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

From	Bo Persson <bo@bo-persson.se>
Newsgroups	comp.lang.c++
Subject	Re: initial size of a std::vector of std::strings
Date	2022-12-07 23:45 +0100
Message-ID	<jvcjd3F7nudU1@mid.individual.net> (permalink)
References	<tmp947$g0jv$1@dont-email.me> <tmptcb$hm5s$1@dont-email.me> <tmquhd$kuud$1@dont-email.me>

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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

Thread

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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