important proposal: scope keyword for class members
Sergey Gromov
snake.scaly at gmail.com
Sun Mar 8 17:02:13 PDT 2009
Sun, 08 Mar 2009 16:06:56 -0700, Andrei Alexandrescu wrote:
> Bill Baxter wrote:
>> On Mon, Mar 9, 2009 at 7:12 AM, John Simon <zildjohn01 at gmail.com> wrote:
>>> Sean Kelly Wrote:
>>>
>>>> John Simon wrote:
>>>>> Sean Kelly Wrote:
>>>>>> Oh, I should mention that I'm not sure how the compiler would handle
>>>>>> this scenario:
>>>>>>
>>>>>> class A { byte[16]; }
>>>>>> class B { byte[32]; }
>>>>>> class C {
>>>>>> this( bool b ) {
>>>>>> if( b ) o = new A;
>>>>>> else o = new B;
>>>>>> }
>>>>>> scope Object o;
>>>>>> }
>>>>>>
>>>>>> If I had to guess I'd say that the compiler would either reserve the max
>>>>>> size necessary to store both A or B, or that in non-trivial cases it
>>>>>> just wouldn't bother with reserving space for o at all.
>>>>> Wrong. The Object is constructed when it comes into scope, and destructed when it leaves scope. Classes can't have an 'opAssign', instead the reference is reassigned. Since the reference is invariant/immutable here, this throws a compile time error.
>>>> I'm talking about a proposed new feature, not an existing one. Please
>>>> take this example in context.
>>> Sorry man, I thought you were disputing with me. My apologies. Let me rephrase.
>>>
>>> I believe that 'scope' declared objects shouldn't allow an assignment of a derived type. Reasoning being that there wouldn't be enough stack space allocated for it.
>
>>>
>>> So in your example above, Object could only recieve a 'new Object', and nothing further down in the hierarchy.
>>
>> I'm pretty sure that assigning to a scope class on the stack is not
>> done by copying the memory. I'm pretty sure what happens is basically
>> something like follows. This scope statement:
>>
>> scope foo = new TheClass;
>>
>> Basically becomes this:
>>
>> static ubyte[sizeof_TheClass] _mem;
>> auto foo = new(_mem.ptr) TheClass;
>
> To that add:
>
> scope(exit) foo.~this();
>
> I don't think there's a syntax for destructor invocation. Never liked
> scope that much. Extremely dangerous for such a cute syntax.
Shouldn't it be sort of
scope(exit) (cast(TheClass)_mem.ptr).~this();
since foo may be re-assigned to something absolutely different?
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