🚀 go-pugleaf

In article ceo@engnews1.Eng.Sun.COM, "joe (j.) halpin" <jhalpin@bnr.ca>
writes:
>
>In 3.7.3.2.4 the January working paper says:
>
>4 A deallocation function can free the storage referenced by the pointer
>  given  as  its  argument and renders the pointer invalid.  The storage
>  can be made available for further allocation.  An invalid pointer con-
>  tains an unusable value:  it cannot even be used in an expression.
>
>This sounds as though, in the following:
>
>char *pc = new char[128];
>delete pc;
>pc = 0;
>
>it makes the final assignment (an expression) invalid.

No. It is not the pointer object that becomes unusable, but the value
of the pointer. You can always store a valid value (like a null pointer)
into the object.

The reason for the odd-looking condition (which follows the C
standard) is to allow architectures which validate pointers in
hardware to be standard-conforming. Example:

Suppose a "tagged" architecture marks each pointer as to whether it
contains a valid value.
	char *p; // p initially tagged as invalid value
	p = new char[100]; // p tagged as valid
	delete [] p; // p tagged as invalid
	p = 0; // p tagged as valid null pointer
The hardware might trap any use of an invalid pointer value, even just
reading the value without dereferencing it.

You can't do anything useful in portable code with uninitialized pointers,
or pointers which point to objects which are no longer available. The
standard allows the implementation a great deal of leeway in trying to
help by identifying such invalid uses.

---
Steve Clamage, stephen.clamage@eng.sun.com
---
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