🚀 go-pugleaf

In article <l8291j9wix9.fsf@neppari.cs.hut.fi>,
"Lassi A. Tuura" <Lassi.Tuura@hut.fi> writes:
>One compiler I need to use has problems with the following code:
>---------------
>#include <new.h>
>class T {
>public:
>    T (int n) : _n (n) { }
>    T (const T &ref) : _n (ref._n) { }
>    ~T () { }
>
>private:
>    int _n;
>};
>
>void f ()
>{
>     T *ptr = new T [10] (1);  // line 14
>}
>---------------
>
>In particular, it complains about the initializer to the vector new:
>  CC: "test.cc", line 14: error: initializer for array of class object
>			  created using `new' (1248)
>
>Another compiler has no problems with the code, and the one giving the
>error message has not proved to be reliable.  The question is: Is this
>legal C++?  I read thru 5.3.4 from the april draft and as far as I
>understood the code was flagged ill-formed.  Did I interpret the draft
>correctly?  If I did, how am I supposed to allocate an array of
>objects of class T?  With explicit calls to operator new and placement
>new?

The code is not legal. The reason is that the default constructor,
which is usually T::T(), will be used when you allocate arrays of T:s.
The compiler will generate a T::T() for you only in one case, namely
when you yourself have explicitly delared no other constructors. In
this case you have such a constructor, T::T(int n). Therefore the
compiler will not make a T::T() for you, which makes it impossible to
allocate arrays of T:s.

The fix is simple: Get yourself a T::T(). Another option is to give
the the int parameter a default value, usually 0 works OK. This would
give you this class:

class T
{
   public:
      T (int n = 0) : _n (n) { }
      T (const T &ref) : _n (ref._n) { }
      ~T () { }

   private:
      int _n;
};

A side note: Don't use "_" as a prefix to identifiers, since that is
almost illegal according to the language. It is much better to use "_"
as a suffix.

Mats Henricson
Stockholm
Sweden
---
[ comp.std.c++ is moderated.  Submission address: std-c++@ncar.ucar.edu.
  Contact address: std-c++-request@ncar.ucar.edu.  The moderation policy
  is summarized in http://dogbert.lbl.gov/~matt/std-c++/policy.html. ]