array size defined from user's input C++ [duplicate]

Question

I am very surprised that the below code compiles fine. I always believed an array size must be a constant at compile time but it seems that I can take the user's input and use it as the array size. I am using GCC with the codeblocks IDE. Has anyone tried this and is there anything wrong with doing it?

int size;
cout<<"Enter array size : "<<endl;
cin>>size;
int arr[size];
// ...more action array with the array after which works fine

Possible duplicate of Why there is no compile error for the array definition with non-constant value? — Shafik Yaghmour
– Shafik Yaghmour, Commented Jun 15, 2015 at 16:54
@OldProgrammer No, it's not valid in C++, it would be valid to declare a VLA in C but C++ doesn't have it. — Some programmer dude
– Some programmer dude, Commented Jun 15, 2015 at 16:56

Sourav Ghosh · Accepted Answer · 2015-06-15 16:55:20Z

1

This feature is called Variable length array and is introduced in C99 standard, just again to make it an optional feature in C11 standard.

I do not have specific idea about C++ standard, but this might be a support feature which comes as a compiler extension. AFAIK, there is nothing in the C++ standard that supports for VLA. Alternatively, using std::vector in C++ is considered a better approach for this.

answered Jun 15, 2015 at 16:55

Sourav Ghosh

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3 Comments

Engineer999 Over a year ago

Would the array be still stored on the stack in this case?

Sourav Ghosh Over a year ago

@Engineer999 In C, gcc allocates the VLA in stack. std::vector uses dynamic memory which is not allocated from stack, AFAIK.

Engineer999 Over a year ago

Thanks. So by using the push_back function on vectors, does this create elements on the heap? Is that effectively the same as having pointers to an element type and using "new"

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