Initialisation of static immutable arrays

Steven Schveighoffer schveiguy at
Wed Oct 6 05:34:17 PDT 2010

On Wed, 06 Oct 2010 08:21:08 -0400, Lars T. Kyllingstad  
<public at kyllingen.nospamnet> wrote:

> On Wed, 06 Oct 2010 07:39:48 -0400, Steven Schveighoffer wrote:
>> Of course.  If you realize that the expression [1,2,3] is not immutable,
>> then it makes sense.
>> Another example to help you think about it:
>> void foo(int x)
>> {
>>     immutable int[3] = [1,2,x];
>> }
>> This must be run on every call of foo, because x can vary.
> I don't think that is a very good reason.  The compiler could detect the
> special (and, might I add, common) case where an array literal whose
> contents are known at compile time is assigned to an immutable variable,
> and treat it as immutable even though [1,2,3] is formally of type int[].

I'm not saying it is a good explanation, but it is why the compiler does  
it.  Any time an array literal occurs anywhere is an allocation.  I have  
never seen a case where the compiler optimizes that out.  Given that  
context, the behavior makes sense.

I don't think any allocation should occur here, even if array literals are  
not made immutable, because that allocation is going to be thrown away  
immediately.  I agree this should be special-cased.  I believe there are  
several bugs on array literals and allocations.


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