Re: design question: per method static initializer code

From:
Alberto Ganesh Barbati <AlbertoBarbati@libero.it>
Newsgroups:
comp.lang.c++.moderated
Date:
Sun, 9 Nov 2008 20:01:09 CST
Message-ID:
<EAARk.99798$Ca.21189@twister2.libero.it>
krisprad@yahoo.co.uk ha scritto:

I have methods in which I need to perform once-only initialization.
This is similar to static initializer blocks in Java, but not exact
since initializer blocks are on per-class basis. I have in mind static
initializer on per-method basis.

struct Foo
{
    void Bar()
    {
          <static initializer code> // this should be executed only
once
          <other code>
    }
};

The way I am doing it is:
struct Foo
{
    void Bar()
    {
          struct StaticBarInitialzer // internal helper class to do
the needful
          {
                <static initializer code>
          }

          static StaticBarInitialzer tmp; // initializer code is
called only once per method
          <other code>
    }
};

One can of course use global singletons, but I do not want to use
global namespace for symbols that are used so locally. I happened to
need this kind of set up several times that I am sure others must have
done it earlier. So, the question is, are there better ways of
achieving this?


It looks like a good idiom to me. It's a bit verbose, but it's not that
bad. You could save some typing by writing it this way:

   void Bar()
   {
     static struct Initializer
     {
       Initializer() { /* static init code */ }
     } init;

     /* ... */
   }

which is essentially the same.

Things are going to look prettier in C++0x with the advent of lambda
expressions, for example:

   // you just need this only once for the entire program
   struct ExecuteOnce
   {
      template <typename T>
      ExecuteOnce(T f) { f(); }
   };

   void Bar()
   {
     static ExecuteOnce init = [] // <- lambda introducer
     {
       /* static init code */
     };

     /* ... */
   }

Nice, isn't it?

HTH,

Ganesh

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