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Groups > comp.lang.c++ > #87776
| From | Juha Nieminen <nospam@thanks.invalid> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: How can I get the type in a C++17 fold function? |
| Date | 2022-12-09 12:57 +0000 |
| Organization | Aioe.org NNTP Server |
| Message-ID | <tmvbci$1n54$2@gioia.aioe.org> (permalink) |
| References | <tmsbtq$fp6$1@gioia.aioe.org> <tmskna$7a9$1@gioia.aioe.org> <tmt0j0$1bnn$1@gioia.aioe.org> |
Muttley@dastardlyhq.com wrote:
> On Thu, 8 Dec 2022 12:18:52 -0000 (UTC)
> Juha Nieminen <nospam@thanks.invalid> wrote:
>>Muttley@dastardlyhq.com wrote:
>>> eg:
>>>
>>> template<typename... T>
>>> auto sum(T&&... args)
>>> {
>>> if (typeid(T) == typeid(string)) { do something }
>>> return (args + ...);
>>> }
>>>
>>> However doing typeid on T gives
>>>
>>> error: expression contains unexpanded parameter pack 'T'
>>>
>>> in clang.
>>>
>>> Is there a way to get the type inside the function?
>>
>>The problem is that T there is not a type, but a parameter pack.
>>You can't 'typeid(T)' when T is a parameter pack (what would that
>>even mean? A parameter pack is essentially a bunch of types).
>>
>>If you want to iterate through all the types in that parameter
>>pack and do one thing or another depending on a particular type,
>>you'll need to do it in the more "traditional" C++11 way of
>>iterating parameter packs. In other words, something along the
>>lines of:
>>
>>void doSomething(const std::string& s) { /* ... */ }
>>
>>template<typename T>
>>void doSomething(const T&) {}
>>
>>template<typename First>
>>auto sum(First&& first) { return first; }
>>
>>template<typename First, typename... Rest>
>>auto sum(First&& first, Rest&&... rest)
>>{
>> doSomething(first);
>> return first + sum(std::forward<Rest>(rest)...);
>>}
>
> Thats just variadic templates with a slightly different syntax. AFAIK the
> point of folds is not having to manually iterate the arguments yourself as
> with 2011 which is little better than C's varargs (and probably less efficient
> due to recursion vs pointer incrementing).
Well, if you can come up with a fold expression that does what you want,
go right ahead.
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How can I get the type in a C++17 fold function? Muttley@dastardlyhq.com - 2022-12-08 09:48 +0000
Re: How can I get the type in a C++17 fold function? Juha Nieminen <nospam@thanks.invalid> - 2022-12-08 12:18 +0000
Re: How can I get the type in a C++17 fold function? Muttley@dastardlyhq.com - 2022-12-08 15:41 +0000
Re: How can I get the type in a C++17 fold function? Juha Nieminen <nospam@thanks.invalid> - 2022-12-09 12:57 +0000
Re: How can I get the type in a C++17 fold function? Öö Tiib <ootiib@hot.ee> - 2022-12-08 04:45 -0800
Re: How can I get the type in a C++17 fold function? Muttley@dastardlyhq.com - 2022-12-08 15:45 +0000
Re: How can I get the type in a C++17 fold function? Öö Tiib <ootiib@hot.ee> - 2022-12-08 13:59 -0800
Re: How can I get the type in a C++17 fold function? Vir Campestris <vir.campestris@invalid.invalid> - 2022-12-14 10:42 +0000
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-14 18:02 +0100
Re: How can I get the type in a C++17 fold function? Muttley@dastardlyhq.com - 2022-12-14 17:17 +0000
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-14 18:23 +0100
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-15 17:59 +0100
Re: How can I get the type in a C++17 fold function? Muttley@dastardlyhq.com - 2022-12-15 17:20 +0000
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-15 18:37 +0100
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-16 05:26 +0100
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-16 05:41 +0100
Re: How can I get the type in a C++17 fold function? scott@slp53.sl.home (Scott Lurndal) - 2022-12-16 15:17 +0000
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-16 18:22 +0100
Re: How can I get the type in a C++17 fold function? scott@slp53.sl.home (Scott Lurndal) - 2022-12-16 17:42 +0000
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-16 19:20 +0100
Re: How can I get the type in a C++17 fold function? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-28 16:41 -0800
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-29 03:40 +0100
Re: How can I get the type in a C++17 fold function? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-28 22:27 -0800
Re: How can I get the type in a C++17 fold function? Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-29 09:36 +0200
Re: How can I get the type in a C++17 fold function? Mason <mason.nobody@gmail.com> - 2022-12-29 17:15 +0000
Re: How can I get the type in a C++17 fold function? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-12-29 19:11 +0100
Re: How can I get the type in a C++17 fold function? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-12-29 19:58 -0800
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-30 08:09 +0100
Re: How can I get the type in a C++17 fold function? Michael S <already5chosen@yahoo.com> - 2022-12-30 03:27 -0800
Re: How can I get the type in a C++17 fold function? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-12-30 14:01 +0100
Re: How can I get the type in a C++17 fold function? Muttley@dastardlyhq.com - 2022-12-30 15:17 +0000
Re: How can I get the type in a C++17 fold function? "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-12-30 20:04 +0100
Re: How can I get the type in a C++17 fold function? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-12-30 13:54 -0800
Re: How can I get the type in a C++17 fold function? Muttley@dastardlyhq.com - 2022-12-31 15:33 +0000
Re: How can I get the type in a C++17 fold function? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2022-12-30 17:31 +0000
Re: How can I get the type in a C++17 fold function? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-01-02 13:30 -0800
Re: How can I get the type in a C++17 fold function? Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-30 18:36 +0100
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