Const

const is keyword that makes something stay constant, unchanged. it can be used to qualify:

• Variables
• Functions
• Structs

Const variables

const is used to make variables immutable, which means that they can only be read but not modified.

const type identifier = value;

Example

const float PI = 3.1415;

PI is now immutable. If you try to modify its value, it will result in compilation error.

Const functions

Just like variable, const also works for functions. If a function is marked as constant, the entire function including parameters and its body should also be constant, which are known at compile time!

Example:

const int sum(int a, int b){
  return a + b;
}

int main(string args[]){

  int result = sum(1, 2); # result = 3

  return 0;
}

The function sum will be run or computed at compile time as it's a const function. Furthermore, sum will be nowhere to be found in the program at run time.

Const struct

A constant struct is a special struct that has all of its member as constant, no exception!

Example:

struct RGB{
  ubyte r, g, b;
  init(self.r, self.g, self.b);
}

static const struct Color{
  static const RGB orange = RGB(255, 165, 0);
  static const RGB purple = RGB(128, 0, 128);
  static const RGB pink   = RGB(255, 192, 203);
  static const RGB gray   = RGB(128, 128, 128);
  static const RGB brown  = RGB(165, 42, 42);
}

This const struct exists is mainly because enum is not enough, as enum only works for primitive types, but here pops another question. And last but not least, this struct Color only exists at compile time, if optimization is on!

The pros & cons

• it reduces some memory usage.
• it reduces the execution time at run time.
• but it increases the compilation time.