Initializing an array in declaration inside the function and using this outside

Question

I’d like to easily initialize an array inside the function:

    int arr[]{1, 2, 3};

It simplifies calculating the size of the array:

    int n  = sizeof(arr) / sizeof(*arr);

However, I need this array to be preserved and be accessible outside the function.

How to do this?

For the start, I have this:


    #include <iostream>

    using namespace std;

    int n{0};
    int* g_arr{nullptr};

    void f()
    {
        int arr[]{1, 2, 3};

        n = sizeof(arr) / sizeof(*arr);
        g_arr = arr; // you can't use g_arr value outside this function, since arr will be freed.
    }

    void b()
    {
        cout << n;

        for (int i = 0; i < n; ++i)
        {
            cout << g_arr[i] << endl; // this is still wrong
        }
    }

    int main()
    {
        f();
        b();
    }

You could have int g_arr[3]; at file scope and assign the values in f — M.M
– M.M, Commented Feb 23, 2020 at 9:26
In the code you have posted there is no need for arr inside the function. In fact it ceases to exist when f returns. If you have other requirements then you will need to detail them — M.M
– M.M, Commented Feb 23, 2020 at 9:30
@M.M Well, I need what I need. It would be very verbose to explain why. It leads to embeded programming and sparing the SRAM using PROGMEM. — zhekaus
– zhekaus, Commented Feb 23, 2020 at 9:36
Well you will need to try. The local array in the question stops existing when the function returns. If you're trying to say you create the array by some other means then obviously that is relevant to the question. Otherwise we just end up with an endless sequence of people suggesting answers and you saying "Oh that won't work for me" — M.M
– M.M, Commented Feb 23, 2020 at 9:40

evgard · Accepted Answer · 2020-02-23 10:51:51Z

2

Make arr[] static:

 void f()
 {
    static int arr[]{1, 2, 3};

    n = sizeof(arr) / sizeof(*arr);
    g_arr = arr;
 }

answered Feb 23, 2020 at 10:51

evgard

363 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Aykhan Hagverdili · Accepted Answer · 2020-02-23 10:23:16Z

1

A possible solution is to copy the stack allocated array:

#include <iostream>
#include <cstring>

int* g_arr = nullptr;
std::size_t n = 0;

void f() {
  int arr[]{1, 2, 3};

  n = sizeof(arr) / sizeof(*arr);

  delete[] g_arr;
  g_arr = new int[n];
  std::memcpy(g_arr, arr, sizeof(arr));
}

void b() {
  std::cout << n << '\n';

  for (std::size_t i = 0; i != n; ++i) {
    std::cout << g_arr[i] << '\n';
  }
}

int main() {
  f();
  b();
}

If you can, use a vector:

#include <iostream>
#include <vector>

std::vector<int> g_arr{};

void f() {
  g_arr = {1, 2, 3};  
}

void b() {
  std::cout << g_arr.size() << '\n';

  for (int const i : g_arr) {
    std::cout << i << '\n';
  }
}

int main() {
  f();
  b();
}

edited Feb 23, 2020 at 10:23

answered Feb 23, 2020 at 9:43

Aykhan Hagverdili

30.5k6 gold badges53 silver badges108 bronze badges

6 Comments

zhekaus Over a year ago

Unfortunately, I can't use vector. Templates library is not available in my environment and it wastes lots of SRAM.

Aykhan Hagverdili Over a year ago

Can you dynamically allocate arr?

zhekaus Over a year ago

I can. However I'll lose the conveniences of using []{…}.

Aykhan Hagverdili Over a year ago

@zhekaus considering []{...} is very important for you, I updated the answer

M.M Over a year ago

In the first case you can use unique_ptr<int[]> rather than a raw owning pointer

|

Collectives™ on Stack Overflow

Initializing an array in declaration inside the function and using this outside

2 Answers 2

Comments

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related