how to initialize pointer with array using constructor C++

Question

creat a pointer in struct creat array initialize -1, 10 times using for loop and print -1, 10 times using method in struct.

struct hasha {
    int* arr;
    int l;
    hasha(int no) {
        arr[no];
        l = no;
        for (int i = 0; i < l; i++) {
            arr[i] = -1;
        }
    }
    void print() {
        for (int i = 0; i < l; i++) {
            cout << arr[i] << " ";
        }
    }
};
int main() {
    hasha a(10);
    a.print();
}

whoever gave you this taks should also have provided you the required basics, see also here: https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list — 463035818_is_not_an_ai, Sep 15 '20 at 13:22
Use `std::vector`, else you have to do allocation/deallocation manually. — Jarod42, Sep 15 '20 at 13:22
`std::vector a(10, -1); for (auto e : a) {std::cout << e << " ";}`. — Jarod42, Sep 15 '20 at 13:24
`arr[no]` does not create an array. Review the things you’ve learned recently. — molbdnilo, Sep 15 '20 at 13:42

score 1 · Accepted Answer · answered Sep 15 '20 at 13:42

I assume this is for learning purposes, otherwise you should be using a std::vector<int> for this.

struct hasha {
    int* arr;
    size_t l;                // size_t is an unsigned int type better suited for sizes

    hasha(size_t no) :       // use the member initializer list (starting with ":")
        arr(new int[no]),    // create the array
        l(no)
    {
        // initialize the array with -1, "l" times using a for loop
        for (size_t i = 0; i < l; ++i) {
            arr[i] = -1;
        }
    }

    // You need to delete the implicitly declared copy ctor and copy assignment operator
    // to forbid copying hasha objects to not free the `int*` multiple times later.
    hasha(const hasha&) = delete;
    hasha& operator=(const hasha&) = delete;

    // You need a destructor to free the memory you allocated:
    ~hasha() {
        delete[] arr;
    }

    void print() {
        for (size_t i = 0; i < l; ++i) {
            std::cout << arr[i] << " ";
        }
    }
};

*"unsigned int type better suited for sizes"*. While I agree, it is opened for debates ;-) — Jarod42, Sep 16 '20 at 15:16
@Jarod42 :-) True. It's also a bit tricky to use it consistently. — Ted Lyngmo, Sep 16 '20 at 15:21

score -1 · Answer 2 · answered Sep 15 '20 at 13:26

-1

The following constructor initialises the pointer with an array: hasha ::hasha (int a[]) : arr (a) {}

answered Sep 15 '20 at 13:26

Christopher Yeleighton

454
2
6

1

There are a lot of things wrong with this. It doesn't initialize an array, it sets the pointer to an external array. It is incompatible with the question's goals. – François Andrieux Sep 15 '20 at 13:31
This is exactly what the author asked for. – Christopher Yeleighton Sep 15 '20 at 13:32
1

This does not create an array of 10 values at -1, and is not compatible with the shown `main` function. You misunderstood the question. – François Andrieux Sep 15 '20 at 13:33
The only part I can understand from the question is the summary, so I replied to what I was able to understand. – Christopher Yeleighton Sep 15 '20 at 13:35
2

In the future, you can ask for clarification in the question's comments section. – François Andrieux Sep 15 '20 at 13:36

how to initialize pointer with array using constructor C++

2 Answers2