clear()

[Max Samukha]

On Fri, 09 Oct 2009 21:50:48 +0200, Yigal Chripun <yigal100 at gmail.com>
wrote:

>On 09/10/2009 19:53, Max Samukha wrote:
>> On Fri, 09 Oct 2009 11:40:43 -0500, Andrei Alexandrescu
>> <SeeWebsiteForEmail at erdani.org>  wrote:
>>
>>> I'm talking with Sean and Walter about taking the first step towards
>>> eliminating delete: defining function clear() that clears the state of
>>> an object. Let me know of what you think.
>>>
>>> One problem I encountered is that I can't distinguish between a default
>>> constructor that doesn't need to exist, and one that was disabled
>>> because of other constructors. Consider:
>>>
>>> class A {}
>>> class B { this(int) {} }
>>>
>>> You can evaluate "new A" but not "new B". So it's legit to create
>>> objects of type A all default-initialized. But the pointer to
>>> constructor stored in A.classinfo is null, same as B.
>>>
>>> Any ideas?
>>>
>>
>> The notion of default constructor is not quite clear.
>>
>> class A
>> {
>>    this(int a = 22) {}
>> }
>>
>> Should A be considered as having a default constructor?
>>
>> class B
>> {
>>    this(int) {}
>> }
>>
>> Should passing int.init to B's constructor be considered default
>> construction? If yes, we could recreate B using the init value. But
>> then:
>>
>> class C
>> {
>>    this(int a) {}
>>    this(int a, int b) {}
>> }
>>
>> Which constructor to call? The one with fewer parameters? What if
>> there are overloaded constructors with identical number of parameters?
>> Should we explicitly mark one of the constructors as default?
>
>I agree. classinfo.defaultConstructor should be replaced by an array of 
>all the constructors. Only when the array is empty you assume the 
>existence of the default compiler generated constructor.

I'd prefer complete runtime information for all members.

The problem is I do not understand what 'default constructor' and
'default construction' means in D for classes that have explicit
constructors with parameters. How to automatically construct this cl
ass:

class B
{
  this(int a = 22) {}
}
?

For example, Object.factory will always return null for B, which is an
arbitrary limitation.

class C
{
  this(int a) {}
}

For C, the "default constructor" should probably be generated like
this:

void C_ctor(C c)
{
    c.__ctor(int.init);  
}
etc.

Otherwise, I cannot see how one can reconstruct an instance of C in
'clear'.