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CS253 Iterator Interface

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Big Picture

• How do you write a container? Something like vector, or list?
• This is CS253, so let’s just assume the Data Structures aspect.
• Specifically, I want a container that works with a for loop:

deque<int> con = {11, 22, 33, 44, 55};
for (auto value : con)
    cout << value << ' ';

11 22 33 44 55 

• The container must have these attributes:
  • a type iterator, with *, !=, and ++.
  • methods .begin() and .end() that return an iterator
• Really, that’s it! The devil is in the details.
• This has nothing to do with templates. That’s a whole separate aspect.

Why

• Why does a for loop need iterator, .begin(), and .end()?
• Because a for loop gets translated, like this:

deque<int> con = {11, 22, 33, 44, 55};
for (auto value : con)
    cout << value << ' ';

11 22 33 44 55 

deque<int> con = {11, 22, 33, 44, 55};
for (auto it=con.begin(); it != con.end(); ++it) {
    auto value = *it;
    cout << value << ' ';
}

11 22 33 44 55 

• Really, it uses the begin() & end() free functions, which call the methods, and it saves the end() value in a variable to prevent computing it multiple times. Details!

Typical structure

    class Container {
      public:
        using value_type = whatever type this contains;
        class iterator {
          public:
            iterator(…);			      // establish a location
            value_type &operator*();		      // get value based on state
            iterator &operator++();		      // go to the next item
            bool operator!=(const iterator &) const;  // compare two iterators
          private:
            declare sufficient data to access elements in the container;
        }
        iterator begin() {
            return iterator(enough data to indicate start of container);
        }
        iterator end() {
            return iterator(enough data to indicate end of container);
        }
    };

Design

• Don’t just start coding like a chicken with its head cut off.
• Stop. Plan.
• Know what you’re doing before you do it. Don’t be an Underwear Gnome.
• It’s all about the iterator type, which encapsulates the current position within the container.
• The essential questions are:
  • What data does the iterator need to establish a position?
  • What iterator value indicates .begin()?
  • What iterator value indicates .end()?
  • How do you get the value?
  • How does the iterator increment?

Answers, for a string

Since a string s is essentially a dynamic array of char, the answers are easy:

• What data does the iterator need to establish a position?
  • char *pointer
• What iterator value indicates .begin()?
  • pointer = &s[0]
• What iterator value indicates .end()?
  • pointer = &s[s.size()]
• How do you get the value?
  • *pointer
• How does the iterator increment?
  • ++pointer

Different answers, for a string

Here’s another way to handle a string s:

• What data does the iterator need to establish a position?
  • string *pointer; size_t index;
• What iterator value indicates .begin()?
  • pointer=&s; index=0;
• What iterator value indicates .end()?
  • pointer=&s; index=s.size();
• How do you get the value?
  • (*pointer)[index]
• How does the iterator increment?
  • ++index

Answers, for a list

A list is a doubly-linked list of nodes, each of which is a struct containing the data and a pointer to the next struct or nullptr.

• What data does the iterator need to establish a position?
  • Node *pointer
• What iterator value indicates .begin()?
  • pointer = the first node in the list, or nullptr
• What iterator value indicates .end()?
  • pointer = nullptr
• How do you get the value?
  • pointer->data
• How does the iterator increment?
  • pointer = pointer->next;

Summary

• Sometimes, all you need is a pointer to the data. For contiguous storage, this is usually sufficient.
• Sometimes, you need a pointer to the container itself, and an index into the data.
• For error-checking, a pointer to the data isn’t good enough. How would you know if you ++ right past the end of the container, or -- before the start? You need the information in the container itself to detect that.
• Don’t just copy the container’s data itself into the iterator. The iterator doesn’t need the actual data, and that would be slow. Instead, the iterator only needs a pointer.