //right
  ++citer2; //error