When to use const and when to use constexpr? What’s the difference between them?
I believe constexpr is initialized during compile time, so it can be used in situations where a compile time constant is needed. Const can be initialized during compile time or runtime.
A good way to figure that out is to try to set a static array’s size using a variable declared as const or constexpr. A const is a constant expression provided that it is initialized with a constant expression, an integer literal for instance. The problem with const is that it can be initialized at run-time and won’t warn you if you assign it to any function that isn’t a constant expression, what I mean by that is
const int i = ceil(1024);
int Array* = {1,2,2,3,3,4,4};
Here you’ll only get an error because i is being set as the array’s size. Whereas if a constexpr was used, it would complain right at the initialization phase. One thing to note however is that a function can also be executed at compile-time so long as its declared constexpr, but there are rules - your compiler should give you a warning if the function will be executed at compile time.
template<typename T>
constexpr T Max(T a, T b)
{
return a < b ? b : a;
}
int main()
{
int a = 1, b = 2;
Max(a, b);
}
For instance in the case above, Max ought to be executed at run-time, the two local variables are passed to registers and pushed on the stack then the call to Max follows. On the other hand if you’d have passed 1 and 2.
template<typename T>
constexpr T Max(T a, T b)
{
return a < b ? b : a;
}
int main()
{
int a = 1, b = 2;
Max(1,2);
}
and check the dissassembly output, then you probably wouldn’t have seen much, and the function should be executed at compile-time. 
Main usage of constexpr from const is, computation burden is going to be taken by developer not user by ensuring value computated at COMPILATION TIME. In this case, the gamer plays your game will not calculate some literals on your game but you do when packaging.
What can be yield as result, more optimized loading screens, more optimized calculations and algorithms on user level.
use constinit: variables must initialize at compile time with a constant expression