Re: forward-decl for Singleton?

From:

werasm <werasm@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 20 Dec 2007 14:18:08 -0800 (PST)

Message-ID:

<b44759c2-3713-4a16-84e7-5d05d58bdf44@w40g2000hsb.googlegroups.com>

On Dec 20, 11:02 pm, Markus Dehmann <markus.dehm...@gmail.com> wrote:

I need a Singleton for general program options so that all classes can
access it.

I use the code below (adapted from the Wikipedia singleton example).
But the problem is if I change one variable in it, all my classes have
to re-compile. But I am planning to add more options often during
development. I tried to solve it through a forward declaration "class
Opts;", but didn't succeed because Opts::instance() results in an
error message about the incomplete type.

One solution might be to use a key-value map, but I would prefer to
use the variables directly, because they can be accessed faster, and
all possible options are known at compile-time.

Does anyone have a hint how to do the forward decl so that I don't
have to recompile every class that includes the Opts decl whenever I
add an option?

Thanks!
Markus

// header singleton.h
template<typename T> class Singleton{
   public:
     static T& Instance(){
         static T theSingleInstance; // assumes T has a protected
default constructor

I've written a little test to see if this works (especially this
assumption stated here above about protected default:

template <class T>
struct Base
{
  static T& instance()
  {
    static T failToCompile;
    return failToCompile;
  }
};

struct Derived
{
  protected:
    Derived(){}
};

int main()
{
  Derived& d = Base<Derived>::instance();
  return 0;
}

Looks like it does not like the base attempting to call
the protected constructor in the derived. ;-)

Regards,

Werner