Uh... destructors?

Steven Schveighoffer schveiguy at yahoo.com
Tue Mar 8 12:17:02 PST 2011


On Tue, 08 Mar 2011 14:28:44 -0500, Bruno Medeiros  
<brunodomedeiros+spam at com.gmail> wrote:

> On 23/02/2011 17:47, Steven Schveighoffer wrote:
>> On Wed, 23 Feb 2011 12:28:33 -0500, Andrei Alexandrescu
>> <SeeWebsiteForEmail at erdani.org> wrote:
>>
>>> On 2/23/11 11:16 AM, Steven Schveighoffer wrote:
>>
>>>> Just because a function is not marked @safe does not mean it is  
>>>> unsafe.
>>>> It just means you can do things the compiler cannot verify are safe,  
>>>> but
>>>> that you know are actually safe. I showed you earlier an example of a
>>>> safe pure function that uses malloc and free.
>>>>
>>>> Programmers are allowed to make conceptually safe functions which are
>>>> not marked as @safe, why not the same for pure functions?
>>>>
>>>> -Steve
>>>
>>> I understand that. My point is that allowing unsafe functions to be
>>> pure dilutes pure to the point of uselessness.
>>
>> And that's not a point. It's an unsupported opinion.
>>
>> pure has nothing to do with safety, it has to do with optimization. Safe
>> functions are no more optimizable than unsafe ones. Safety has to do
>> with reducing memory bugs.
>>
>> The two concepts are orthogonal, I have not been convinced otherwise.
>>
>> -Steve
>
> pure has something to do with @safety. (Also, it has more to do with  
> than just optimization, it also affects code readability.)
>
> In order to gain any benefit from calling pure functions (whether the  
> benefit is programmer code readability or compiler optimization) it  
> needs to be determined from the pure function's signature what is the  
> transitively reachable mutable state that the function may access.  
> Normally this state is whatever is transitively reachable from the  
> parameters. However, if you allow *arbitrary* _pointer arithmetic_ you  
> could legally manipulate any mutable data in your program from within  
> the pure function. This would make the pure attribute useless because it  
> would not offer any additional guarantees whatsoever over an unpure  
> function. So such a rule is necessary such that, for example, the  
> following function should not be allowed to be pure:
>
> pure int func(int* ptr, int ix) {
>    return (ptr + ix)++;
> }

If I want to make my own array type, then it will be quite unusable in  
such pure functions.

Assuming that such code is illegal assumes the compiler fully understands  
the semantic meaning of that code.

Keep in mind that an array is semantically understood by the compiler, but  
I can't convey that same semantic knowledge to the compiler for my custom  
type.  I may know that some pointer arithmetic is perfectly safe and pure,  
even when the compiler cannot grok that.  It doesn't make sense that pure  
functions shouldn't be allowed to be streamlined/optimized as I can normal  
functions.  To me the concepts of @safe and pure are still orthogonal.

@safe is not so much about "safety" as it is about "compiler-verified  
safety".  I feel somewhat the words are getting in the way here.  I don't  
mean memory unsafe code, I mean memory safe code that cannot be compiler  
verified via @safe.  I feel that because the compiler can't verify pure  
functions are memory safe doesn't make pure functions unusable or useless.

-Steve


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