D newb

[Steven Schveighoffer]

"JAnderson" wrote
> bearophile wrote:
>> Joel Anderson:
>>>> bearophile:
>>>>> My question: does the compiler raise a compilation error if the 
>>>>> 'scope' keyword is used where the compiler can't actually allocate the 
>>>>> class on the stack? (if not, then I think it may be better for the 
>>>>> compiler to raise such error, avoiding the illusion of efficiency).
>>> If the class has a component sub-class the component may be heap 
>>> allocated.  I don't agree with making that an error though.
>>
>> May I ask you why you don't like that as an error?
>> Generally I don't like undefined situations, if I add the 'scope' keyword 
>> I want the class to be stack-allocated, and I assume the compiler does 
>> that. Then the layout of the memory used by the code is different, and I 
>> can for example take care to avoid a stack overflow. If the compiler at 
>> compile time is able to tell me that the class can't be allocated on the 
>> stack, then I think the program has to not compile until I remove the 
>> 'scope' keyword there. This makes the code reflect what the program 
>> actually does.
>>
>> Bye,
>> bearophile
>
> What I'm talking about is:
>
> class A
> {
>
>
> }
>
> class B
> {
> ...
> A a = new A(); // Heap Allocated
> ...
> };
>
>
> ...
>
> scope B b = new B; //B is scope allocated but A is not
>
> Say your using someone else's class.  You get latest and you using one of 
> there classes with the word scope.  They've decided to add something that 
> is heap allocated.  Now suddenly your code is not working because someone 
> made a slight change to their interface.
>

The compiler cannot know when a constructor will decide to heap-allocate. 
So it cannot predict whether the entire class will be stack-allocated.  Once 
the constructor is called, you cannot extend the previous stack frame 
(without a lot of extra work), and even then, it cannot be sure that the 
calling function won't use that stack space later in the function.  I think 
what you ask for is impossible to implement, and impossible to detect.

-Steve