c++11 foreach syntax and custom iterator

Question

I am writing an iterator for a container which is being used in place of a STL container. Currently the STL container is being used in many places with the c++11 foreach syntax eg: for(auto &x: C). We have needed to update the code to use a custom class that wraps the STL container:

template< typename Type> class SomeSortedContainer{     std::vector<typename Type> m_data; //we wish to iterate over this     //container implementation code };     class SomeSortedContainerIterator{     //iterator code }; 

How do I get auto to use the correct iterator for the custom container so the code is able to be called in the following way?:

SomeSortedContainer C; for(auto &x : C){     //do something with x...  } 

In general what is required to ensure that auto uses the correct iterator for a class?

Answer 1

To be able to use range-based for, your class should provide const_iterator begin() const and const_iterator end() const members. You can also overload the global begin function, but having a member function is better in my opinion. iterator begin() and const_iterator cbegin() const are also recommended, but not required. If you simply want to iterate over a single internal container, that's REALLY easy:

template< typename Type> class SomeSortedContainer{      std::vector<Type> m_data; //we wish to iterate over this     //container implementation code public:     typedef typename std::vector<Type>::iterator iterator;     typedef typename std::vector<Type>::const_iterator const_iterator;      iterator begin() {return m_data.begin();}     const_iterator begin() const {return m_data.begin();}     const_iterator cbegin() const {return m_data.cbegin();}     iterator end() {return m_data.end();}     const_iterator end() const {return m_data.end();}     const_iterator cend() const {return m_data.cend();} };     

If you want to iterate over anything custom though, you'll probably have to design your own iterators as classes inside your container.

class const_iterator : public std::iterator<random_access_iterator_tag, Type>{     typename std::vector<Type>::iterator m_data;     const_iterator(typename std::vector<Type>::iterator data) :m_data(data) {} public:     const_iterator() :m_data() {}     const_iterator(const const_iterator& rhs) :m_data(rhs.m_data) {}      //const iterator implementation code }; 

For more details on writing an iterator class, see my answer here.

Answer 2

You have two choices:

• you provide member functions named begin and end that can be called like C.begin() and C.end();
• otherwise, you provide free functions named begin and end that can be found using argument-dependent lookup, or in namespace std, and can be called like begin(C) and end(C).