Dynamically allocating a structure in C++ using malloc

Question

#include <iostream>
#include <stdlib.h>
#include <string.h>
using namespace std;
struct product{
        string productName;
        float price;
};

int main()
{
    struct product *article;
    int n=2; // n represent here the number of products
    article= (product*) malloc(n * sizeof(product));
    for(int i=0;i<n;i++)
    {
        cin >> article[i].productName; // <=> (article+i)->productName;
        cin >> article[i].price;
    }

    for(int i=0;i<n;i++)
    {
        cout << article[i].productName <<"\t" << article[i].price << "\n";
    }
    return 0;
}

My question is why this is wrong, because when I try to run this I get a segmentation fault. I used the GDB debugger to see which line is causing the problem, and this is the line that was causing this:

cin >> article[i].productName;

Why? This was bothering me for days...

Treat this as an object lesson in why you don't use `malloc()` et al in C++ programs. — Jonathan Leffler, Feb 28 '15 at 17:07
yes i can compile it, and @JonathanLeffler yes this is so helpful; i can't imagine how i could figure this out by my self thanks guys — Yassine Bakkar, Feb 28 '15 at 17:12

score 6 · Accepted Answer · edited Feb 28 '15 at 19:07

6

When you allocate memory with the new operator, it does two things:

it allocates memory to hold an object;
it calls a constructor to initialize the objects.

In your case (malloc), you do only the first part so your structure members are uninitialized.

edited Feb 28 '15 at 19:07

Jonathan Leffler

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answered Feb 28 '15 at 17:04

Andrey Derevyanko

550
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score 1 · Answer 2 · answered Feb 28 '15 at 16:59

1

article[0] is not initialized (i.e. the constructor of struct product has not been called for article[0]). Therefore, article[0].productName has not been initialized either.

Use new product[n] instead of (product*) malloc(n * sizeof(product)) to initialize the array elements (and by transitivity the members of the elements).

answered Feb 28 '15 at 16:59

Oswald

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@YassineBakkar use `std::vector article(n);` . Then the memory will be freed automatically. – M.M Mar 02 '15 at 00:05

Kajgies · Answer 3 · 2015-03-01T23:40:12.727

Try using this:

#include <iostream>
#include <string>
using namespace std;
struct product{
    string productName;
    float price;
};

int main()
{
    int n = 2; // n represent here the number of products
    product *article = new product[n];
    for (int i = 0; i<n; i++)
    {
        cin >> article[i].productName; // <=> (article+i)->productName;
        cin >> article[i].price;
    }

    for (int i = 0; i<n; i++)
    {
        cout << article[i].productName << "\t" << article[i].price << "\n";
    }
    return 0;
}

article[0] was not initialized when you used cin. If you use new[] instead, it should work.

nmserve · Answer 4 · 2015-03-03T07:05:19.443

0

Instead of malloc-ing object, use new

product *article = new product[n];

edited Mar 03 '15 at 07:05

answered Feb 28 '15 at 17:00

nmserve

11
5

1

It would have to be an array `new`, wouldn't it? – Jonathan Leffler Feb 28 '15 at 17:06

Dynamically allocating a structure in C++ using malloc

4 Answers4