Re: template const trick versus myers singleton - global data revisited

"Alf P. Steinbach" <>
Mon, 22 Jun 2009 03:32:38 +0200
* Technical:

On Jun 20, 10:40 pm, "Alf P. Steinbach" <> wrote:

Depends what you mean. The templated const trick is mostly for where you need a
/non-integral/ type constant. That is, except for possible static initialization
fiasco issues you can safely do this, in a header file:

   template< typename Dummy >
   struct Names_ { static std::string const cppLanguageCreator; };

   template< typename Dummy >
   std::string const Names_<Dummy>::cppLanguageCreator = "Bjarne Stroustrup";

   typedef Names_<void> Names;

It's slowly coming together for me. Last(hopefully) quesiton.

  template < typename dummy >
  struct consts // constants
    static double const PI;
    inline static std::string const& creator()
      static std::string const cppLanguageCreator = "Bjarne
      return ( cppLanguageCreator );

  template< typename dummy > double const consts<dummy>::PI =
  typedef consts<void> constants;

Would the creator method within the struct be a a suitable alternative
to circumventing the possible static initialization fiasco?


But on the other hand, the member routine gives more verbose and less readable
notation, possible nano-second-inefficiency (I'm mentioning that just for
completeness), and it can't deliver a compile time constant, that is, one that
can be used in declarations requiring compile time constant.

The main utility of the templated const trick has so far been in /debates/ about
allowing definitions of constants of any type for which a constant can be
defined, within a class definition, because via the templated const trick any
such in-class constant definition can be mechanically rewritten to standard
C++98, demonstrating that there's no particular technical show-stopper and that
current compilers already implement the basic machinery to handle them. I'm not
familiar enough with C++0x to say whether it's got those constants. But my
impression is that it has, that is, that C++0x will support such syntax (if you
want to check: the draft is available from the committe pages in PDF format).

Cheers & hth.,

- Alf

PS: a member routine defined in the class definition is implicitly 'inline', so
that keyword is superfluous in the example above.

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