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

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On 12/7/2022 4:45 PM, Bo Persson wrote:
> On 2022-12-07 at 21:54, 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);
> 
> You are not allowed to use round parenthesis in default member 
> initializers, because of possible confusion with function declarations 
> ("Most vexing parse").
> 
> Like
> 
>     std::vector <std::string> component_names ();
> 
> is a member function returning a vector, not a default initialized 
> variable.
> 
>> 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.
> 
> Define "work".  :-)
> 
> This creates an array of vectors of strings. Probably not what you want, 
> anyway.
> 
> 
> You CAN use an {} initializer for the vector
> 
>    std::vector <std::string> component_names {max_ncp, ""};
> 
> but have to be REALLY careful not to trigger an initializer_list 
> constructor when things inside the {} is convertible to the vector value 
> type. Because then it will do something completely different!
> 
> Like
> 
>     vector<int> v {max_ncp, 0};
> 
> will create a vector with two elements, max_ncp and 0. Sigh!
> 
>>
>> Thanks,
>> Lynn

Thanks,
Lynn

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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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