Is the size of an array determined at compile time?

Question

When I was reading about the array initialization in this tutorial. I found out this note.

type name [elements];

NOTE: The e

Answer 1

The declaration T a[N] requires that N be a converted constant expression.

Converted constant expression is an expression implicitly converted to prvalue of type T, where the converted expression is a core constant expression. If the literal constant expression has class type, it is contextually implicitly converted to the expected integral or unscoped enumeration type with a constexpr user-defined conversion function.

An int literal such as 5 is a prvalue, so can be used in the declaration T a[5], but an lvalue, for example int n = 5 cannot be used in the declaration T a[n], unless the lvalue under-goes an implicit lvalue-to-rvalue conversion where the lvalue:

a) has integral or enumeration type, is non-volatile const, and is initialized with a constant expression, or an array of such (including a string literal)

b) has literal type and refers to a non-volatile object defined with constexpr or to its non-mutable subobject

c) has literal type and refers to a non-volatile temporary, initialized with a constant expression

Therefore the following are valid:

const int n = 5; 
int a[n]; 
constexpr int n = 5; 
int a[n];