On Wed, Jan 14, 2009 at 2:48 PM, Phil Endecott < spam_from_boost_dev@chezphil.org> wrote:
That's what the functor versions are there for: sorting classes or
structures based upon a key. All you have to do is define the correct functor.
I still believe that "rightshift" is the wrong name for this functor in these cases.
It's an arithmetic right shift, or alternatively division by a power of 2. What do you want to call it?
Fixed-point types can sort as an integer of the same size in bytes. The
only real requirement is that they must return a data type that when subtracted from an item of the same type returns something convertible to a size_t, that if (b - a) < (c - a) then b < c, and that bitshifting works as division by a power of two would, reducing the difference by a power of two per shift.
Can you clarify that a bit? Say I have this:
class fixed { .... some_integer_type impl; .... bool operator<(const fixed& other) const; fixed operator>>(unsigned int shift_mt) const; };
What do I need to do?
You < operator is fine, and usable without modification. If your - operator returns something castable to a size_t, then you don't need any functors, that will do (return impl - right.impl;) If your - operator doesn't return something castable to a size_t, then you need to define a rightshift functor that does. How about: struct rightshift { some_integer_type operator()(const fixed &x, unsigned offset) { return x.impl >> offset; } };
I would
have thought that the floating-point algorithm would just be an instance of the generic algorithm with custom functors, with a bit of specialisation to cope with the reverse-order-for-negatives thing.
Positive floats can be sorted just like integers, with the right functor. Negative floats have to be sorted backwards. I didn't see any efficient way to write a functor to handle negative floats.
Can't you just pass a "greater" functor instead of "less"?
Yes, but that would hurt efficiency.