Re: Array of objects from class without default constructor

"P. Lepin" <>
Mon, 30 Mar 2009 17:14:43 +0300
Matthias wrote:

As far as I know, it's not possible to get an array new-expression call
a class constructor with parameters. Thus, it is non trivial to build an
array of object belonging to a class, say A, without default constructor.

To bypass this limitation, one can add a trivial constructor to class
A. The resuting code looks like the following:


I don't feel confortable with this solution: from my program design it's
non-sense to build an A object without specifying its size...
Are there other solutions to the `array of objects from class without
default constructor' problem? Does my solution looks horrible to C++

(not a C++ expert, take everything below with a biiig grain of

Well, you could always decide to over-engineer and do something
like this (assuming you have control over the class interface):

#include <iostream>
#include <ostream>
class NextSizeCallback {
   public: virtual int operator()() = 0; };
class IncCallback: public NextSizeCallback {
   public: IncCallback(): n_(1) { }
            virtual int operator()() { return n_++; }
   private: int n_; };
class A {
   public: static void setNextSizeCallback(
              NextSizeCallback* c) { A::c_ = c; }
            static int getNextSize() { return (*A::c_)(); }
            A(): n_(getNextSize()) { }
            int getSize() const { return n_; }
   private: static NextSizeCallback* c_;
            int n_; };
NextSizeCallback* A::c_ = NULL;
int main() {
   IncCallback i;
   A* t = new A[10];
   for (int i = 0; i != 10; ++i)
     std::cout << t[i].getSize() << std::endl;
   return 0; }

I doubt this is any better in terms of readability and
maintainability than using placement new and creating your
objects explicitly, though.

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