2.32.txt

Exercise 2.32: Is the following code legal or not? If not, how might you make
it legal?

	int null = 0, *p = null;

By Faisal Saadatmand

Illegal: though initialized from a literal, i.e. a constant, null is a plain
int object with a memory address, and thus, it is not a constant expression. As
such, it cannot be used to initialize p, which is a pointer to an int.

Assuming that the intention of the code were simply to define p as a null
pointer, the best way to do so is to use the standard library defined literal
type nullptr:

	int *p = nullptr; // C++ style; preferred

Alternatively, we could use the C-style NULL or 0 (NULL may require cstdlib or
a macro definition):

	int *p = NULL; // C-style
	int *p = 0;

Finally, we can make the above code legal by defining null as constexpr const
pointer to an int (low-level const is implied).

	constexpr int *const null = 0; // evaluated at compile time
	int *p = null; // OK: assign 0 to p

This approach, however, has one major drawback. It is not generic, and thus,
works only with pointers to int. For example we would be able to assign null to
a pointer to a double:
	
	constexpr int *const null = 0; // null is a const pointer to a const int
	double *p = null; // error: p is a pointer to a double

Note: with the class temples (chapter 16) this drawback can be overcome.