Do equality checks for dynamic arrays attempt to shortcut with pointer comparison

[simendsjo]

On 04/03/2014 09:23 AM, dnspies wrote:
> On Thursday, 3 April 2014 at 07:16:53 UTC, simendsjo wrote:
>> On 04/03/2014 09:03 AM, dnspies wrote:
>>> If two dynamic arrays point to the same place in memory, is it fast to
>>> compare them for equality? or does every element still have to be
>>> compared?
>>
>> Equals will first check for memory location.
>> This is from the runtime:
>>
>> bool opEquals(Object lhs, Object rhs)
>> {
>>     // If aliased to the same object or both null => equal
>>     if (lhs is rhs) return true;
>>
>>     // If either is null => non-equal
>>     if (lhs is null || rhs is null) return false;
>>
>>     // If same exact type => one call to method opEquals
>>     if (typeid(lhs) is typeid(rhs) || typeid(lhs).opEquals(typeid(rhs)))
>>         return lhs.opEquals(rhs);
>>
>>     // General case => symmetric calls to method opEquals
>>     return lhs.opEquals(rhs) && rhs.opEquals(lhs);
>> }
>
> But this only applies to objects with a class type.  A dynamic array
> isn't an object with a class type, it's a builtin type. Does it use this
> opEquals?  Can a dynamic array even be cast to an Object?  I don't see
> that "lhs is rhs" will return true since the array startptr-length pairs
> may still occupy different places in memory even if the contents of
> those arrays occupy the same place.

If the arrays have different lengths, they are obviously not equal.
Here's a quick test of the semantics of is on arrays.

     auto a = [1, 2];
     auto b = a;
     assert(a is b);
     b = a[0..$];
     assert(a is b);
     b = a[0..1];
     assert(a !is b);
     assert(a.ptr == b.ptr);