C++ question ;

Question

In the following code , why should we use dynamic memory allocation with new ? We know the “length” in advance, so why not using a normal array with the size of “length” ? is there any difference ?!
Help plz 🙏🏻

#include <iostream>

using namespace std;

int main()
{
int length, sum = 0;
cout << "Enter the number of students in the group" << endl;
cin >> length;
int *marks = new int[length];
cout << "Enter the marks of the students" << endl;
for( int i = 0; i < length; i++ ) // entering marks of students
{
cin >> *(marks+i);
}
for( int i = 0; i < length; i++ ) // calculating sum
{
sum += *(marks+i);
}
cout << "sum is " << sum << endl;
return 0;
}

Answer 1

Without the use of `new` operator, you would be creating a Variable Length Array (VLA).

https://en.m.wikipedia.org/wiki/Variable-length_array

A VLA is created on the stack, which was not meant for large memory reservation or memory reservation where size needed is unknown until run-time.

Also VLA is not part of the standard, meaning it may or may not be supported (depends on compiler).

An advanced coder might suggest you to study the use of smart pointer. A kind of data structure which is smart in that it does a clean-up chores in the event of error (regular heap-based arrays don't).

https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard

Hth, cmiiw

Answer 2

thank u both 🙏🏻

Answer 3

I think they are same is it?

Answer 4

i think this is because if u want to create an array in stack, the size need to be constant..
since the size is determined at runtime, the array is created in heap using the new keyword