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

"Alf P. Steinbach" <>
Sun, 21 Jun 2009 19:18:50 +0200
* Technical:

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

Internal linkage is the default for a const at namespace scope.

And so above the keyword 'static' is redundant -- unlike my typo below...

Correct! I tend to forget the 'redundancy' from time to time, you'd
think by now I'd learn.

Not sure what you mean about "static keyword" but the templated const allows you
to define constants in a header file and for each such constant have only one
occurrence of the value in the final program, without relying on optimizations.

Well I was referring to 'form b'. The c_constants class. The one
advantage I see with the singleton approach is static keyword is not
required on for the variable PI and PI can be initialized using the
constructor. That's perhaps the only drawback I see with the
templated const trick. I can't use a constructor to initialize PI
while achieving the objective.

I'm still not sure exactly what you mean.

Givent the two forms: form (a)and form (b)
  ///form a
  template < typename dummy >
  struct consts // constants
  { static double const PI; };
  template< typename dummy > double const consts<dummy>::PI =
  typedef consts<void> constants;

  ///form b
  class c_constants {
  public :
    double const PI ;
    static int const MAX_INT = INT_MAX ;
     : PI ( 3.14159 )
    static c_constants& instance() {
      static c_constants inst ;
      return inst ;

With the Myers singleton PI is declared as 'double const PI' and is
initialized in the constructor. With the templated const trick PI is/
must be declared as 'static double const PI' and is defined outside of
the struct i.e. template< typename dummy > double const
consts<dummy>::PI = 3.14159;. Just trying to summarize the difference
(initialization list with Myers end elimination of the 'static'
keyword, versus using the static keyword and providing the definition
outside of the strut) in my mind and more importantly see if theres
any advantage.


There are notational and efficiency differences. Functionally the main
difference is that the templated const can provide a compile time constant, e.g.
one that can be used to dimension a raw array type, but (for other usage) the
templated const is subject to possible static initialization order fiasco.

The first 'static' you added, however, would make this function have internal
linkage, and so would (modulo optimization) place a copy of both the function
and its wrapped constant in every compilation unit where it's used.

Interesting, this is also true for the 'form b'. i.e
    static c_constants& instance() {
      static c_constants inst ;
      return inst ;

No, for a class member declaration the keyword 'static' does not imply internal

instance has internal linkage and would place a copy of the function
and it's wrapped contents in every complilation unit where its used.

The 'inline' is to allow multiple definitions of the function (one per
compilation unit, only one of them being picked for the final program).

Got it! Learned something new today.


Cheers & hth.,

- Alf

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