How is a dynamic array different from statically bound array in C++?

Question

I am learning C++ and I just read about dynamic arrays and how it lets you set the length of an array during runtime rather than during compile time. However, you don't need a dynamic array to do this. So what is the point of a dynamic array; when would you use it? I feel like I am missing something obvious so any insight is much appreciated. Thanks!

// Static binding.
int size = 0;

cout << "Enter size of array:" << endl;
cin >> size;

int array[size];

int array_length = sizeof(array) / sizeof(int);
cout << "Number of elements in array: " << array_length << endl;

// I just set the length of an array dynamically without using a dynamic array.
// So whats the point of a dynamic array then?

I think the compiler that you are using lets you assign a variable as a size of an array as an extra feature. It still uses dynamic arrays in the backend to accomplish that, as you are technically not allowed to assign variable sizes to arrays. — ankit
– ankit, Commented Sep 17, 2011 at 0:56

Mysticial · Accepted Answer · 2011-09-17 01:00:49Z

3

I don't think you can do that in C++. Only C99 allows variable-length arrays.

Does this even compile? Were you talking about the vector class?

EDIT:

It does not compile in Visual Studio 2010:

1>..\main.c(207): error C2057: expected constant expression
1>..\main.c(207): error C2466: cannot allocate an array of constant size 0
1>..\main.c(207): error C2133: 'array' : unknown size
1>..\main.c(209): error C2070: 'int []': illegal sizeof operand

edited Sep 17, 2011 at 1:00

answered Sep 17, 2011 at 0:55

Mysticial

473k46 gold badges343 silver badges337 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

ab217 Over a year ago

It works just like you think it would. I am running whatever compiler comes with Xcode 4 on the mac and no I am talking about int *array = new int[dynamic_size]; type of dynamic arrays.

Mysticial Over a year ago

Then it's definitely a compiler extension. I'm close to 100% positive that this isn't specified in any C++ standard to date.

ab217 Over a year ago

Okay so the code I wrote above probably wouldn't work with another compiler; I would actually have to use a legitimate dynamic array instead?

Mysticial Over a year ago

Correct, you'll have to dynamically allocate an array with new. (and delete it later)

ab217 Over a year ago

Okay thanks. I understand you would have to use a dynamic array with any other compiler; my compiler just misled me because it supports variable length array sizes.

oh whatever · Accepted Answer · 2011-09-17 00:56:26Z

1

You would need a dynamically allocated array for cases where you don't know ahead of time how many items you will have.

Another (and better) option is to use std::vector.

answered Sep 17, 2011 at 0:56

oh whatever

20.3k2 gold badges38 silver badges50 bronze badges

Comments

adpalumbo · Accepted Answer · 2011-09-17 01:04:57Z

1

As per the standard, an array defined "statically" would:

have a constant size, defined at compile time
Be allocated on the stack rather than the heap.

So the reasons you'd opt for a dynamically allocated array (using new type[]) are because you don't always know the size you need for an array at compile time, and the stack is a limited resource that you often need to be careful not to exhaust.

But in practical terms, you'd often be best served by using std::vector or other STL container instead of a builtin array of any sort.

answered Sep 17, 2011 at 1:04

adpalumbo

3,03115 silver badges12 bronze badges

Comments

Don Reba · Accepted Answer · 2011-09-17 01:12:55Z

1

Your example would not compile under a standards-compliant compiler. For instance, GCC says this:

http://codepad.org/Kvq2PfNx

In function 'int main()':
Line 13: error: ISO C++ forbids variable-size array 'array'
compilation terminated due to -Wfatal-errors.

There is a way of dynamically allocating stack memory using the alloca function. However, this memory is invalidated when the function terminates. You should generally prefer new[]/delete[] or std::vector for dynamic memory allocation.

answered Sep 17, 2011 at 1:12

Don Reba

14.1k3 gold badges51 silver badges65 bronze badges

Comments

Jan S · Accepted Answer · 2011-09-17 01:02:28Z

0

You can dynamically create an array using the keyword new.. Lets say you don't know ahead of time how many values you need. It's a waste of space to declare an array a[100] when the user might just enter a few values. Also the user might enter more values, and then you would have a array overflow error. You can create an array dynamically like - int a = new int[]
Also, statically created arrays are created on the stack where as dynamically created arrays are created on the heap-which means the memory is globally available even after the function goes out of scope.

answered Sep 17, 2011 at 1:02

Jan S

1,83715 silver badges21 bronze badges

Collectives™ on Stack Overflow

How is a dynamic array different from statically bound array in C++?

5 Answers 5

5 Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

5 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related