At Sunday 2004-09-26 02:19, you wrote:
From: "Victor A. Wagner Jr."
At Saturday 2004-09-18 03:44, Terje Slettebø wrote:
Even for + and -, there are differences, such as that you can subtract two pointers, but not add them.
The erroneous assumption that this is true is something that has bothered me for some time (and indeed I have worked on systems where it simply isn't true, that the result of a+b would be useful even if both were pointers). In an abstract sense it's not true on any system.
Consider:
T* a; T* b; .... get them assigned meaningfully to something in a sequence "container"
The midpoint between them is (this is legal using your constraints):
a + (b-a)/2
1st year algebra teaches us that this is equivalent to:
(a+b)/2
While this is interesting (and I understand it was not meant to pick on me in particular, as you said, and it wouldn't really make any sense, as Dave Abrahams pointed out), my point was strictly about what is legal _C++_, and the above is not legal C++, which I think you will agree with:
"5.7 Additive operators [...] For addition, either both operands shall have arithmetic or enumeration type, or one operand shall be a pointer to a completely defined object type and the other shall have integral or enumeration type."
The context was detecting addability, and if the trait had reported "yes" for pointers, it would have caused errors elsewhere.
You might also argue that the language should allow the expression above, but that's a different discussion.
yes... I was trying to catch up on EMail while waiting for the regression test to fail for mingw and just responded. It's a different discussion, and yes, I believe the language is 'in error'.
I'll also note, in passing, that for integers the two are equivalent also, but very few compilers will generate code to guarantee the correct result if you attempt to calculate (a+b)/2 which _does_ exist on every computer out there.
With "correct result", I guess you mean the possibility of overflow in this expression, compared to the other form.
either form can overflow, (different circumstances, of course) therefore the simple "average two things" just isn't all that easy in C++ (or C...or any other HLL I'm aware of)
Regards,
Terje
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Victor A. Wagner Jr. http://rudbek.com The five most dangerous words in the English language: "There oughta be a law"