c++ - Why is NULL/0 an illegal memory location for an object?

Question

I understand the purpose of the NULL constant in C/C++, and I understand that it needs to be represented some way internally.

My question is: Is there some fundamental reason why the 0-address would be an invalid memory-location for an object in C/C++? Or are we in theory "wasting" one byte of memory due to this reservation?

Answer 1

The null pointer does not actually have to be 0. It's guaranteed in the C spec that when a constant 0 value is given in the context of a pointer it is treated as null by the compiler, however if you do

char *foo = (void *)1;
--foo;
// do something with foo

You will access the 0-address, not necessarily the null pointer. In most cases this happens to actually be the case, but it's not necessary, so we don't really have to waste that byte. Although, in the larger picture, if it isn't 0, it has to be something, so a byte is being wasted somewhere

Edit: Edited out the use of NULL due to the confusion in the comments. Also, the main message here is "null pointer != 0, and here's some C/pseudo code that shows the point I'm trying to make." Please don't actually try to compile this or worry about whether the types are proper; the meaning is clear.