C++: Implementing Range Based For Loops for a custom class and retaining backwards compatibility

I'm implementing range-based FOR loops for a custom class, but I need to retain compatibility with older compilers. To that end, my implementation goes like this

class FooArrayIterator
{
public:
    FooArrayIterator(const FooArray& container, unsigned int index = 0)
        :_container(container),
        _index(index)
        {}

bool operator != (const FooArrayIterator& other)
{
    return (_index != other._index);
}

int operator*() const;

const FooArrayIterator& operator++()
{
    _index++;
    return *this;
}

private:
    const FooArray  _container;
    unsigned int    _index;
};

int FooArrayIterator::operator*() const
{
    return _container[_index];
}

FooArrayIterator begin(FooArray& container)
{
    return FooArrayIterator(container, 0);
}

FooArrayIterator end(FooArray& container)
{
    return FooArrayIterator(container, container.length());
}

However, when testing:

FooArray c;
c.append(0);
...
c.append(9);

for (auto n : c)
{
    std::cerr << "Number is: " << n << endl;
    fflush(stderr);
}

It fails to compile, saying "this range-based 'for' statement requires a suitable "begin" function and none was found". I was under the impression that I could get away with a free-standing begin() and end() functions. Is this not possible?