Set an element of a `const` array as the length of another array in c++

I defined a const int array like this :

const int arr[] = { 100 , 200, 300, 400 };

Now i want to set one of the elements of above array as the length of another array like the following :

char buffer[arr[3]];

But it gave me a compile time error :

non-constant arguments or reference to a non-constant symbol

I studied this This question to solve my problem but i became confuse about these questions :

• Why can't i set an element of a const array as the length of another array?
• Are the elements of a const array constant or read only?
• What is the differences between a const and read only statements in c?

Regards!