通过 vector.begin() 命令访问 vector 的第一个元素

Question

I am trying to understand a part of a code in which

(*this).bond.assign(mck.bond.begin(), mck.bond.end())

I want to understand the role of begin() and end () command. I read at different places that it is an iterator index but i couldn't understand its meaning. I tried to understand it by writing a short code but it is not working. Can someone please help me to understand the above line of the code and the role of begin() and end() command.

int main()
{
  vector<int> vec_name;

  vec_name.push_back(10);
  vec_name.push_back(20);
  vec_name.push_back(30);
  vec_name.push_back(40);

  cout << vec_name.size() <<endl;
  cout << vec_name.begin() <<endl;
}

Answer 1

.begin() returns an iterator, not the element or a reference to the element. It's not the same as printing vec_name[i] or using vec_name::front() which returns a reference. So to print the returned value, you need to declare an iterator which receives the return value of vec_name.begin() and then print the iterator.

**EDIT: ** Using your example code, it would be something like this:

int main()
{
  vector<int> vec_name;
  vector<int>::iterator it;

  vec_name.push_back(10);
  vec_name.push_back(20);
  vec_name.push_back(30);
  vec_name.push_back(40);

  cout << vec_name.size() <<endl;
  //cout << vec_name.begin() <<endl; //cannot print iterators directly
  it = vec_name.begin();  //Pass return value to iterator.
  cout << *it << endl;    //Print dereferenced iterator 

}