Question

I was wondering recently what the requirements were for range based for loops in c++11, as I had only ever seen examples of intended usage:

for (auto person : people)
{
  cout << person.name << endl;
}

But given that a container need only have begin and end methods, but need not contain anything at all, would the below be considered ‘bad practice’ in any way? If nothing else, it is a fresh answer if someone asks you for a Fibonacci sequence in an interview!

#include <string>
#include <iostream>
#include <Windows.h>

using namespace std;

struct FibItr
{
    FibItr(int cur = 1, int prev = 0) : mCur(cur), mPrev(prev) {}
    FibItr & operator++()
    {
        mCur += mPrev;
        mPrev = mCur - mPrev;
        return *this;
    }

    int operator*(){ return mCur; }

    bool operator!=(const FibItr & _rhs)
    {
        return mCur != _rhs.mCur || mPrev != _rhs.mPrev;
    }   

    unsigned int mCur, mPrev;
};

struct Fib
{
    FibItr begin() { return FibItr(); }
    FibItr end() { return FibItr(0, 0); }
};

int main( int argc, char* argv[] ) 
{   
    for (auto num : Fib())
    {
        cout << num << endl;
        Sleep(500);
    }

    return 0;
}

Answer 1

The question is not really about the auto for-loop but if it is reasonable to implement stranger kind of iterators. While there are corner-cases you can make a perfect good argument for implementing some operations as iterators (memoized fibonacci being a good example).

There are whole libraries devoted to turning iterators in something more, so some other people also think it is a good idea.

As an aside: Implementing an iterator is tricky business, which is why methods like this should be used with care. Boost.Iterator is a good set of helpers that can make that easier.