Stroustrup is disappointed with D :(

[Steven Schveighoffer via Digitalmars-d]

On 9/22/15 3:38 PM, Ali Çehreli wrote:
> On 09/22/2015 11:58 AM, Tourist wrote:
>> "D disappointed me so much when it went the Java way".
>> https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#to-do-unclassified-proto-rules
>>
>>
>>
>> It's something about virtual calls, but I didn't understand what he
>> means. What does he mean?
>
> It is about virtual calls in ctors and dtors. Here is the problem:
>
> import std.stdio;
>
> class B {
>      this() {
>          foo();
>      }
>
>      void foo() {
>          writeln("base");
>      }
> }
>
> class D : B {
>      this() {
>          writeln("derived is only now complete");
>      }
>      override void foo() {
>          writeln("derived");
>      }
> }
>
> void main() {
>      auto b = new D;
> }
>
> Although we are in the middle of consructing a D, the call foo() inside
> B's ctor is dispatched to D's virtual foo() even though the D part of
> the object has not been constructed yet. This is in contrast to C++,
> where the object goes through multiple personalities during its
> construction: First B, then D, etc.
>
> The program above prints
>
> derived
> derived is only now complete
>
> As can be seen, D.foo is called before D is ready for use.
>
> Here is the equivalent C++ program:
>
> #include <iostream>
>
> using std::cout;
>
> class B {
> public:
>
>      B() {
>          foo();
>      }
>
>      virtual ~B() {}
>
>      virtual void foo() {
>          cout << "base\n";
>      }
> };
>
> class D : public B {
> public:
>
>      D() {
>          cout << "derived is only now complete\n";
>      }
>
>      virtual void foo() {
>          cout << "derived\n";
>      }
> };
>
> int main()
> {
>      D d;
> }
>
> The output of the C++ program:
>
> base
> derived is only now complete
>
> C++'s approach is better from the point of view of corretness. However,
> it is slower because the object's vtbl pointer must be stamped several
> times during construction. (I am not aware of available compiler
> optimizations there.)
>
> Ali
>

Yeah, but you can't do this in C++ though:

class D : B {
    this()
    {
       writeln("derived is only now complete");
       super();
    }
}

I find the ability to control the construction order far more important 
than virtual calls for base constructors.

-Steve