Re: How to iterate all k-subsets of a range in a specific order?

[Tommi]

Although, the only case, where this would be
a problem is with a range of type T, where:
1) It's impossible to provide random access to T
2) T can't return a reference from its 'front' property
3) T is a finite range (not infinite)
4) 'front' property may return the same value at different indexes

Something like:

struct R
{
    int _value = 0;
    int _round = 1;

    @property bool empty() const
    {
        return _value == 100 && _round == 2;
    }

    @property int front() const
    {
        return _value;
    }

    void popFront()
    {
        if (_value == 99)
        {
            if (_round == 1)
            {
                _value = 0;
                _round = 2;
            }
            else
            {
                _value = 100;
            }
        }
        else
        {
            ++_value;
        }
    }
}

Albeit, in this simple example it would be possible to provide 
random access to R, because the sequential definition of it could 
be easily replaced with an algebraic one. But for a more complex 
sequential definition, it might not be possible. So, the 
situation, where this (potential) defect of the range concept 
would be a problem, seems very rare, but it's nevertheless 
possible.