important proposal: scope keyword for class members

[Sergey Gromov]

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?