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

Technical <>
Sat, 20 Jun 2009 18:31:12 -0700 (PDT)
On Jun 20, 8:18 pm, "Alf P. Steinbach" <> wrote:

In a single file program this will work, but if you do it in a header fil=


which is the main point of the templated const trick, then you risk multi=


definitions, that is, running afoul of the One Definition Rule.

Instead do

   template< typename Dummy >
   double const consts<Dummy>::PI = 3.14159;

There is a special exemption for this, or put another way, the ODR allows=


in order to make it possible to define templates in header files.

Is there another name for the 'templated const trick'? With regards
to the ODR (this is where I get confused at times), why wouldn't i run
the risk of violation if I do:
 // header.h
 #ifndef HEADER
 #define HEADER
   static const double PI = 3.14159;

 // foo.cpp - assume there's a foo.h
 # include "header.h"
 { std::cout << PI << '\n' }

 // bar.cpp - assume there's a bar.h
 # include "header.h"
 { std::cout << PI << '\n' }

Not sure what you mean about "static keyword" but the templated const all=

ows you

to define constants in a header file and for each such constant have only=


occurrence of the value in the final program, without relying on optimiza=

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.

Not for this concrete example, because the static initialization fiasco i=

s about

dynamic initialization, initialization that involves execution of user co=

de, and

your constants are of built-in type:

   First, static initialization is performed, which consists of, in o=


      zero-initialization, and then
      initialization with constant expressions (compile time consta=


      dynamic initialization.

And the problem is that C++ does not strictly define the order that diffe=


compilation units' dynamic initialization is performed in.

So with e.g. std::string constants the templated const trick in general r=

uns the

risk of a static initialization order fiasco, since it just defines an or=


namespace scope constant, which might be accessed (from some other compil=


unit's dynamic initialization) before its constructor runs.

One way to avoid that is to wrap the constant in a function:

   inline std::string const& piChars()
       std::string const theValue = "3.141592654";
       return theValue;

That should really be:
     inline static std::string const& piChars()
    { static std::string const theValue = "3.141592654"; return
theValue; }

  instead of
   std::string const piChars = "3.141592654";

Of course only static ocnst integral types can be defined within the
composite type 'consts' but given the context point taken

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