Re: How to distinguish between heap/stack pointers

From: Thomas Matthews (Thomas_MatthewsSpitsOnSpamBots_at_sbcglobal.net)
Date: 02/09/04 

Date: Mon, 09 Feb 2004 15:55:07 GMT

E. Robert Tisdale wrote:
> Sayan wrote:
>
>> How do I distinguish between a heap pointer and a stack pointer?
>> I want the following to work:
>>
>> template<class T>
>> bool isHeapPtr(T* p1, T* p2);//the function I want to write
>> //...
>> int a = 5;
>> int *pas = &a;
>> int *pah = new int;
>> *pah = 10;
>> cout << isHeapPtr(pas, pah);
>
>
> One pointer, pas, points to an object allocated from automatic storage.
> The other pointer, pah, points to an object allocated from free storage.
> Which one should isHeapPtr test? Try this:
>
> bool isStackPointer(void* p) {
> return &p < p;
> }

How can one use the '<' operator on pointers?
There is no portable method to compare the ordering sequence
on pointers.

There is also no guarantee that a pointer will point to a stack
object or a heap object. There is also no requirement for an
implementation to provide a stack or heap.

Some people will try to cast the pointer to an integer value
hoping that the integral value is a unique value in the address
space. If this works, it will be platform specific.

The only comparison operators that are valid on pointers
are equality (==) and inequality (!=). The ordering comparison
operators (<, <=, >, >=) when applied to pointers assume a
a sequence or ordering; which is only valid on contiguous
sequences like arrays. If one has automatic memory at a high
address and dynamic in a low address, comparing pointers from
each section to each other is meaningless.

If one needs to know if a variable is of local, automatic or
dynamic storage, then either the program is designed wrong
or the program is platform specific. 

-- 
Thomas Matthews
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