Why the statement int null = 0, *p = null is illegal?

Question

I'm new to C++ and am trying the learn the concept of pointer. Could someone tell me why the C++ statement below is illegal? It seems to me to be legit but I have been told its illegal.

int null = 0, *p = null;

Answer 1

The C++ standard allows pointers to be assigned the value 0 as a constant.

However, the code:

int null = 0;
int *p = null; 

does not set p to the constant 0, it sets it to the value of null, which is an integer variable.

If we generalize a little bit, and put int null = 0; on a line, and int *p = null; in a completely different line, with some code in between. There is nothing saying that the code in between doesn't do null = 4; - however, I don't think that is the main reason for not allowing this, but rather that it is easier to write a compiler that checks "is this the integer constant 0" than "is this named constant of the value zero". What if the constant is from another compile unit (a link-time constant)?

Also, reading 0 is much easier than having 46 different coding standards, each of which uses a different name for null.

Note that even if you make const int null = 0;, it is still not the constant 0 - it's a constant of the same value as 0, but not the same, lexically, as 0.