CODEWITHSUNDEEP
c++pointers
SlackerByNature
SlackerByNature

Reputation: 107

Issue with understanding how pointers and 'new' work

I'm currently learning about pointers in my C++ book (Programming: Principles and Practice using C++ by Stroustrup). The book had me do the following 'drill' to become accustom to pointers and arrays. I've commented parts of the drill that aren't relevant to my issue.

int num = 7;
int* p1 = #

// output p1 address and content...

int* p2 = new int(10);

// initialise each element, and output content...

int* p3 = p2;
p1 = p2;

// output p1 and p2 address and content...

delete[] p1;

/* As all pointers now point to the same array created in the free store, 
   I was under the impression that I only needed to use delete for 1 of 
   the pointers to deallocate memory,as above, but the program crashes 
   if I don't do it for all 3 and execute next section of code? */

p1 = new int(10);
p2 = new int(10);

// Initialise each array to a different range of numbers using a loop,
// output each array, change elements in p2 to be the same as p1, output...

delete[] p1;
delete[] p2;

The last part is where I am having trouble. When outputting each array, the elements values are the same. My guess is that p1 still == p2, due to the code a few lines before. I thought that when you use the 'new' keyword it returns an address, referencing a different, newly allocated block of memory and therefore p1 would no longer == p2. The only way I got it to work was to directly create 2 arrays and have p1 and p2 reference them using the & operator. Any explanation as to what I'm doing wrong is appreciated.

Upvotes: 0

Views: 100

Answers (2)

Lubo Antonov
Lubo Antonov

Reputation: 2318

The problem probably stems from the fact that when you say

p = new int(10)

you are allocating just ONE integer and initializing it to 10, not an array of size 10.

Upvotes: 2

Luchian Grigore
Luchian Grigore

Reputation: 258558

int* p2 = new int(10);

// initialise each element, and output content...

int* p3 = p2;
p1 = p2;

// output p1 and p2 address and content...

delete[] p1;

This code leads to undefined behavior, because you allocate with new and free the memory with delete[]. 

int* p2 = new int(10);
//allocates a single int with value 10

is different from

int* p2 = new int[10];
//allocates an uninitialized array of 10 ints

That aside (although a serious issue, as all undefined behavior), the problem was this:

int* p2 = new int(10);
int* p3 = p2;
p1 = p2;
//all pointer point to the same memory location

delete[] p1;
//delete that memory
//all three pointers are now invalid

Attempting to free the memory again via delete p2 or delete p3 will again lead to undefined behavior, and probably a crash, since you already deleted that memory. That is why allocating new memory will fix the crash.

Bottom line: don't free the same memory multiple times.

Upvotes: 2

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