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Re: initial size of a std::vector of std::strings

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On 2022-12-07 at 22:34, Michael S wrote:
> On Wednesday, December 7, 2022 at 10:54:23 PM UTC+2, Lynn McGuire wrote:
>> On 12/7/2022 5:28 AM, Alf P. Steinbach wrote:
>>> On 7 Dec 2022 06:42, Lynn McGuire wrote:
>>>> Is there a way to set the size of std::vector <std::string> formulas
>>>> to max_ncp size ?
>>>>
>>>> Something like "std::vector <std::string> formulas [max_ncp];"
>>>
>>> The most reasonable interpretation to me is that you're asking how to
>>> set the initial /capacity/ of the vector in the declaration, to avoid an
>>> explicit .reserve() invocation as a separate following statement.
>>>
>>> Presumably because you declare such vectors in many places.
>>>
>>> To do that you can to wrap the vector, either via a factory function or
>>> via a simple class, like
>>>
>>>      struct Capacity{ int value; };
>>>
>>>      struct Fast_stringvec:
>>>          public vector<string>
>>>      {
>>>          template< class... Args >
>>>          Fast_stringvec( const Capacity capacity, Args&&... args ):
>>>              vector<string>( forward<Args>( args )... )
>>>          {
>>>              reserve( capacity.value );
>>>          }
>>>      };
>>>
>>> Then do e.g.
>>>
>>>      auto formulas = Fast_stringvec( Capacity( max_ncp ) );
>>>
>>> ... where I believe the use of `auto` helps avoid the Most Vexing Parse.
>>> ;-)
>>>
>>> If `max_ncp` is a constant you can add a constructor that uses that by
>>> default.
>>>
>>> ---
>>>
>>> A not so reasonable interpretation is that you're asking how to make the
>>> vector contain `max_ncp` strings initially, for which you can just do
>>>
>>>      vector<string> formulas( max_ncp, ""s );
>>>
>>> ---
>>>
>>> Crossing the line to perhaps unreasonable interpretation is that you're
>>> asking how you can make it so that when you add a new to string to the
>>> vector, the new string item will guaranteed be of size `max_ncp`.
>>>
>>> For that you need to make the item type one that holds a `string` of
>>> exactly that size.
>>>
>>> It would probably be necessary to take control of copying to such items,
>>> to avoid inadvertent size changes.
>>>
>>>
>>> - Alf
>> I am trying to declare a vector with max_ncp strings in a struct.
>> struct component_data {
>> std::vector <std::string> component_names [max_ncp];
>> doublereal heatoffusionatmeltingpoint [max_ncp];
>> doublereal meltingpointtemperature [max_ncp];
>> doublereal sublimationtemperature [max_ncp];
>> doublereal triplepointtemperature [max_ncp];
>> doublereal triplepointpressure [max_ncp];
>> };
>>
>> Visual Studio 2015 does not like
>> std::vector <std::string> component_names (max_ncp);
>> or
>> std::vector <std::string> component_names (max_ncp, "");
>>
>> But it does like
>> std::vector <std::string> component_names [max_ncp];
>> But it does not work.
>>
>> Thanks,
>> Lynn
> 
> Works fine on godbolt's vs2015
> 
> https://godbolt.org/z/8WPo33Yn1

That's for a local variable inside a function, where

std::vector <std::string> bar(x);

works.

However, as a struct member parenthesis are not allowed. Because reasons.

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initial size of a std::vector of std::strings Lynn McGuire <lynnmcguire5@gmail.com> - 2022-12-06 23:42 -0600
  Re: initial size of a std::vector of std::strings James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-12-07 00:52 -0500
  Re: initial size of a std::vector of std::strings Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-07 08:17 +0100
  Re: initial size of a std::vector of std::strings Öö Tiib <ootiib@hot.ee> - 2022-12-07 00:23 -0800
    Re: initial size of a std::vector of std::strings Juha Nieminen <nospam@thanks.invalid> - 2022-12-07 09:13 +0000
      Re: initial size of a std::vector of std::strings Öö Tiib <ootiib@hot.ee> - 2022-12-07 06:03 -0800
        Re: initial size of a std::vector of std::strings Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-07 15:37 +0100
          Re: initial size of a std::vector of std::strings Öö Tiib <ootiib@hot.ee> - 2022-12-07 09:33 -0800
            Re: initial size of a std::vector of std::strings Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-07 18:45 +0100
      Re: initial size of a std::vector of std::strings Paavo Helde <eesnimi@osa.pri.ee> - 2022-12-07 16:34 +0200
        Re: initial size of a std::vector of std::strings Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-07 15:38 +0100
        Re: initial size of a std::vector of std::strings Juha Nieminen <nospam@thanks.invalid> - 2022-12-08 07:58 +0000
    Re: initial size of a std::vector of std::strings Lynn McGuire <lynnmcguire5@gmail.com> - 2022-12-07 16:28 -0600
      Re: initial size of a std::vector of std::strings Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-12-07 14:53 -0800
        Re: initial size of a std::vector of std::strings Lynn McGuire <lynnmcguire5@gmail.com> - 2022-12-07 16:56 -0600
        Re: initial size of a std::vector of std::strings Juha Nieminen <nospam@thanks.invalid> - 2022-12-08 08:03 +0000
  Re: initial size of a std::vector of std::strings "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-12-07 12:28 +0100
    Re: initial size of a std::vector of std::strings Lynn McGuire <lynnmcguire5@gmail.com> - 2022-12-07 14:54 -0600
      Re: initial size of a std::vector of std::strings "daniel...@gmail.com" <danielaparker@gmail.com> - 2022-12-07 13:16 -0800
      Re: initial size of a std::vector of std::strings "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-12-07 22:33 +0100
        Re: initial size of a std::vector of std::strings Bonita Montero <Bonita.Montero@gmail.com> - 2022-12-07 22:37 +0100
      Re: initial size of a std::vector of std::strings Michael S <already5chosen@yahoo.com> - 2022-12-07 13:34 -0800
        Re: initial size of a std::vector of std::strings Bo Persson <bo@bo-persson.se> - 2022-12-07 23:48 +0100
      Re: initial size of a std::vector of std::strings Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-12-07 13:49 -0800
        Re: initial size of a std::vector of std::strings Lynn McGuire <lynnmcguire5@gmail.com> - 2022-12-07 16:01 -0600
          Re: initial size of a std::vector of std::strings Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-12-07 14:47 -0800
            Re: initial size of a std::vector of std::strings Lynn McGuire <lynnmcguire5@gmail.com> - 2022-12-07 17:09 -0600
      Re: initial size of a std::vector of std::strings Bo Persson <bo@bo-persson.se> - 2022-12-07 23:45 +0100
        Re: initial size of a std::vector of std::strings Lynn McGuire <lynnmcguire5@gmail.com> - 2022-12-07 17:08 -0600
  Re: initial size of a std::vector of std::strings Lynn McGuire <lynnmcguire5@gmail.com> - 2022-12-08 01:44 -0600

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