Multi tool use

How can I define iterator and const_iterator in my class while I uses std::vector as field in my class?

Given the following class:

template<class T>
class A {
vector<T> arr;
public:
A(int size);
A(vector<T> arr);
int size() const;
A& operator=(const A& p);

template<class S>
friend ostream& operator<<(ostream& os, const A<S>& p);

template<class S>
friend bool operator==(const A<S>& p1, const A<S>& p2);
};


How can I define iterator and const_iterator for my class? (I want to uses the iterators of the vectors instead to implement class iterator and class const_iterator)..

iterator

const_iterator

class iterator

class const_iterator

I want (for example) to support in something like that:

A a(5);
for(A<int>::iterator it = a.begin() ; it != a.end() ; it++) { /* .. */ }


2 Answers
2

You can simply do this, in C++11:

template<class T>
class A {
vector<T> arr;
public:

using iterator = typename vector<T>::iterator;
using const_iterator = typename vector<T>::const_iterator;

const_iterator begin() const { return arr.begin(); }
iterator begin() { return arr.begin(); }
const_iterator end() const { return arr.end(); }
iterator end() { return arr.end(); }
};


or in C++14:

template<class T>
class A {
vector<T> arr;
public:
using iterator = typename vector<T>::iterator;
using const_iterator = typename vector<T>::const_iterator;

auto begin() const { return arr.begin(); }
auto begin() { return arr.begin(); }
auto end() const { return arr.end(); }
auto end() { return arr.end(); }
};


Then you can both support iterator-based iteration:

A<int> a(5);
for(A<int>::iterator it = a.begin() ; it != a.end() ; it++) { /* .. */
}


And ranged-based for loop:

A a(5);
for(auto v : a) { /* .. */
}


Further explanations on how to support range-based for loop are available here : How to make my custom type to work with "range-based for loops"?

(thanks @JohnML for the edit suggestion to make the answer c++11 compliant!)

Simply typedef the vector's existing iterator types into your own class:

template<class T>
class A {
...
public:
typedef vector<T>::iterator iterator;
typedef vector<T>::const_iterator const_iterator;
...
};


Or

template<class T>
class A {
...
public:
using iterator = typename vector<T>::iterator;
using const_iterator = typename vector<T>::const_iterator;
...
};


typename

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