Moving pointer with Dynamic Memory C++

Question

this is my first question in this forum, sorry my bad english. I have a question about pointers and dynamic memory in c++.

Example, this code:

#include <iostream>

using namespace std;

int main(int argc, char const *argv[])
{
  int *a = new int;

  for (int i = 0; i < 5; i++)
    cout << a++ << endl;

  return 0;
}

Output:

0x11d4c20
0x11d4c24
0x11d4c28
0x11d4c2c
0x11d4c30

My question, is why can I move more than that 'single' block of memory that I created with new.

• What is a pointing to?

Same occurs with new int[], even if I specific the size:

#include <iostream>

using namespace std;

int main(int argc, char const *argv[])
{
  int *a = new int[2];

  for (int i = 0; i < 5; i++)
    cout << a++ << endl;

  return 0;
}

Output:

0x2518c20
0x2518c24
0x2518c28
0x2518c2c
0x2518c30

Again, what is happening?

• What is a pointing to?

Does all of this mean I'm violating memory?

Answer 1

a is an int*, not an int. What you are printing is actually the pointer, i.e. the memory address of the pointed object. Use the dereference operator * whenever you want to modify the pointed value, i.e.

cout << (*a)++ << endl;

NB: Likewise, you can get a pointer to an int using the reference operator, &, not to be mixed up with a reference (e.g. a int& type).

This may print 0 1 2 3 4. may because you are not initializing the new int created in dynamic memory. This means reading from *a (dereferenced a) is undefined behavior, which means your program may misbehave. You have to change your line using new:

int *a = new int();

This will initialize *a to 0 and now 0 1 2 3 4 will be printed correctly.

Note that int *a = new int[2]; does create a dynamic array of 2 entries in dynamic memory, which means *(a + 1) can be used as well (as if it was a regular array). It does not initialize *a to 2.

Do remember to delete a; when you've done using it. In a real application, you could get a memory leak if you don't - i.e. your program would still use memory it doesn't need anymore. Caution, when you have to delete a dynamically-allocated array (i.e. new int[2]), you need to use delete[] a; instead, or you will trigger undefined behavior.

You may also use a unique_ptr (or a shared_ptr) in C++11 as an alternative to this kind of memory allocation, i.e. :

#include <memory>
// ...
std::unique_ptr<int> a = std::make_unique<int>(0);

Thanks to this solution, you do not need to delete a because the unique_ptr will do this for you, when itself dies (i.e. out of the scope, here).

Edit: Bonus:

0x2518c20
0x2518c24
0x2518c28

Why is the number incremented by 4 if you just used ++?

Using ++ on an address will actually increment it by sizeof(T), which here is sizeof(int) and not 1. This explains why you can, as previously stated, use *(a + 1) if you used new int[2].

Answer 2

i think that it is ,because a points to an integer , the size of an integer is 4 bytes (sizeof(int) == 4) after executing a++ ,a points to the next integer,try char *a, and a++ to be more sure

Answer 3

It is legal to point at an object or the spot right after an object. A new int[2] creates two adjacent objects (in an array) and returns a pointer to the first one.

So yes, what you did above is not permitted. The behaviour of adding 5 to a pointer to a single object is not defined by the C++ standard; the compiler can generate assembly that does anything at all.

As it happens, on a flat memory architecture, pointers are basically unsigned integers. And incrementing a pointer by 1 is just incrementing it by the size of the pointed-to object.

So what often happens is you just get pointers to whatever happens to be there.

Now this is not guaranteed and should not be relied upon. Many of the rules of C++ that make actions undefined permit certain optimizations to occur over the "naive" mapping you might think the compiler does. For example, pointers to short can never point to an int and change its value in a defined way, which means if a function has both an int and short pointer it can assume that a write to the short does not modify the int.

The naive "write to the two words in an int" method of using shorts can work, then not work for seeming no reason, because the behaviour was undefined and the compiler was free to optimize assuming it could not happen.

In short, your actions are not legal, the behaviour you got is not surprising, but you can never rely on it.

Pointers are not just unsigned integers, even if that is what your compiler implements them with. They are an abstraction. Their behaviour is determined not by their implementation, but rather what the standard permits you do with them. When you act in ways the standard does not permit, the behaviour you get is undefined by the standard. Compilers can, and have been known to, exploit that fact to assume undefined behaviour cannot and does not occur. The program could behave unexpectedly on lines of code prior to the undefined behaviour as the compiler reorders and optimizes based on the assumption your code has well defined behaviour.

Answer 4

Pointers in c++ (and c) are just addresses to the memory (32/64bit numbers). There is no problem with increasing them or decreasing any way you want and you are not violating the memory or any other rule. You would be however violating the memory if you tried to read or write to the address pointed to by A after going through the for cycle.

As for what it is pointing to, most likely it's just some more space allocated to you by the new (and malloc under it) because it tends to give more space than you ask for, though this behaviour is not guaranteed. It might also point to heap data or just unassigned memory.