"the last change" for ranges

[Andrei Alexandrescu]

In wake of a few discussion I've witnessed, I'm thinking of a last 
change for ranges. (In fact there's one more, but that's minor.)

The problem is that input ranges and forward ranges have the same 
syntactic interface, but different semantic interfaces. Consider the 
problem of finding the first two identical adjacent items in a range:

R adjacentFind(R)(R r)
{
     if (r,empty) return r;
     R last = r;
     r.popFront;
     for (; !r.empty && last.front != r.front; last.popFront, r.popFront)
     {
     }
     return r;
}

This will work properly on lists and vectors, but horrendously on files 
and sockets. This is because input ranges can't be saved for later use: 
incrementing r also increments popFront and essentially forces both to 
look at the same current value.

I'm thinking a better design is to require any range that's forward or 
better to define a function save(). Ranges that don't implement it are 
input ranges; those that do, will guarantee a brand new range is 
returned from save(). So then adjacentFind would look like this:

R adjacentFind(R)(R r)
{
     if (r,empty) return r;
     R last = r.save;
     r.popFront;
     for (; !r.empty && last.front != r.front; last.popFront, r.popFront)
     {
     }
     return r;
}

Obviously, when you pass a range that doesn't support save, adjacentFind 
will not compile, which is what we want.

Andrei

P.S. There is a way to implement adjacentFind for forward ranges by 
saving data instead of ranges. I've used a limited version above for 
illustration purposes.