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Rationale behind user provided ctor requirement for const objects of non-POD class types

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(Originally posted to comp.std.c++, but that newsgroup is dead.)

Recently, I've seen a compiler (clang) that will not compile code like this:

   struct A {
         A() {}
   };

   class B {
         A a_;
   };

   int foo()
   {
         const B b;
         return 0;
   }

It will not compile it because 'b' is const B, which does not have a 
user defined ctor.  C++'03 [dcl.init] p9 says:

   If no initializer is specified for an object, and the object
   is of (possibly cv-qualified) non-POD class type (or array
   thereof), the object shall be default-initialized; if the
   object is of const-qualified type, the underlying class type
   shall have a user-declared default constructor.

Why is that?  Why can't a const B constructed the same way as a 
non-const B, using the implicitly defined default constructor?

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Thread

Rationale behind user provided ctor requirement for const objects of non-POD class types Nikos Chantziaras <realnc@arcor.de> - 2011-05-23 12:07 +0300
  Re: Rationale behind user provided ctor requirement for const objects of non-POD class types "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-05-23 12:11 +0200
    Re: Rationale behind user provided ctor requirement for const objects of non-POD class types Nikos Chantziaras <realnc@arcor.de> - 2011-05-23 13:32 +0300
      Re: Rationale behind user provided ctor requirement for const objects of non-POD class types "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-05-23 13:36 +0200
        Re: Rationale behind user provided ctor requirement for const objects of non-POD class types "Balog Pal" <pasa@lib.hu> - 2011-05-23 14:44 +0200
          Re: Rationale behind user provided ctor requirement for const objects of non-POD class types Öö Tiib <ootiib@hot.ee> - 2011-05-24 16:04 -0700
            Re: Rationale behind user provided ctor requirement for const objects of non-POD class types "Alf P. Steinbach /Usenet" <alf.p.steinbach+usenet@gmail.com> - 2011-05-25 02:37 +0200

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