Adding memory allocation to a C++ array

Question

Question

What is the most efficient way to add memory allocation to an already declared variable?

For example:

int n=3;
int m = 4;
int * p;
p = new int[n];
for(int i = 0;i<n;i++) {/*function that stores values in p*/}

From there, how to get to:

p = new int[n+m];

By copying

I've thought about making a new variable (y for example) with the required memory allocation (n+m) and copy the value of p to it and then copy y to p. Is that the most efficient way to do this ?

Example:

int * y;
y = new int[n + m];
for(int i = 0; i < n;i++) {y[i] = p[i];}
p = y;
delete [] y;

Related question:

Is there a purpose for memory allocation without an array?

Example:

int * p;
p = new int;

What would require this method instead of declaring int p?

The most efficient way is to use `std::vector`, where "efficient" is about both computing and programmer effort. — , Mar 03 '16 at 05:16
Why would I keep both y and p if I only needed the final result ? — JoGooD, Mar 03 '16 at 05:25
Because y and p are both pointing to the same space in memory. If you delete y, you also delete p. You need to allocate a whole new array for p, copy the contents of the array at y to the array at p, and then you can safely delete y. but there isn't much need for this. so long as p is still pointing at y's memory, y can go the way of the dodo and later you can delete p. the real problem is in deleting p's memory before you lose the pointer to it by copying y over it. — user4581301, Mar 03 '16 at 05:55

score 0 · Accepted Answer · answered Mar 03 '16 at 05:18

0

Is that the most efficient way to do this ?

No, this is no good because you freed what to use and caused memory leak by not freeing what will be no longer used. At least it should be

int * y;
y = new int[n + m];
for(int i = 0; i < n;i++) {y[i] = p[i];}
delete [] p;
p = y;

There may be more efficient method, depending on the usage. For example, linked list may be more efficient if you don't need randam access for elements.

answered Mar 03 '16 at 05:18

MikeCAT

73,922
11
45
70

So instead of using p, I should just delete p and use y from there ? (there is a mistake on the last line of your code) – JoGooD Mar 03 '16 at 05:29
@JoGooD No error was found in [my test](https://ideone.com/srNmWL). Please tell me the mistake you found. Which of `p` or `y` should be used may depend what you want, but I guess typically using `p` is better because it may be declared for larger scope. – MikeCAT Mar 03 '16 at 05:34
`delete [] p;` `p = y;` `p` is deleted then `y` is assigned to `p` – JoGooD Mar 03 '16 at 05:38

score -1 · Answer 2 · answered Mar 03 '16 at 05:14

I think what you want is std::vector. You can use this like an array that you can resize dynamically.

int n=3;
int m = 4;
std::vector<int> p(n);
for(int i = 0;i<n;i++) { ... }

then later

p.resize(n+m);

The memory will also freed automatically when p goes out of scope.

Adding memory allocation to a C++ array

2 Answers2