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 20:01:19 CST

Message-ID:

<z1BRk.99811$Ca.21332@twister2.libero.it>

blargg ha scritto:

krisprad@yahoo.co.uk wrote:

I have methods in which I need to perform once-only initialization.

[...]

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

        // initializer code is called only once per method
        static StaticBarInitialzer tmp;

        <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?

Anything wrong with the straightforward approach of using a static
variable and if statement?

    struct Foo
    {
        void Bar()
        {
              static bool initialized = false;
              if ( !initialized )
              {
                  <static initializer code>
                  initialized = true;
              }

              <other code>
        }
    };

This code isn't thread-safe. Well... to be fair, even the OP's code is
not thread-safe according to the C++03, but it will be guaranteed to be
thread-safe in C++0x.

It can even be wrapped in a macro (insert standard disclaimer about
evilness of macros). The odd structure is to have initialized set AFTER
the block is executed, and without requiring the user to put another macro
invocation after the block.

    #define ONE_TIME_BLOCK \
        static bool initialized = false;\
        if ( !initialized )\
            goto initialize;\
        for ( ; 0; initialized = true )\
        initialize:

That's nice, really. Just a question, why use goto? This works in the
same way:

     #define ONE_TIME_BLOCK \
         static bool initialized = false;\
         for ( ; !initialized; initialized = true )\

or, even better (replacing initialized with init just to avoid line
wrapping in emails):

     #define ONE_TIME_BLOCK \
         for (static bool init = false; !init; init = true )\

To avoid conflicts, you could also uglify the name of the variable by
using something like init##__LINE__.

HTH,

Ganesh

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