c++ initialize array with declared size as a value of an integer

Question

I want to initialize an array with a size using a value I read into an integer variable. I cant seem to understand why it works in Dev-C++ but not in Turbo C++. Here's the code to help make things clear

int arr_size; //cin max value for lets say number of students or something...
cin >> arr_size;
int array[arr_size]; // declares array with size (assume 10 or 100) with range 0 to 9 or 0-99

The compiler shows an error in Turbo C++ (really old, I know, but my school uses it unfortunately). Dev-C++ and codeblocks doesnt.

Why is that so? I know its bad practice "as they define it in some books" to have an array size the same as an int value, but is there a work around for this in Turbo C++? I want to know why the error happens and how I can get a work around it ... thanks a lot!

Always thought variable length arrays are not allowed in standard c++. You can do it in C. See en.wikipedia.org/wiki/Variable-length_array — dubnde
– dubnde, Commented Jun 13, 2011 at 13:00

Oliver Charlesworth · Accepted Answer · 2011-06-13 13:15:52Z

9

The C++ standard only permits arrays to be sized with a constant expression. (However, some compilers may offer it as a non-standard language extension.)

You could use a std::vector instead:

std::vector<int> array(arr_size);

Or you could dynamically-allocate memory manually:

int *const array = new int[arr_size];

...

delete [] array;  // Remember to delete when you're done

edited Jun 13, 2011 at 13:15

answered Jun 13, 2011 at 12:58

Oliver Charlesworth

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2 Comments

Puppy Over a year ago

Uh, a vector is dynamically allocated memory?

Ben Voigt Over a year ago

@DeadMG: We all know he meant "or you could dynamically allocate memory manually".

dubnde · Accepted Answer · 2011-06-13 13:03:14Z

7

Variable length arrays are not allowed in standard c++. You can do it in C99. Consider using C++ std::vector as :

std::vector<int> array(arr_size);

And you can index it exactly like the array if you have to.

answered Jun 13, 2011 at 13:03

dubnde

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1 Comment

Sarfaraz Nawaz Over a year ago

Specifically, C99. Other version of C doesn't allow it either.

Kik · Accepted Answer · 2011-06-13 13:01:43Z

2

The workaround is to dynamically allocate this array, making sure to delete the memory when done.

int arr_size; //cin max value for lets say number of students or something...
cin >> arr_size; 
int *arr = new int[arr_size];
//use the array as needed
delete [] arr;

answered Jun 13, 2011 at 13:01

Kik

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1 Comment

Rhexis Over a year ago

with: delete [] arr; inside the [] what do you put?

Sarfaraz Nawaz · Accepted Answer · 2011-06-13 12:58:45Z

1

You want variable length array (VLA) which is not allowed in C++. Its allowed in C99.

Use std::vector<int> instead, as:

int arr_size; 
cin >> arr_size;
std::vector<int> array(arr_size);

answered Jun 13, 2011 at 12:58

Sarfaraz Nawaz

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