copying a string in C++ "Error reading characters of string."

Question

I am implementing a stack and I would like to push and pop string data. can I not just set

string name = originalString ???

here is what I have for push:

void push(StackNode** top_ref, int nAttack, int nDefense, string nName, string nType) {
    StackNode* stackNode = (StackNode*)malloc(sizeof(StackNode));
    stackNode->attack = nAttack;
    stackNode->defense = nDefense;

    stackNode->name = nName; //not working
    stackNode->type = nType; //not working

    stackNode->next = NULL;
    stackNode->next = *top_ref;
    *top_ref = stackNode;
}

where stackNode->name

and stackNode->type is already defind as a string

I keep getting: "Error reading characters of string."

Is this for an academic class where you're required to use `malloc`? If not, can I introduce you to `new[]`? And then after you've gotten accustomed to that, can I introduce you to `std::stack`? https://en.cppreference.com/w/cpp/container/stack — JohnFilleau, Mar 18 '20 at 03:08
Yepp new made it work, although I am not quite sure why it is so different to malloc. Thanks for the help! I replaced malloc with this for future people StackNode* stackNode = new StackNode; — conglomerate, Mar 18 '20 at 03:20
`malloc` is dumb and only gives you that amount of memory. The constructor isn't called. With `new`, the constructor is called. That's the difference. — JohnFilleau, Mar 18 '20 at 03:20
But to be clear - this is for an academic exercise where you're required to do your own memory management? Because the C++ best practice is to use STL containers when they're appropriate. Here, you'd use `std::stack`. — JohnFilleau, Mar 18 '20 at 03:21

score 0 · Accepted Answer · answered Mar 18 '20 at 03:16

The issue is malloc doesn't play well with objects. See Does malloc create a new instance of the class or not?

malloc allocates enough space to hold a StackNode, but doesn't call the constructor on it. Nor does it create the underlying member objects, your strings included. To do so, you need to do like described in this answer:

A* a = (A*)malloc(sizeof(A));
new (a) A();

a->~A();
free(a);

But at this point, you really should just be using new anyway. There's very little reason to use new in C++ (there's a time and a place, but minimize it). There's even fewer times when it's right to call malloc.

Do

StackNode* stackNode = new StackNode;

which will call the constructor appropriately. Remember to call delete on the memory when you're done with it!

thanks John, it was definitely "new" that was needed thanks for the explanation as well! — conglomerate, Mar 18 '20 at 03:22

copying a string in C++ "Error reading characters of string."

1 Answers1