It is my understanding that in C and C++ , we create data-structures whose size is known at compile time on the stack and we use the heap (malloc-free / new-delete) for things whose size is not known at compile time and is decided at run-time.Why then , does my g++ compiler allow me to do something like the following code snippet.
int main(void)
{
int n ;
cin >> n ; // get the size of array.
int arr[n] ; // create a variable sized array.
.... do other stuff ...
}
Specifically , in the case of an array:
An array is assigned a contiguous block of memory on the stack and there are variables above and below it on the stack , so the array's size must be known so that the variable above the array on the stack , the array itself , and the variables below the array on the stack can all fit into memory neatly. How then are variable sized arrays on the stack implemented ? Why are they even necessary? Why can't we just use the heap for variable sized buffers?
EDIT:
I understand from the comments , that C and C++ have different rules regarding whether VLA's are standard or not and also Neil Butterworth's comment that it is generally not a good idea to ask a question about two languages at once. Thank you people , so I am removing the C tag from my question , as I intended to mainly ask about C++ , as evident by the code snippet syntax. Sorry for the confusion , and thanks for the responses.
