CODEWITHSUNDEEP
c++
Tony Issac
Tony Issac

Reputation: 29

C++ What does delete pointer,pointer=0; statement work ? Does it clear memory twice?

int *ptr = new int(10);    
printf("%d \n",*ptr);    
delete ptr,ptr=0;    
printf("%d",ptr);

Output:
10
0

My question here is how this statment "delete ptr,ptr = 0" works ? Does it free the memory twice ?

Upvotes: 2

Views: 146

Answers (6)

mission.liao
mission.liao

Reputation: 338

No, it won't. In C++, it's comma operator . Quote from wiki:

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).

Therefore, in that statement

delete ptr, ptr = 0;

is equivalent to

delete ptr; // executed first, its return value is ignored
ptr = 0;    // the return value of '=', which is '0' is returned by ',' operator

Upvotes: 3

PYA
PYA

Reputation: 8636

Delete ptr will free up dynamically allocated memory, whereas ptr=0 makes sure that pointer is now a "null" pointer so you dont get any weird/unexpected behavior

Upvotes: 0

cwfighter
cwfighter

Reputation: 502

The statement delete ptr will make the pointer become the overhang pointer. This stands for you can't use the pointer anymore, but now actually it still point to a valid memory address. So you need to use ptr=0 for making it be a nullptr. This is always a safe way to use the ptr. I hope this can help you.

Upvotes: 0

John3136
John3136

Reputation: 29266

The pointer and the memory the pointer points to are 2 different things. Setting the pointer to 0 after you delete it is just an added safety mechanism to ensure you don't try to use a memory address that you shouldn't.

int *ptr = new int(10);

ptr will have a value like 0xabcd1234, but *ptr will be 10 You can "do stuff" with the memory address 0xabcd1234 because it's allocated to you.

printf("%d \n",*ptr);
delete ptr,ptr=0;

delete ptr "gives back" the memory, but you still have it's address (that's dangerous_. ptr = 0 means you forget the address, so all is good.

I guess the "trick" is the comma operator: delete ptr,ptr=0; which as other have said means "do the left hand part, then the right hand part." If you try to get a result (int i_know_it_is_a_stupid_example = 10,20; the result is the RHS [20])

Upvotes: 4

Inisheer
Inisheer

Reputation: 20794

delete ptr

Frees the memory at the address pointed to by ptr. However, ptr still points to that memory address.

ptr = 0

ptr no longer points to a specific memory address.

Therefore, the point of ptr = 0 is to enable the programmer to test whether the pointer is still usable (in use). If you don't set ptr = 0, but only delete ptr, ptr will still point to a location in memory which may contain garbage information.

Upvotes: 3

Amadan
Amadan

Reputation: 198324

Frees the memory, then nulls the pointer so no-one could get confused that the pointer is still pointing at a valid location. Note that x, y is evaluated the same as x; y, the only difference being that the former doesn't work with statements (and is an expression, itself).

Upvotes: 1

Related Questions