🚀 go-pugleaf

Alex Martelli (martelli@cadlab.cadlab.it) wrote:

|> This specific problem highlights ANOTHER sore need: a *portable* way
|> to test for "is this pointer inside this array, or not".  Not only
|> would this be precious for pre/post/invariant assertions, it would
|> come in handy in all sorts of low-level programming situations --
|> a memory allocation subsystem could check to see whether it's
|> being passed a valid pointer for deallocation, for example (a more
|> general solution to this latter issue would probably also need a
|> portable way to test whether a given pointer is, or isn't, suitably
|> aligned for a given type).

|> bool isPointerInside(const void* pointerBeingTested,
|>           const void* startOfArray,
|>           size_t sizeOfArray);
|> ideally with startOfArray/sizeOfArray allowed to be, in fact, a
|> _sub_-array, e.g., give "double A[20];",

|> isPointerInside((const void*)&A[11],(const void*)&A[8], 5*sizeof(double))

|> would be valid (NOT undefined behaviour) and yield true,

|> isPointerInside((const void*)&A[14],(const void*)&A[8], 5*sizeof(double))

|> would be valid (NOT undefined behaviour) and yield false.


|> and maybe also:

|> template<class T> isPointerAligned(const void* pointerBeingTested);
|> (to be called as, e.g., isPointerAligned<double>((const void*)myptr))

|> would, I feel, be worthy additions, mostly for low-level programming.

What you want can be done portably.  Consider:

    template< class Iterator >
    bool
    isContained( Iterator ptr , Iterator start , Iterator finish )
    {
        bool            found( false ) ;
        for ( ; ! found && start != finish ; start ++ )
            found = (start == ptr) ;
        return found ;
    }

Regretfully, the performance of the above leaves much to be desired, at
least for large arrays.  But it is standard conforming, and I believe
that it is the only way to do this in a standards conforming manner.
-- 
James Kanze           (+33) 88 14 49 00          email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
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