strange behaviour of structures in C++[part 2]

Question

This is in continuation with Strange behaviour of structures in C++ part 1

If I do this

#include<iostream>

using namespace std;

struct box
{
    //int simple_int;
    int arr[3];
};
int main()
{
    box a={10,20,30};
    //box b={100};
    //cout<<b.simple_int<<"\n";
    cout<<a.arr[0];
}

OUTPUT : 10 which is correct.

But if i remove the comments.

#include<iostream>

using namespace std;

struct box
{
    int simple_int;
    int arr[3];
};
int main()
{
    box a={10,20,30};
    box b={100};
    cout<<b.simple_int<<"\n";
    cout<<a.arr[0];
}

OUTPUT: 100 20 //instead of 100 10

Why?

It should be a={X, {10,20,30}} where X is whatever you want simple_int to be. I don't use this syntax often so I don't know why the first works, but with only an array it should be a={{10,20,30}} — Neil Kirk, Jul 29 '13 at 14:25
It works because inner brackets are optional (see also Part 1 of this question, same answer) — MSalters, Jul 29 '13 at 15:09

score 10 · Answer 1 · answered Jul 29 '13 at 14:25

10

box a = {10, 20, 30};

will initialize a as:

a.simple_int = 10;
a.arr = {20, 30, 0};

When you output a.arr[0], it will output 20, as expected.

answered Jul 29 '13 at 14:25

Nemanja Boric

21,627
6
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2

Not `{20, 30, 30}`, but `{20, 30, 0}` – John Dibling Jul 29 '13 at 14:26
That is correct syntax? It shouldn't be as I say in the comments? – Neil Kirk Jul 29 '13 at 14:26

Pierre Fourgeaud · Answer 2 · 2013-07-29T14:33:14.740

3

Your code give the correct output.

Your struct will be initialized at :

a.simple_int = 10;
a.arr = {20, 30, 0};

So a.arr[0] will be 20.

Output of your program : 100 20 as expected.

If you want to avoid this kind of misunderstanding, initialize your structure more like :

box a = { 100, { 10, 20, 30 } };

With that, the result is :

a.simple_int == 100;
a.arr[0] == 10;

edited Jul 29 '13 at 14:33

answered Jul 29 '13 at 14:28

Pierre Fourgeaud

14,290
1
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62

score 0 · Answer 3 · answered Jul 29 '13 at 14:27

You can not initialize arrays like that. In the second example, your struct has an int and an array of ints. Your initialization of a doesn't correspond to anything and so the compiler partially fills it for you.

You could write a = {5, {10, 20, 30}} to make a.simple_int be 5 and make a.arr be an array of those 3 values.

strange behaviour of structures in C++[part 2]

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