Definition

const: this cannot be changed or mutated

• Used much more in C++ that in C
• Signal of intent to compiler, meaningless at hardware level
  • Results in compile-time errors

Preventing Being Changed By Pointers

const can be used to prevent either or both of pointer behaviors (changing the value of the pointer & changing the thing the pointer points to)

• const next to pointer name means you can’t change the value of the pointer
• const next to data type pointed to means you can’t use this pointer to change the thing being pointed to
• Tip: read variable declaration from right-to-left

const in Function Parameters

• A const parameter cannot be mutated inside the function
  • Therefore it does not matter if the argument can be mutated or not
• A non-const parameter could be mutated inside the function
  • It would be BAD if you could pass it a const var
  • Illegal regardless of whether or not the function change the variable

Examples

Preventing Being Changed By Pointers

int main(int argc, char** argv) {
		int x = 5;                  // int
		const int y = 6;            // (const int)
		y++;                        // compiler error

		const int *z = &y;          // pointer to a (const int)
		*z += 1;                    // compiler error
		z++;                        // ok

		int *const w = &x;          // (const pointer) to a (variable int)
		*w += 1;                    // ok
		w++;                        // compiler error

		const int *const v = &x;    // (const pointer) to a (const int)
		*v += 1;                    // compiler error
		v++;                        // compiler error

		return EXIT_SUCCESS;
}