zero an array inside a struct in c++

Question

I have a struct defined in my program.

struct A{
    int arr[10];
}

Lets say I have a pointer to it. A * a = new A;

I can zero it in two ways:

memset(&a->arr,0,sizeof(A));
memset(a->arr,0,sizeof(A));

both work and look the same!

which one is more correct?

i know i can use a class and a constructor, this code snippet is an small demo that demonstrate an example so using a class or a different function instead of memset will not help — rafi wiener, Jun 29 '15 at 15:31

score 2 · Answer 1 · answered Jun 29 '15 at 15:19

2

which one is more correct?

I'd argue neither. The easiest way would be to value initialize the allocated object:

A * a = new A();

Of course, this assumes that you actually have a good reason to new this object.

answered Jun 29 '15 at 15:19

juanchopanza

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so for C which is better? – rafi wiener Jun 29 '15 at 15:23
@rafiwiener Neither one is better, and neither one is worse. The compiler will most likely generate the exact same code for both variants. – Some programmer dude Jun 29 '15 at 15:27
@rafiwiener No clear "better", but I prefer the first one since the second one actually converts to the first one, and therefore would make the first one more "the better way" – KABoissonneault Jun 29 '15 at 15:27
1

@rafiwiener It is a matter of opinion. I would go for the second variant. – juanchopanza Jun 29 '15 at 15:27
this is an argument during a code review with a co-worker, i did the second way – rafi wiener Jun 29 '15 at 15:35
1

@rafiwiener My way is better. I win :-) – juanchopanza Jun 29 '15 at 15:35
Good luck using `new` in C :). – Derek Jun 29 '15 at 17:09

score 2 · Answer 2 · answered Jun 29 '15 at 15:33

Since you are using C++ I would take advantage of C++11 features and use:

#include <iostream>
#include <cmath>
using namespace std;

struct A{
    int arr[10]{};  // initializes array with all 0's
};

int main() {
    A * a = new A;
    for (auto e : a->arr) // ranged based for loop to show all of the 0's
        cout << e << "\n";
    return 0;
}

You can see it running with this Live Example

Some programmer dude · Answer 3 · 2015-06-29T15:29:45.947

1

While the type of each expression is different, the actual result, the pointer you pass to memset, will be equal in both cases.

Personally I would probably use std::fill instead of memset in a C++ program:

std::fill(std::begin(a->arr), std::end(a->arr), 0);

Also note that if you have more members in the structure, sizeof(A) will be different from sizeof(a->arr) (or sizeof(A::arr)).

edited Jun 29 '15 at 15:29

answered Jun 29 '15 at 15:24

Some programmer dude

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kiviak · Answer 4 · 2015-06-29T15:41:28.087

1

you can define a default construct function

struct A{
    int arr[10];
    A():arr(){}
};

edited Jun 29 '15 at 15:41

answered Jun 29 '15 at 15:26

kiviak

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There's really no need for that loop! Use value initialization: `A () : arr() {}`. – juanchopanza Jun 29 '15 at 15:26
@ juanchopanza i dont kown new features of c++11 – kiviak Jun 29 '15 at 15:35
1

It is an old C++03 feature – juanchopanza Jun 29 '15 at 15:37
@ juanchopanza i got it ,thank you – kiviak Jun 29 '15 at 15:40

score 0 · Accepted Answer · answered Jun 29 '15 at 15:26

This is the correct way for just the array

memset(a->arr,0,sizeof(a->arr))

Picked out the arr member just in case there are other structure members that do not need to be touched. Makes no difference in your example the following will do likewise

memset(a->arr,0,sizeof(A));

zero an array inside a struct in c++

5 Answers5