TDPL: Manual invocation of destructor

[Steven Schveighoffer]

On Sun, 08 Aug 2010 23:16:48 -0400, Andrei Alexandrescu  
<SeeWebsiteForEmail at erdani.org> wrote:

> On 08/08/2010 01:09 PM, Andrej Mitrovic wrote:
>> On Page 187 it states (shortened here for convenience):
>> "By calling clear(b), you invoke b's destructor, obliterate the  
>> object's state with Buffer.init, and call Buffer's default constructor"
>>
>> On the next page there's a unittest, and the entire example is this:
>>
>> import core.stdc.stdlib;
>>
>> class Buffer
>> {
>>      private void* data;
>>
>>      // constructor
>>      this()
>>      {
>>          data = malloc(1024);
>>      }
>>
>>      // destructor
>>      ~this()
>>      {
>>          free(data);
>>      }
>> }
>>
>> unittest
>> {
>>      auto b = new Buffer;
>>      auto b1 = b;    // extra alias for b
>>      clear(b);
>>      assert(b1.data is null);
>> }
>>
>> void main() { }
>>
>> The assert fails, regardless if there is another reference to the  
>> object or not (b1 in this case).
>>
>> I've added some writeln()'s (not shown here), and just like the book  
>> said, after clear(b), b's destructor gets called, the fields get  
>> initialized to .init, and then b's constructor gets called. But the  
>> constructor will allocate memory for b1.data again, which means data is  
>> not null anymore.
>>
>> So I'm guessing the assert code is wrong in the example?
>
> Yes, the assert is in error.

Wait wait, when destroying an object manually, the runtime will re-call  
the constructor?  I thought it was just going to assign the TypeInfo's  
initial value to the class?

Re-calling the constructor on destruction seems like a horrible  
mis-feature.  Nobody will ever use it.

-Steve