Confusion about calling operator new function directly

Question

I know that we normally don't call operator new & operator delete function directly. Because If I do so constructor & destructor won't be called & it likely ends up with memory leak. But for primitive types this seems not problematic. Correct me If I am wrong.

Consider following program:

#include <iostream>
int main()
{
    int* p=(int*)operator new (3);
    std::cout<<*p;
    operator delete(p);
}

Why it prints garbage value as output? What I have done wrong here? I am curious to know about how to correctly give value by calling operator new function directly. I want the output of program 3 as a output, but by not using new operator like int* p=new int(3);. What should I do?

_"Because If I do so constructor & destructor won't be called & it likely ends up with memory leak."_ Where did you hear this? Where should a memory leak come from? — Lightness Races in Orbit, Sep 19 '15 at 16:42
You misunderstood it. That says that the destructor won't be invoked, which is problematic _if the destructor has work to do_. In this case you use `int`, which has no members which need clearing up. No memory leak. — Lightness Races in Orbit, Sep 19 '15 at 16:43
Also, you said yourself that the constructor won't be called so I don't understand why you think `3` will function as a constructor argument here. Read those links yourself. — Lightness Races in Orbit, Sep 19 '15 at 16:43
Create the integer normally? `int* ptr = new int(3)` or `int x = 3`? I don't understand what you're trying to do. You're saying "I really want to use this strange syntax that doesn't invoke the constructor. How do I use it to invoke the constructor?" — Lightness Races in Orbit, Sep 19 '15 at 16:46
It's better to say there is no memory leak for `int` because `int` doesn't manage any dynamically-allocated resources. It's just a number. — Lightness Races in Orbit, Sep 19 '15 at 16:47
No thanks; I must charge a fee for one-on-one mentoring/teaching. — Lightness Races in Orbit, Sep 19 '15 at 16:49

score 3 · Accepted Answer · edited May 23 '17 at 12:23

You have two problems:

You only allocate three bytes
You don't initialize the memory you allocate.

Both of these will lead to undefined behavior.

The argument to the operator new function is the number of bytes to allocate, not what to initialize the allocated memory to.

The new operator, and the function operator new are two different things that behaves differently.

See this old question and answers for more details.

score 1 · Answer 2 · answered Sep 19 '15 at 16:50

I think the main misunderstanding in your code is that you think that here:

int* p=(int*)operator new (3);

You are doing this:

int* p = new int(3);

It prints garbage value because your int to which p points has not been initialized.

What you actually do is you allocate 3 bytes of memory for storing and int. So int pointer p will now point to memory are with 3 bytes allocated (on most platforms you need 4 bytes to store an int).

See the new C++ reference for more information (first overload is what you should check).

Confusion about calling operator new function directly

2 Answers2