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c++pointersstruct

Problems with pointers and struct (C++)

Can someone tell me what is wrong with the following code?

#include <iostream>
using namespace std;

typedef struct point{
    int key;
    struct point * key0;
} pnt;

int main() {
    pnt * p = new pnt;
    p->key = 1;
    p->key0->key = 2;
}

I'm kinda new to C++ and I'm still trying to understand these concepts.

I was expecting to be able to set some arbitrary value to the structure I defined. I was just experimenting if I understood how that work.

The compiler just says I used an illegal instruction.

Solution

  • This line:

    pnt * p = new pnt;

    Allocates a pnt struct, but does not initialize its contents. The address of the allocated struct is assigned to p.

    In this line:

    p->key = 1;

    You initialize the key field.

    But at this point, the key0 field is still uninitialized.

    But, in this last line:

    p->key0->key = 2;

    You attempt to dereference the uninitialized key0 field (with key0->key). This is causing undefined behavior.

    To avoid it, you need to initialize key0, either by allocating another pnt struct and using its address, or by using the address of an existing pnt object.

    Side notes:

    1. In C++, it is usually better to avoid using raw new.
      To begin with, it requires a delete (without it, you get a memory leak - as in your code).
      Also, there are better alternatives, like using smart pointers, or in other cases - using standard containers.

    2. It is better to avoid using namespace std;. See here.

    3. In C++, there's no need for typedef to create your struct type (it's common in C). You can simply use:

      struct pnt{
    ...
    pnt * key0;
};