Updating an array (passed as parameter) inside a C++ function

Question

I have the following declaration and function call:

unsigned int myArray[5] = {0, 0, 0, 0, 0};
ModifyArray(&myArray[0]);

The above code cannot be modified, it is given as is.

I need to write the implementation for ModifyArray to update myArray to contain 1, 2, 3, 4, 5.

I have written it as:

void ModifyArray(unsigned int * out_buffer) 
{
  unsigned int updatedValues[5] = {0, 0, 0, 0, 0};
  updatedValues[0] = 1;
  updatedValues[1] = 2;
  updatedValues[2] = 3;
  updatedValues[3] = 4;
  updatedValues[4] = 5;
  *out_buffer = &updatedValues;
}

This doesn't really seem to work. I have a feeling that I'm not doing the assignment correctly on the last line of code, however I have tried multiple variations, and it still doesn't seem to update just element 0 of the array.

What am I doing wrong? Thanks!

P.S.: Please note that the scenario is more complex than presented here. I have simplified for readability purposes, but the code should look very similar to this, just the last assignment should be updated to a correct one if possible.

*out_buffer = &updatedValues; You need to copy the new values to the old array. But really, there's no need for updatedValues at all. Just use out_buffer directly. — 001
– 001, Commented Nov 23, 2021 at 15:38
Why don't you just sets the values directly to the elements of out_buffer? — NathanOliver
– NathanOliver, Commented Nov 23, 2021 at 15:38
Either just set the updated values directly to the out_buffer without the detour through updatedValues, or use std::memcpy. Also keep in mind that your function needs to know the array length. — perivesta
– perivesta, Commented Nov 23, 2021 at 15:38
You cannot assign a C-style array. (you can copy one using any number of methods, but = is not one of them). It isn't clear why you need an intermediate array though. "My code is more complex" is not a valid excuse. — n. m. could be an AI
– n. m. could be an AI, Commented Nov 23, 2021 at 15:43
"The above code cannot be modified" Sad, as signature is bad (unsigned int * out_buffer, int size) or unsigned int (& out)[5] or std::span<unsigned int, 5> would be better. — Jarod42
– Jarod42, Commented Nov 23, 2021 at 15:45

molbdnilo · Accepted Answer · 2021-11-23 15:42:34Z

3

*out_buffer is an unsigned int.
&updatedValues is an unsigned int(*)[5] - a pointer to an array of five elements - which you can't assign to an int.

You should not assign any arrays (it's impossible), you should modify the contents of the given array:

void ModifyArray(unsigned int *out_buffer) 
{
    out_buffer[0] = 1;
    out_buffer[1] = 2;
    out_buffer[2] = 3;
    out_buffer[3] = 4;
    out_buffer[4] = 5;
}

which you can simplify to

void ModifyArray(unsigned int *out_buffer) 
{
    for (int i = 0; i < 5; i++)
    {
        out_buffer[i] = i+1;
    }
}

answered Nov 23, 2021 at 15:42

molbdnilo

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1 Comment

Jano Rajmond Over a year ago

The first implementation is the one I need. Definitely tried that before, however I may have accidentally run the WRONG test case :D Anyways, it works now! Thank you!

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