Question

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What is difference between instantiating an object using new vs. without

This is probably a basic question, and might have already been asked (say, here); yet I still don’t understand it. So, let me ask it.

Consider the following C++ class:

class Obj{
    char* str;
public:
    Obj(char* s){
        str = s;
        cout << str;
    }
    ~Obj(){
        cout << "Done!\n";
        delete str;        // See the comment of "Loki Astari" below on why this line of code is bad practice
    }
};

what’s the difference between the following code snippets:

Obj o1 ("Hi\n");

and

Obj* o2 = new Obj("Hi\n");

Why the former calls the destructor, but the latter doesn’t (without explicit call to delete)?

Which one is preferred?

Answer 1

Both do different things.

The first creates an object with automatic storage duration. It is created, used, and then goes out of scope when the current block ({ ... }) ends. It’s the simplest way to create an object, and is just the same as when you write int x = 0;

The second creates an object with dynamic storage duration and allows two things:

• Fine control over the lifetime of the object, since it does not go out of scope automatically; you must destroy it explicitly using the keyword delete;

• Creating arrays with a size known only at runtime, since the object creation occurs at runtime. (I won’t go into the specifics of allocating dynamic arrays here.)

Neither is preferred; it depends on what you’re doing as to which is most appropriate.

Use the former unless you need to use the latter.

Your C++ book should cover this pretty well. If you don’t have one, go no further until you have bought and read, several times, one of these.

---

Your original code is broken, as it deletes a char array that it did not new. In fact, nothing newd the C-style string; it came from a string literal. deleteing that is an error (albeit one that will not generate a compilation error, but instead unpredictable behaviour at runtime).

Usually an object should not have the responsibility of deleteing anything that it didn’t itself new. This behaviour should be well-documented. In this case, the rule is being completely broken.