How to make my custom type to work with “range-based for loops”?

Question

Like many people these days I have been trying the different features that C++11 brings. One of my favorites is the \"range-based for loops\".

I understand that:

Answer 1

Inspired by BitTickler's comment about how to make it work for non-"container" types, here's a minimal example of something that works for doubles:

class dranged {
    double start, stop, step, cur;
    int index;

public:
    dranged(double start, double stop, double step) :
        start(start), stop(stop), step(step),
        cur(start), index(0) {}

    auto begin() { return *this; }
    auto end() { return *this; }

    double operator*() const { return cur; }

    auto& operator++() {
        index += 1;
        cur = start + step * index;
        return *this;
    }

    bool operator!=(const dranged &rhs) const {
        return cur < rhs.stop;
    }
};

Note that the use of < in the != operator maintains the correct invariant, but obviously assumes step is positive and wouldn't be appropriate everywhere a more general range would be. I've used an integer index to prevent propagation of floating point error, but have aimed for simplicity otherwise.

This can be used as:

double sum() {
    double accum = 0;
    for (auto val : dranged(0, 6.28, 0.1)) {
        accum += val;
    }
    return accum;
}

GCC and Clang both produce very reasonable code when compiled with optimisations (i.e. either -Os or above -O1 for GCC or -O2 for Clang).