10-229
To: J3
From: Nick Maclaren
Subject: Interop TR: CFI_is_contiguous
Date: 2010 October 07
Reference: 10-165r2.pdf


One comment that was made by at least Aleks Donev and Reinhold Bader in
N1766 was that CFI_is_contiguous should return the contiguity rather
than assigning it through an argument.  Exactly like CFI_address (as
will be proposed in a paper by John Reid to address requirement 1 of
N1820), it is a pure function that returns a simple value, which is
unlike all of the other CFI_functions.  It would be better to match
that.

It has been said that it might want to return an error indication
for unallocated or disassociated arrays, but that argument applies
identically to CFI_address.  It is also extremely user-hostile, and
means that the following simple use:

    if (CFI_is_contiguous(cdesc)) ...

changes into:

    int temp;

    if (CFI_is_contiguous(cdesc,&temp) && temp) ...

There are also subtle gotchas in the C language to do with calling
functions that have side-effects in arbitrary expressions, and they
regularly cause trouble in combination with optimisation.  They are
unlikely to affect the other functions, as those are 'statement-like',
unlike CFI_address and CFI_is_contiguous.

A related point is that it is a mistake to specify the assumption that
all functions that return int will NECESSARILY be returning an error
indicator.  One cannot predict the future, but there is no need to
restrict future enhancements unecessarily.


Edits to 10-165r2:

[11:9] Change the line to:

"If an error occurs during execution of a function that returns an int
value as an error indicator, the returned value is".

[11:14-17] Replace the specification of CFI_is_contiguous by:

"5.2.6.4 int CFI_is_contiguous ( const CFI_desc_t * );

Description.  CFI_is_contiguous returns 1 if the object described by the
C descriptor can be determined to be contiguous, and 0 otherwise."

[12:7] Append to the end of the paragraph "The result is an error
indicator."

[12:13] Append to the end of the paragraph "The result is an error
indicator."

[12:17] Append to the end of the paragraph "The result is an error
indicator."

[12:23] Append to the end of the paragraph "The result is an error
indicator."

[12:29] Append to the end of the paragraph "The result is an error
indicator."