Dynamic and continuous array

Question

Is there anyway to make a 1D dynamic and continuous array using C++11?

I will take in the size of the array via stdin and once it's created it won't need to be resized.

Right now I am using a 1D vector of enums, and my biggest performance issue is the vector [] operator.

If it can't be done in C++, I am open to ways of doing it with malloc and C, just please note the best way to delete it.

Edit: Didn't realize vectors were night and day with debug and release. I reran callgrind with -O3 and now the issue is ostream not vectors - thanks to everyone who made me second guess myself before I rewrote it all using arrays when not needed.

That operation on a vector is generally not more costly than an index into an array. — sje397, Feb 02 '14 at 06:56
Is the issue that you are using `enums`? Why are you not using regular integers? — Floris, Feb 02 '14 at 07:00
I am using enums for readability, it's a 1 byte enum and I do bitwise operations on it. — ParoX, Feb 02 '14 at 07:03

score 3 · Answer 1 · answered Feb 02 '14 at 06:58

You can always dynamically create contiguous homogeneous storage of a certain type from the heap using the new operator

Type *pType = new Type[size](initial_value)

In Order to delete the storage, you need to explicitly invoke the array delete operator

delete[] pType

But, when you say, and my biggest performance issue is the vector [] operator., I doubt. Did you profile the retail version of your code? How do you know the vector subscript is your performance bottleneck?

+1 "I doubt. Did you profile the retail version of your code?" — Paul Draper, Feb 02 '14 at 07:00

score 3 · Accepted Answer · answered Feb 02 '14 at 07:00

    int size = 10;
    std::unique_ptr<int[]>  a(new int[size]);

Follows RAII (that is array is autodestructed) However I don't think vector [] operator should be performance issue. In debug compilation it may be checked, however in release it should not.

In MSVC there is feature called checked iterators which may "kill" performance. However you're able to switch this feature off (just google it)

score 2 · Answer 3 · answered Feb 02 '14 at 06:59

std::vector has enough performance to use in production. It is used for solving problems on programming contests. Maybe, you forget to compile into release?

Also, you can use new and delete operators.

To free allocated memory use free(void *ptr) function.

score 0 · Answer 4 · answered Feb 02 '14 at 06:58

0

In C you create a 1-D array with

int *ptr=NULL;
int num_elements = 0;
printf("how many elements?\n");
if(scanf("%d", &num_elements)==1) {
  ptr = malloc(sizeof(int)*num_elements);
  if(ptr == NULL) {
    printf("unable to allocate %d elements\n", num_elements);
    exit 0;
  }
}

and when you're done, you call

free(ptr);

Make sure you call it only once! Some people will do

if( ptr != NULL ) {
  free(ptr);
  ptr = NULL;
}

as defensive coding - prevents accidentally freeing it twice.

answered Feb 02 '14 at 06:58

Floris

45,857
6
70
122

the if block is unnecessary - freeing a null pointer is totally okay. – Andreas Grapentin Feb 02 '14 at 07:26
@AndreasGrapentin see http://stackoverflow.com/questions/1938735/does-freeptr-where-ptr-is-null-corrupt-memory - "it was not always so". I recall running into crashed on freeing NULL (Borland C on PC in early 90's maybe). I suspect the "do nothing on `free(NULL)` standard wasn't always there. – Floris Feb 02 '14 at 13:29
the accepted answer in your linked question seems to support my point :) – Andreas Grapentin Feb 02 '14 at 13:38
1

@andreas - yes. But further down there are other observations... We both have a point but you are absolutely correct about the standard. – Floris Feb 02 '14 at 14:42

score 0 · Answer 5 · edited Feb 02 '14 at 21:35

0

If you are using C++, using malloc is probably a poor (inferior) idea.

int length;
cin >> length;

MyType * array = new MyType[length];

Then to delete,

delete[] MyType;

edited Feb 02 '14 at 21:35

Thanatos

42,585
14
91
146

answered Feb 02 '14 at 06:59

Paul Draper

78,542
46
206
285

Dynamic and continuous array

5 Answers5