Benefit of using pointers for dynamic memory allocation with arrays

Question

I am trying to understand the benefit using this...

double *x = new double[n];

instead of just using this...

double x[n];

Thanks

#include <iostream>

using namespace std;

main()
{
    int n;
    cout<<"# of elements in array"<<endl;
    cin>>n;

    double *x = new double[n]; //or double x[n]

    int i;

    for(i=0;i<n;i++)
    {
        cout<<x[i]<<endl;
    }
    return 0;
}

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Rontogiannis Aristofanis
– Rontogiannis Aristofanis

2012-10-11 16:32:11 +00:00
Commented Oct 11, 2012 at 16:32 — Rontogiannis Aristofanis
– Rontogiannis Aristofanis, Commented Oct 11, 2012 at 16:32

Community · Accepted Answer · 2020-06-20 09:12:55Z

5

Mandatory note:

std::vector<double> x(n);

beats what you have there.

Actual answer:

The benefit is that

double *x = new double[n];

is legal, whereas

double x[n];

is not, unless n is a compile-time constant (in your case, it's not).

C++ doesn't support variable-length-arrays.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Oct 11, 2012 at 16:19

Luchian Grigore

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2 Comments

indivisible.invisible.indivi.. Over a year ago

In this context what does it mean for something to be legal? Both methods work. What I'm trying to understand is the reason why I would use the pointer?

Nemanja Trifunovic Over a year ago

@Rambi: the second one does not work on most C++ compilers. You are probably using g++ where it works.

tomahh · Accepted Answer · 2012-10-11 16:30:40Z

0

Dynamic allocation allow you a lot of things.

First of all, you can return this array from the function where it is create.
Strict C(C89) does not allow dynamic stack allocation. double x[n] will throw error on many compilers.

edited Oct 11, 2012 at 16:30

answered Oct 11, 2012 at 16:20

tomahh

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2 Comments

KitsuneYMG Over a year ago

"Strict C does not allow dynamic stack allocation" incorrect. C99 added variable length arrays

Michał Górny Over a year ago

I believe that Tom meant C89 «Strict C». Not to mention the post is about C++, obviously. And VLAs are really poorly defined, that's one of the easiest ways to obtain self-destructive behavior in programs.

Roddy · Accepted Answer · 2012-10-11 16:37:07Z

0

Dynamically allocated arrays won't cause Stack Overflow if they're too big.

The pointer lives on the stack, but the contents live on the heap. The downsides, however, include slower access due to an indirection, possible memory leaks and more impenetrable code.

answered Oct 11, 2012 at 16:37

Roddy

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