C++ Logo

STD-PROPOSALS

Advanced search

Subject: Re: [std-proposals] Allowing access to object representations
From: sdkrystian (sdkrystian_at_[hidden])
Date: 2019-08-21 18:38:05


> It's nothing to do with the value of the int object as a whole, but of the leftmost byte. Why on earth would it not depend on endianness?Because the standard says so. Here are some citations:In that particular cast, the value of the pointer is unchanged. http://eel.is/c++draft/expr.static.cast#13.sentence-4Since the value is unchanged, it still points to the int object. Therefore, when the indirection operator is used, it yields an lvalue of type `char` that denotes the object the pointer points to. http://eel.is/c++draft/expr.unary.op#1.sentence-1Now, when an lvalue-to-rvalue conversion is applied to the lvalue (such as during initialization), the result is the value of the object (which has NOT been truncated), and since that value is within the representable range of `char`, the behavior is well defined, and will ALWAYS yield 42. (If it's outside the representable range of char, the behavior is undefined http://eel.is/c++draft/expr.pre#4)
-------- Original message --------From: Jake Arkinstall via Std-Proposals <std-proposals_at_[hidden]> Date: 8/21/19 19:24 (GMT-05:00) To: std-proposals_at_[hidden] Cc: Jake Arkinstall <jake.arkinstall_at_[hidden]> Subject: Re: [std-proposals] Allowing access to object representations It's nothing to do with the value of the int object as a whole, but of the leftmost byte. Why on earth would it not depend on endianness?On Wed, Aug 21, 2019 at 11:49 PM sdkrystian via Std-Proposals <std-proposals_at_[hidden]> wrote:> On big-endian systems we'll end up with `ch==0`; on little-endian systems we'll have `ch==42`.I think not - the value of the int object does not truncate. It's value is 42.  -------- Original message --------From: Arthur O'Dwyer via Std-Proposals <std-proposals_at_[hidden]> Date: 8/21/19 17:30 (GMT-05:00) To: std-proposals_at_[hidden] Cc: Arthur O'Dwyer <arthur.j.odwyer_at_[hidden]> Subject: Re: [std-proposals] Allowing access to object representations On Wed, Aug 21, 2019 at 4:37 PM language.lawyer--- via Std-Proposals <std-proposals_at_[hidden]> wrote:On 21/08/2019 22:51, language.lawyer_at_[hidden] wrote:
> On 21/08/2019 22:44, Timur Doumler via Std-Proposals wrote:
>> So you're saying that, even without any pointer arithmetic, just this code:
>>
>> int x = 100000;
>> std::cout << *reinterpret_cast<char*>(&x);
>>
>> has undefined behaviour?
>>
>> If that's the case then this is even more insane than I thought. Please clarify whether this is really what you're saying here!
>
> I suspect you was trying to answer my mail, but anyways yes, I think that your example has UB.

UB if 100000 is outside of the range representable by char, ofc.
On a platform where char represents the same range of values as int the code obviously won't have UB.Well, regardless of your views on UB, we can all agree that the behavior of    int x = 42;    char ch = *reinterpret_cast<char*>(&x);is not portable. On big-endian systems we'll end up with `ch==0`; on little-endian systems we'll have `ch==42`.  (This contradicts something sdkrystian said earlier; he seemed to be under the impression that `ch==42` always.)Whether the integer value of `x` happens to be less than CHAR_MAX doesn't matter at all; the behavior is implementation-defined at best — and UB at worst. I'm inclined to believe the people who say it's UB, because in my experience pretty much anything involving `reinterpret_cast` is UB.However, all is not lost — it'll still do what you expect, in practice! :)–Arthur

-- 
Std-Proposals mailing list
Std-Proposals_at_[hidden]
https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals



STD-PROPOSALS list run by herb.sutter at gmail.com

Standard Proposals Archives on Google Groups