Re: [boost] [AFIO] Formal review

Subject: Re: [boost] [AFIO] Formal review
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2015-09-02 19:04:48

On 02.09.2015 21:55, Niall Douglas wrote:
> On 2 Sep 2015 at 5:31, John Bytheway wrote:
>
>>> Please do correct me if I am wrong, but I had thought that this is
>>> defined behaviour:
>>>
>>> int a=5;
>>> void *b=(void *)(size_t) a;
>>> int c=(int)(size_t) b;
>>> assert(c==a);
>>>
>>> This is certainly a very common pattern in C.
>>
>> I believe this is implementation-defined.  Based on N3797
>> [expr.reinterpret.cast], paragraph 5:
>>
>> "A value of integral type or enumeration type can be explicitly
>> converted to a pointer. A pointer converted to an integer of sufficient
>> size (if any such exists on the implementation) and back to the same
>> pointer type will have its original value; mappings between pointers and
>> integers are otherwise implementation-defined."
>>
>> The behaviour that is defined (pointer to integer and back) is the
>> reverse of the one you want.
>
> That's the whole point of the intermediate size_t cast. int to size_t
> is defined behaviour, size_t to void * is defined behaviour, void *
> to size_t is defined behaviour, size_t to int is defined behaviour.

Only that the original and resulting ints are equal is not guaranteed.

> Older code, and still a lot of C code, directly casts between int and
> void * and back again, even though strictly speaking that's a no no.

Horrible code exists. No need to add yet more.