Re: Good way to write integer overflow checks?
On 11/9/2013 10:28 AM, Alf P. Steinbach wrote:
This code is in support of some API functionality:
[code]
inline
auto can_inflate( gdi::Rect const& r, int const dx, int const dy )
-> bool
{
CPPX_XASSERT( INT_MIN/2 < dx && dx < INT_MAX/2 );
CPPX_XASSERT( INT_MIN/2 < dy && dy < INT_MAX/2 );
typedef unsigned long Unsigned_long;
auto const msb = ULONG_MAX - (ULONG_MAX >> 1);
return
(r.left & msb) == ((Unsigned_long( r.left ) - dx) & msb) &&
(r.top & msb) == ((Unsigned_long( r.top ) - dy) & msb) &&
(r.right & msb) == ((Unsigned_long( r.right ) + dx) & msb) &&
(r.bottom & msb) == ((Unsigned_long( r.bottom ) + dy) & msb);
}
[/code]
Can this be written in an even gooder way, for bestest possible code?
Disclaimer: the code has not been tested or even called.
What is the code supposed to do? Are you checking if the size of 'r' is
not going to under- or overflow if you add dx and dy to it? Isn't this
the usual way to check if (b+a) is not going to overflow:
if (INT_MAX - b > a) // a+b will NOT overflow
..
?
V
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