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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c++
Subject: Re: initial size of a std::vector of std::strings
Date: Wed, 07 Dec 2022 14:47:11 -0800
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Lynn McGuire <lynnmcguire5@gmail.com> writes:
> On 12/7/2022 3:49 PM, Keith Thompson wrote:
>> Lynn McGuire <lynnmcguire5@gmail.com> writes:
>> [...]
>>> 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, "");
>> How does it express its dislike?  Both work for me:
>> #include <vector>
>> #include <string>
>> #include <iostream>
>> int main() {
>>      int max_ncp = 42;
>>      {
>>          std::vector<std::string> component_names(max_ncp);
>>          std::cout << component_names.size() << ' ';
>>      }
>>      {
>>          std::vector<std::string> component_names(max_ncp, "");
>>          std::cout << component_names.size() << '\n';
>>      }
>> }
>> The output is "42 42".
>> 
>>> But it does like
>>> 	std::vector <std::string> component_names [max_ncp];
>>> But it does not work.
>> That defines an array (not a std::array, just a C-style array
>> object) of
>> max_ncp vectors.  I think you want a single vector.
>> Do you want to set the initial size or the initial capacity?
>
> One vector with max_ncp strings in it.

So the initial state of the vector should be that it has a size() of
max_ncp, and each of the max_ncp strings it contains is initialized to
-- what, the empty string?

Then either this:
    std::vector<std::string> component_names(max_ncp);
or this:
    std::vector<std::string> component_names(max_ncp, "");
should give you want you want.  You said Visual Studio 2015 doesn't like
either of those, but you didn't say what it's complaint was.

Is the size of the vector going to change after it's been created?

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for XCOM Labs
void Void(void) { Void(); } /* The recursive call of the void */