Question

I have a variable defined in one of my files, It may be manipulated by the code in the file itself, but it is always a constant value to any external file.

How do i declare the variable as constant without raising errors on assigning this variable a value inside the file it is defined in while allowing compiler to optimize it’s read as if it is a constant in those external units ?

Answer 1

A rvalue can’t be modified. Use an accessor function to access it guarantees you only offer an rvalue, e.g.

static int value;

extern int getconst();

int getconst() {
  return value;
}

This makes:

getconst() = -1; // Compiler error

Alternatively you can expose your value via a const pointer to a const int:

#include <stdio.h>

static int value = -1;

extern const int * const public_non_modifiable;
const int * const public_non_modifiable = &value;


int main() {
  printf("%d\n", *public_non_modifiable); // fine
  *public_non_modifiable = 0; // compiler error
  return 0;
}