Re: 'global' data

Fri, 22 May 2009 17:45:28 -0700 (PDT)
On May 19, 8:30 am, Jeff Schwab <> wrote: wrote:

Anyway, if you want to code the discardable thing explicitly you can u=

se the

template const tric:

Could you elaborate on the template ocnst trick? Did a google search=


thumbed through Josuttis, C++ template text but I'm coming up short.

Static members of class templates may be defined in headers, and appear
in multiple translation units.

I suspect there's two parts of this that's throwing me for a loop
(template has a tendency to do that to me though :) )

 template< typename Dummy >
 double const Math_<Dummy>::pi = 3.14;

Here dummy is not deducible. T/F?

That's still just part of the template definition. Dummy is a
typename-id, not actually a typename.

typedef Math_<void> Math;
void? I suspect is all part of the trick?

The actual type doesn't matter. The point is that when the template is
instantiated, Math::pi will refer to Math_<your_type_here>::pi.

Taken astep further ..
   template< typename Dummy >
   struct Math_
     static double const pi;
     static int const test ;
     // and so on.. ownder how far i could go with this.. unnamed

As far as you want. You can even do it with non-const, non-integral,
non-primitive, non-trivial types.

Template member definitions are instantiated only if they are accessed,
so you only pay for whichever variables or functions actually get used.
  For example:

#include <iostream>

namespace app {

     bool was_here1, was_here2;

     struct kilroy { kilroy( bool& was_here ) { was_here = true; =

} };

     namespace internal {

         template<typename Dummy>
         struct math { static kilroy k1, k2; };

         template<typename Dummy> kilroy math<Dummy>::k1( was_h=

ere1 );

         template<typename Dummy> kilroy math<Dummy>::k2( was_h=

ere2 );


     typedef internal::math<void> math;

     void main() {
             << &math::k1 << '\n' // Instantiate k1, but=

 not k2.

             << was_here1 << '\n' // 1
             << was_here2 << '\n'; // 0


int main() { app::main(); }

[OT] I'm reviewing coding standards that caution against declaring
global objects in the outer scope of a program or namespace. The
standard also states that global constants shall be declared as
'static const' within the public scope of a class. Global variable
data objects shall be avoided, but when used, they shall be
encapsulated in a class and accessed through a method call (Ex:
Singleton design pattern).

Not sure if I agree with this. Not all 'global' is bad and as such
requires 'encapsulation in a class and accessed through a singleton'

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