constexpr.txt

is a keyword that can be used to indicate that a function or variable is a compile-time constant. This means that its 
value can be determined at compile time, rather than at runtime. This allows the compiler to make certain optimizations
and also allows for the use of the variable or function in contexts where only a compile-time constant is allowed. such
as in the size of an array.

When a function or variable is marked as "constexpr", it means that the compiler GUARANTIES that the function or variable
can be evaluated at compile-time, and can be used in constant expressions and template arguments.

A "constexpr" function is guaranteed to be evaluated at compile-time if it meets certain requiremnts, such as // 1a.

the function must be declared as "constexpr"
the function must contain only a single return statement
the return statement must be a constant expression 
the function must not contain any loops or switch statements
the function must not contain any mutlable variables

if a "constexpr" function meets these requirements, then the compiler is guaranteed that it can evaluate the function at
compile-time. THis means that the function can be used in contexts where only a constant value is allowed, such as in the
size of an array or as a template argument.

A "constexpr" variable has similar to functions, its guaranteed to be evlauated at compile-time if it meets certain    
requiremtns, such as:			// 1b.

the variable must be initialized with a constant expression
the type of the variable must be a literal type.


So the difference between "const" and "constexpr" that const does not completely guarantee that is going to be 
evaluated during compile-time while "constexpr" guarantees that it is going to be evaluated during compile-time 
under certain conditions for variables read (1b.) for functions read (1a.)

const int number = 2; - it will be evaluated during compile-time.
const int number = getNumber(); - wont be evaluated during compile-time only during run time 

const value doesn't guarantee compile-time evaluation

constexpr int number = 2; - will get evaluated during compile-time
constexpr int number = getNumber(); - will get evaluated during compile-time