Re: singleton

From:

Rahul <sam_cit@yahoo.co.in>

Newsgroups:

comp.lang.c++

Date:

Tue, 25 Mar 2008 01:21:09 -0700 (PDT)

Message-ID:

<c3ea4242-6778-40c7-86d5-c209f5f257e4@e39g2000hsf.googlegroups.com>

On Mar 25, 1:14 pm, Colin <song6...@gmail.com> wrote:

class A{
private:
    A(){}
    static A* a=null;
public:
    static A *getInstance(){
      if(a==null)
      {
          a=new A(); //remeber delete it..
      }
      return a;
    }

};

On Mar 25, 10:17 am, thomas <FreshTho...@gmail.com> wrote:

hi guys,
-----code-----
class A{
private:
    A(){}
public:
    static A *getInstance(){
       static A *a = new A(); //-------L1----
       return a;
    }};

-----code-----

I use the static keyword to declare a static instance A,
I wonder if getInstance() is called multiple times, will L1 be
executed multiple times?

generally a initialization sentence of static variable will be
executed only once, but I'm not sure what will the c++ compiler expand
for L1.

What if the static member function getInstance() is invoked by two
threads and when one thread enters into the if condition and a thread-
switch happens causing the other thread too to enter into the if
condition? It would become a race condition and would result in a
memory leak...

I hope you got the point...