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Subject: Re: [ub] Justification for < not being a total order on pointers?
From: Gabriel Dos Reis (gdr_at_[hidden])
Date: 2013-10-16 10:37:12


Nevin Liber <nevin_at_[hidden]> writes:

| On 16 October 2013 10:06, Gabriel s too subtle for even most committee members,
| let alone mere mortals, to get right.Dos Reis <gdr_at_[hidden]> wrote:
|
|
| |       I would not have trouble telling people (especially notices):
| "Ignore expect
| | talks
| |       about operator< on pointers.  Prefer std::less<T>, unless you meant
| a
| | relationship
| |       between objects pointed to, in which use operator<.  Mean what you
| say
| | and say
| |       what you mean."
| |
| |
| |
| | So what about std::less<void>?  Should people be using it?
|
| What is wrong about it?
|
|
| It requires operator<.  If people specialize std::less<T> instead adding an
| operator<, it won't work.

That is an issue with the definition of std::less<>, not necessarily
with operator< on pointers.

| It would be surprising that:
|
| set<T, less<T>> works, but
| set<T, less<>> does not.

Make T = std::less<std::complex<MyFloat>> and instantiate the argument :-)

|
| One of the two motivations for this in n3421 is to not require that the
| argument type in a comparator functor be explicitly specified, but that assumes
| there is an operator< for the type, or possible compiler magic for pointer
| comparisons because of the ub issue.


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