Append const to array

Yuxuan Shui via Digitalmars-d-learn digitalmars-d-learn at puremagic.com
Tue Sep 20 17:04:04 PDT 2016


On Tuesday, 20 September 2016 at 22:38:33 UTC, Jonathan M Davis 
wrote:
> On Tuesday, September 20, 2016 22:23:08 Yuxuan Shui via 
> Digitalmars-d-learn wrote:
>> struct A {
>>   ulong[] x;
>> }
>> struct B {
>>   ulong x;
>> }
>> void main() {
>>   B[] b;
>>   const(B) xx = B(1);
>>   b ~= xx; // Works
>>
>>   A[] c;
>>   const(A) yy = A([1]);
>>   c ~= yy; // Does not
>> }
>>
>> What gives?
>
> const(A) means that the ulong[] inside is const(ulong[]). When 
> yy is copied
> to be appended to c, it goes from const(A) to A, which means 
> that
> const(ulong[]) would need to be sliced and and set to ulong[], 
> which would
> violate const, because it would mean that the last element in c 
> could mutate
> then elements of its x, which would then mutate the elements in 
> yy.
>
> - Jonathan M Davis

That makes sense, thanks.


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