Initialize a struct's pointer in c++

Question

Please help me understand what I'm doing wrong here.

Struct is

struct node
{
int value;
stack <int>losersStack;
};

Main is

int main()
{
    int size;
    printf("Enter size of arrary\n");
    scanf("%d", &size);
    node *arr = new node[size];
    for(int i=0; i<size; i++)
    {
        printf("Enter a value\n");
        scanf("%d", arr[i].value );
    }
}

I get an error during the value entering. "Unhandled exception at 0x55e5effe (msvcr100d.dll) in blabla: 0xC0000005: Access violation writing location 0xcdcdcdcd."

Thanks in advance!

@Barmar I'm more used to it than to cout.. Is that part of the issue? — Ran Lavi, Mar 27 '18 at 18:37
You seem to know how to use `scanf` when you get the size (but why use `scanf` in the first place in C++?), but then you *forget* how to use it in the loop? — Some programmer dude, Mar 27 '18 at 18:37
If you're more used to it, then you should know that you need to give the address of the variable. — Barmar, Mar 27 '18 at 18:37
Firstly, this is more C than C++, and I would recommend choosing one over the other since modern C++ code style diverges significantly from C. Also, if it helps, `0xCDCDCDCD` is a sign that your compiler has inserted code to mark memory as being unitialized. [see more on magic numbers here](https://stackoverflow.com/questions/127386/in-visual-studio-c-what-are-the-memory-allocation-representations) — alter_igel, Mar 27 '18 at 18:38
@Barmar OMG!! I was wondering why it worked a few hours ago and now it started with this error! Probably in one of my edits I've erased that by mistake! Thank you so much! — Ran Lavi, Mar 27 '18 at 18:39
@Some programmer dude That's what happens when you work on the same code for too many hours.. You start changing and skipping stuff.. Lost my focus — Ran Lavi, Mar 27 '18 at 18:43
@alter igel Thanks! I already saw it in under a question somebody else posted. But I was sure that the mistake is before the loop for some reason, with the "new node". Got totally off guard with the "&" — Ran Lavi, Mar 27 '18 at 18:45
@Rosh Then it's time for either coffee, coke, or bed. Perhaps a coke while finishing up and then bed? Or at least go outside and get some fresh air, walk of your tensions, clear your mind. Always good to do a couple of times a day. :) — Some programmer dude, Mar 27 '18 at 18:47
@Some programmer dude You're definitely right. I haven't even been outside today, and it's 10PM. I just really wanted to finish writing several codes by midnight and submit on time. But I guess I'll take the penalty of submitting late, cause in the current concentration state, I'll do more harm than good. Thanks dude! — Ran Lavi, Mar 27 '18 at 19:03

score 3 · Accepted Answer · answered Mar 27 '18 at 18:40

3

You need to give the address of the variable:

scanf("%d", &arr[i].value);

But since you're writing C++, it's easier to use cin:

cin >> arr[i].value;

answered Mar 27 '18 at 18:40

Barmar

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score 2 · Answer 2 · answered Mar 27 '18 at 18:51

In the code you posted, you need an ampersand & before the arr expression.

scanf("%d", &arr[i].value );

But don't use all that low level stuff. In particular, avoid using new. Here (minus error-checking on the input) is the C++ way to do it.

#include <iostream>
#include <stack>
#include <vector>
struct node
{
int value;
std::stack <int>losersStack;
};

int main()
{
    std::cout <<  "Enter size of arrary\n";
    int size;
    std::cin >> size;
    std::vector<node> arr(size);
    for(int i=0; i<size; i++)
    {
        std::cout << "Enter a value\n";
        std::cin >> arr[i].value;
    }
}

Initialize a struct's pointer in c++

2 Answers2