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Subject: Re: [boost] [AFIO] Formal review
From: John Bytheway (jbytheway+boost_at_[hidden])
Date: 2015-09-04 05:52:37

On 2015-09-02 14:55, 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.

To be clear, do you mean standard-defined (as opposed to
implementation-defined)? I'm assuming so.

Then, as Andrey pointed out, the result is not defined. Only integer
values that were obtained from a previous cast from a pointer can be
safely cast to a pointer type (unless you have evidence from elsewhere
in the standard to suggest more).

If you actually want defined behaviour here, I think you should use a
union { int; void*; }. That ought not to have any performance penalty.

That said, I imagine that in practice the implementation-defined
behaviour of what you're doing will be what you want in any
implementation where using AFIO makes sense.


> 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.
> Niall
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