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Groups > comp.lang.c++ > #82709
| From | Andrey Tarasevich <andreytarasevich@hotmail.com> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: A counterintuitive breaking change related to new comparisons |
| Date | 2022-01-03 14:24 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <sqvt2d$as4$1@dont-email.me> (permalink) |
| References | <sqvcts$h94$1@dont-email.me> |
On 1/3/2022 9:48 AM, Andrey Tarasevich wrote:
> A colleague discovered that switching from `-stc=c++17` to `-std=c++20`
> in their project resulted in a different behavior from some associative
> containers. A bit of research allowed to narrow down the culprit to what
> can be demonstrated by the following minimalist example
>
> #include <iostream>
>
> struct S
> {
> char c;
>
> operator const char *() const { return &c; }
>
> friend bool operator <(const S& lhs, const S& rhs)
> { return lhs.c < rhs.c; }
> };
>
> int main()
> {
> std::pair<int, S> p1{}, p2{};
> std::cout << (p1 < p2) << (p2 < p1) << std::endl;
> }
>
> C++17 compilers output `00`. C++20 compilers output `01` (or, perhaps,
> `10`).
>
> The obvious guess is that comparisons for `std::pair` work differently
> in C++20 after the introduction of `<=>`. And indeed, the `<=>` operator
> for `std::pair` in this case ignores the user-defined `<` and instead
> opts for raw pointer comparison through conversion to `const char *`.
> This is a rather surprising and counterintuitive breaking change, to put
> it mildly...
>
> If we remove the user-defined conversion to `const char *`, C++20 will
> use the user-defined `<` and also output `00`.
>
> The above results were obtained with GCC. Clang 10 outputs `00` even in
> `-std=c++20` mode, but Clang 11 and later output `01`.
To take out of the picture the irrelevant matter of undefined pointer
comparison, here's another example
#include <iostream>
struct S
{
int a;
S(int a) : a(a) {}
operator int() const { return a; }
friend bool operator <(const S& lhs, const S& rhs)
{ return lhs.a > rhs.a; }
};
int main()
{
std::pair<int, S> p1{ 0, 1 }, p2{ 0, 2 };
std::cout << (p1 < p2) << (p2 < p1) << std::endl;
}
This program outputs `01` in C++17 mode (and earlier)
http://coliru.stacked-crooked.com/a/98f0b6b57e9caf45
but outputs `10` in C++20 mode
http://coliru.stacked-crooked.com/a/3f8c3e96cd39f596
In C++17 the user-defined comparison operator is used. In C++20
conversion to `int` and subsequent comparison of `int`s is used.
--
Best regards,
Andrey Tarasevich
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A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 09:48 -0800
Re: A counterintuitive breaking change related to new comparisons Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-01-03 21:06 +0000
Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 14:13 -0800
Re: A counterintuitive breaking change related to new comparisons Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-01-04 00:23 +0000
Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 16:33 -0800
Re: A counterintuitive breaking change related to new comparisons Juha Nieminen <nospam@thanks.invalid> - 2022-01-04 08:27 +0000
Re: A counterintuitive breaking change related to new comparisons "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-01-04 17:24 +0100
Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-04 10:26 -0800
Re: A counterintuitive breaking change related to new comparisons Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-01-04 00:39 +0000
Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 14:24 -0800
Re: A counterintuitive breaking change related to new comparisons "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-01-03 23:55 +0100
Re: A counterintuitive breaking change related to new comparisons Manfred <noname@add.invalid> - 2022-01-04 00:56 +0100
Re: A counterintuitive breaking change related to new comparisons Manfred <noname@add.invalid> - 2022-01-04 00:59 +0100
Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-03 16:59 -0800
Re: A counterintuitive breaking change related to new comparisons Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2022-01-04 01:27 +0000
Re: A counterintuitive breaking change related to new comparisons Öö Tiib <ootiib@hot.ee> - 2022-01-04 14:49 -0800
Re: A counterintuitive breaking change related to new comparisons Öö Tiib <ootiib@hot.ee> - 2022-01-04 15:16 -0800
Re: A counterintuitive breaking change related to new comparisons Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-05 09:06 -0800
Re: A counterintuitive breaking change related to new comparisons "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-01-03 18:51 -0800
Re: A counterintuitive breaking change related to new comparisons wij <wyniijj@gmail.com> - 2022-01-04 03:53 -0800
Re: A counterintuitive breaking change related to new comparisons wij <wyniijj@gmail.com> - 2022-01-04 21:59 -0800
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