Re: Dropping C++03 support (again)

From: Peter Dimov (pdimov_at_[hidden])
Date: 2023-02-20 04:09:42

Steven Robbins wrote:
> It breaks the idea that "==" is an equivalence relation, which seems to me to
> unnecessarily complicate things for the user.

Heterogeneous comparisons aren't equivalence relations. Values of
different types can't be equivalent.

But the question "who decides the meaning of x == y" doesn't have
anything to do with equivalence anyway. You have to have an answer
to it.

The principled approach to heterogeneous comparisons is to define
x == y as C(x) == C(y), where C is the common type of X and Y, i.e. a
type that can represent all values of X and all values of Y. But this
(a) only shifts the question to "who decides C" and (b) doesn't at all
work for any op== that doesn't follow the principled approach, such
as boost::function::operator== (which considers x == y true when
the boost::function x contains y, but for which x == x doesn't compile),
or bind(f, _1) == v, which constructs a lambda expression that returns
f(x) == v. (Or for _1 == v when using Lambda/Lambda2, for that
matter.)

There are tons of existing C++ code that works perfectly well without
adhering to principled approaches to defining op==, and breaking this
willy-nilly was irresponsible.