How to initialize an std::array of strings?

Question

I have an std::array<std::string, 4> that I want to use to hold path names for text files that I want to read into the program. So far, I declare the array, and then I assign each std::string object the value.

#include <array>
#include <string>
using std::array;
using std::string;
...
array<string, 4> path_names;
path_names.at(0) = "../data/book.txt";
path_names.at(0) = "../data/film.txt";
path_names.at(0) = "../data/video.txt";
path_names.at(0) = "../data/periodic.txt";

I am using this approach because I know in advance that there are exactly 4 elements in the array, the size of the array will not change, and each path name is hard coded and also cannot change.

I want to declare and initialize the array and all its elements in just one step. For example, this is how I would declare an initialize an array of ints:

array<int, 10> ia2 = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};  // list initialization

How would I do the same but for std::string? Creating these strings would involve calling the constructor. It's not as simple as just list-initializing a bunch of ints. How to do that?

Did you try to use string literals instead of numbers in the initializer? — Cheers and hth. - Alf, Sep 17 '18 at 05:12
Possible duplicate of [Using std::array with initialization lists](https://stackoverflow.com/q/8192185/608639) — jww, Sep 17 '18 at 05:52
`at` does range checking - obsolete if you are sure that indices are in range (which applies in given case), so prefer index operator `[]`... — Aconcagua, Sep 17 '18 at 06:00
@Aconcagua Is the index operator `[]` more efficient that `at`? Does `at` do a runtime check? — Galaxy, Sep 19 '18 at 04:38

P.W · Accepted Answer · 2018-09-17T11:18:27.083

1

This can be done in two ways:

Using string literals in list initialization

array<string, 4> path_names = {"../data/book.txt", "../data/film.txt", "../data/video.txt", "../data/periodic.txt"};

Assigning path names to strings and using the strings in initialization.

string s1 = "../data/book.txt";
string s2 = "../data/film.txt";
string s3 = "../data/video.txt";
string s4 = "../data/periodic.txt";
array<string, 4> path_names = {s1, s2, s3, s4};

And if you want to avoid copies here, you can use std::move:

array<string, 4> path_names = {std::move(s1), std::move(s2), std::move(s3), std::move(s4)};

edited Sep 17 '18 at 11:18

answered Sep 17 '18 at 05:40

P.W

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Second variant doesn't look any better to me than the one in the question; at least, I'd move the strings instead of copying (`std::move(sN)`)... – Aconcagua Sep 17 '18 at 06:12
Updated the answer suggesting `std::move` as well. – P.W Sep 17 '18 at 11:19

How to initialize an std::array of strings?

1 Answers1