How can an declare an array in a function that returns an array in c++

Question

// functions that converts a number into an array

int *initialiser(int number)
{
    int array[20];

    for (int i = 19; i >= 0; i--)
    {
        array[i] = number % 10;
        number /= 10; // number = number/10
    }

    return array;
}

//I get this error

primaryAddSub.cpp: In function 'int* initialiser(int)':
primaryAddSub.cpp:21:9: warning: address of local variable 'array' returned [-Wreturn-local-addr]
     int array[20];
         ^~~~~

I can use static int array[20]; but the function will return the same result each time I call it.

or std::vector<int> initialiser(int number) { std::vector<int> array(20); ... } and the rest would work too. Only the call site would perhaps need some adjustment - depending on what it looks like now. — Ted Lyngmo
– Ted Lyngmo, Commented May 14, 2020 at 16:16
@user4581301 Arrays are always passed by reference WAT??? Arrays decay to a pointer and that pointer is passed by value. To pass an array by reference, you need to explicitly do so like void foo(int (&x)[100]); — NathanOliver
– NathanOliver, Commented May 14, 2020 at 16:21

cigien · Accepted Answer · 2020-05-14 16:27:28Z

1

As the comment says, prefer to use a std::vector instead of an int*. An example of how this works would be:

std::vector<int> initialiser(int number)
{
    std::vector<int> array;

    // ... fill up array

    return array;
}

If you absolutely want to return an int*, then as the error says, you are returning a pointer to a local variable. You need to allocate memory instead:

int *initialiser(int number)
{
    int *array = new int[20];

    // ... fill up array 

    return array;
}

Don't forget to delete this memory when you are done with it. Also, the caller really has no way to know how many elements are in the array, so they might read/write out of bounds.

In general this is error prone, so prefer the first version, then you don't have to worry about deleting anything, and are less likely to make out of bounds errors.

edited May 14, 2020 at 16:27

answered May 14, 2020 at 16:15

cigien

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2 Comments

Gostar Over a year ago

you are a life and time saver thanks alot....it took me hours but I didn,t find any solution. Thanks again

cigien Over a year ago

No worries, glad to help. Please do take the suggestion about using vector instead, even if the array version is working now for you.

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