static const - top level of program does not allow static const - Dart

Question

I have looked at some other similar questions on SO, but they don't appear to address the following specifically.

What I want to achieve is to have compile-time constants that cannot be altered.

I have a program which I reorganized a little in order to de-clutter. The program had some const declarations prior to "main()". I moved these to a class, however it required that I declare them as "static const". I then thought, ok those other "const" declarations prior to "main()" should probably also be "static const". However when I attempted that, the Editor advised "Top-level declarations cannot be declared to be 'static'". EG.

static const int    I_CORRECT_YN       = 12;   // prompt nr.

So, I am a little confused. I thought that a "const" was static. Why do I have to declare "static" in the class? Why can't I declare a "top level" const as "static"? Also, what is the difference between:

static const int I_CORRECT_YN = 12;
const  int       I_CORRECT_YN = 12;
static final int I_CORRECT_YN = 12;
final int        I_CORRECT_YN = 12;    ?

What is the best or only way to declare compile-time values that cannot be altered?

I guess I am looking at the literal meaning, but I presume there is a more complex meaning.

Answer 1

Why do I have to declare "static" in the class?

Because instance variables/methods can't be const. This would mean their value could be different per instance, which can't be the case for compile-time constants. (Source)

Why can't I declare a "top level" const as "static"?

The static modifier marks variables/methods as class-wide (same value for every instance of the class). Top-level stuff is application-wide and doesn't belong to any class, so marking them as class-wide doesn't make any sense and is not allowed. (Source)

What is the best or only way to declare compile-time values that cannot be altered?

You are already doing it - add static when defining class constants. Don't add it when defining top-level constants. Also, use const. final vars aren't compile-time values.

What is the difference between [source code with different constant definitions omitted]

static const and const is the pretty much the same, usage depends on context. The difference between const and final is that const are compile-time constants - they can only be initialized using literal values (or expressions constisting of operators and literal values) and can't be changed. final variables also can't be changed after being initialized, but they are basically normal variables. This means any kind of expression can be used, and the value can be a different one for every class instance:

import "dart:math";

Random r = new Random();
int getFinalValue() {
  return new Random().nextInt(100);
}

class Test {
  // final variable per instance.
  final int FOO = getFinalValue();
  // final variable per class. "const" wouldn't work here, getFinalValue() is no literal
  static final int BAR = getFinalValue();
}

// final top-level variable
final int BAZ = getFinalValue();
// again, this doesn't work, because static top-level elements don't make sense
// static final int WAT = getFinalValue();

void main() {
  Test a = new Test();
  print(Test.BAR);
  print(BAZ);       // different from Test.BAR
  print(a.FOO);
  a = new Test();
  print(Test.BAR);  // same as before
  print(BAZ);       // same as before
  print(a.FOO);     // not the same as before, because it is another instance, 
                    // initialized with another value
  // but this would still be a syntax error, because the variable is final.
  // a.FOO = 42;
}

I hope this helped, and I didn't descibe it too confusing. :]