Re: enum type in template declaration

From	Soviet_Mario <SovietMario@CCCP.MIR>
Newsgroups	comp.lang.c++
Subject	Re: enum type in template declaration
Date	2019-12-06 22:42 +0100
Organization	A noiseless patient Spider
Message-ID	<qsei0f$7eo$1@dont-email.me> (permalink)
References	(1 earlier) <qscii5$5an$1@dont-email.me> <qsdgou$paa$1@dont-email.me> <qsdib1$2j6$1@dont-email.me> <qsdv4a$fa2$1@dont-email.me> <qse6ma$1j7$1@dont-email.me>

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Il 06/12/19 19:29, Paavo Helde ha scritto:
> On 6.12.2019 18:20, Soviet_Mario wrote:
>> Il 06/12/19 13:42, Paavo Helde ha scritto:
>>>
TNX
  (for the 1st part)

> However, 'SizedField<RestrictedValues(3)> foo;' compiles. I 
> would not worry about that, in C++ one can always find ways 
> to shoot himself.

ok, this kind of mistakes worries me less than simple slight

> 
>>
>> If so, okay ...
>>
>>
>> Oh, but BTW, what "helper" functions about run-time 
>> INTROSPECTION are
>> more or less standard ?
>>
>>
>> I've found some is_enum<EnumType> (), is_array<typename> 
>> () and so.
>>
>> But a simple function like : bool is_of_type <typename> ()
>>
>> in other languages such operators are named like "TypeOf" 
>> (or typename,
>> also a string representation could be usefull)
> 
> C++ is pretty statically typed, so you typically do not need 
> things like TypeOf to find out the type at run time. For 
> polymorphic types there is typeid() though which is similar, 
> and also provides name() (which is not guaranteed to be 
> useful).
> 
>>
>> This question is for a concrete case.
>> I'm  trying to build a 2D table container not particularly 
>> generic : it
>> supports just two type of contained items, either double 
>> either a "fixed
>> size string", but I'm not sure I'd be able to write really 
>> generic
>> functions as the two basic types contain some equivalent 
>> operators,
>> constructors and comparison op, but also some semantically 
>> different
>> accessors for higher level function on a container.
>>
>> So I'd appreciate some way a container could look inside 
>> himself (at
>> runtime) and be aware of its type and call proper 
>> specialized functions
>> where the syntax are not equivalent and I cannot rely only 
>> on overload
>> for automatic selection of the best fit, or even in case I 
>> have to
>> choose to MIX data, thing that will end in ambiguity, as 
>> the two types
>> (fixedstrings and double) are largely convertible and 
>> interchangeable,
>> so a generic function with identical signatures could not 
>> know what is
>> the best to be called)
> 
> You have to make up your mind if you have the type (double 
> or string) fixed at compile time or at run time. 

they will exist both, and interact from time to time (mutual 
conversions, plus some calculations done in string mode, 
like dates comparisons and some sorting in names/surnames 
fields, and other in "double mode", statistics on data. The 
two tables have to stay synced when one is sorted and the 
other driven together)

> If at 
> compile time, then there is no need for the container to 
> check the type at run time, as the type was already fixed at 
> the compile time and cannot change. E.g. a 
> std::vector<double> does not need to check at run-time if 
> its elements have suddenly turned into strings.

it is the template function that is generic and has to know 
to cope with both in different semantically ways, or no ?

> 
> If the item type only becomes known only at run time, then 
> it most probably means you have some variant type. 

uhm ... the allocation of proper types is known, but at 
"CODE-TIME" some functions have duplexed code, and must know 
which part to use according to the actual type of the 
container caller.

Maybe this arises from bad design of the functions, dunno :(

anyway I'll study typeid and type_info keywords. Tnx

> For this 
> custom variant type there are no overloaded functions 
> present, you have to write them yourself, and there you have 
> to figure out the correct type at run-time and act accordingly.
> 


-- 
1) Resistere, resistere, resistere.
2) Se tutti pagano le tasse, le tasse le pagano tutti
Soviet_Mario - (aka Gatto_Vizzato)

Thread

enum type in template declaration Soviet_Mario <SovietMario@CCCP.MIR> - 2019-12-05 23:20 +0100
  Re: enum type in template declaration James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-05 22:39 -0500
    Re: enum type in template declaration Soviet_Mario <SovietMario@CCCP.MIR> - 2019-12-06 13:15 +0100
      Re: enum type in template declaration Paavo Helde <myfirstname@osa.pri.ee> - 2019-12-06 14:42 +0200
        Re: enum type in template declaration Soviet_Mario <SovietMario@CCCP.MIR> - 2019-12-06 17:20 +0100
          Re: enum type in template declaration Paavo Helde <myfirstname@osa.pri.ee> - 2019-12-06 20:29 +0200
            Re: enum type in template declaration Soviet_Mario <SovietMario@CCCP.MIR> - 2019-12-06 22:42 +0100
              Re: enum type in template declaration Paavo Helde <myfirstname@osa.pri.ee> - 2019-12-07 00:51 +0200
                Re: enum type in template declaration Soviet_Mario <SovietMario@CCCP.MIR> - 2019-12-07 18:54 +0100
      Re: enum type in template declaration Öö Tiib <ootiib@hot.ee> - 2019-12-06 06:00 -0800
      Re: enum type in template declaration James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-06 09:56 -0500
      Re: enum type in template declaration Manfred <noname@add.invalid> - 2019-12-06 15:56 +0100
  Re: enum type in template declaration Soviet_Mario <SovietMario@CCCP.MIR> - 2019-12-06 18:43 +0100
  Re: enum type in template declaration saturnman2008@gmail.com - 2019-12-07 01:53 -0800
    Re: enum type in template declaration Soviet_Mario <SovietMario@CCCP.MIR> - 2019-12-07 18:57 +0100

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