Re: Singleton_pattern and Thread Safety

From:

Leigh Johnston <leigh@i42.co.uk>

Newsgroups:

comp.lang.c++

Date:

Tue, 14 Dec 2010 14:48:29 +0000

Message-ID:

<FIudnQYZOI8pHprQnZ2dnUVZ8j4AAAAA@giganews.com>

On 14/12/2010 10:07, James Kanze wrote:

On Dec 13, 7:30 pm, Leigh Johnston<le...@i42.co.uk> wrote:

On 13/12/2010 18:47, Leigh Johnston wrote:

[...]

This is your code:

namespace {

Singleton* ourInstance =&Singleton::instance();

Singleton&
Singleton::instance()
{
if (ourInstance == NULL)
ourInstance = new Singleton;
return *ourInstance;
}
}

The ourInstance *pointer* is a global object (albeit with internal
linkage) which you are initializing with a dynamic allocation wrapped in
a function. If you have more than such initialization in more than one
TU the order of the initializations is unspecified.

Just as confirmation that what I am saying is correct I created two
singletons using your method in the files a.cpp and b.cpp and here is
the result:

leigh@leigh-VirtualBox:~/dev/singleton$ g++ a.cpp b.cpp
leigh@leigh-VirtualBox:~/dev/singleton$ ./a.out
singleton B constructed
singleton A constructed
leigh@leigh-VirtualBox:~/dev/singleton$ g++ b.cpp a.cpp
leigh@leigh-VirtualBox:~/dev/singleton$ ./a.out
singleton A constructed
singleton B constructed
leigh@leigh-VirtualBox:~/dev/singleton$ g++ a.cpp b.cpp
leigh@leigh-VirtualBox:~/dev/singleton$ ./a.out
singleton B constructed
singleton A constructed
leigh@leigh-VirtualBox:~/dev/singleton$

That's not the point. The point is that it is impossible to use
the singleton before it has been constructed.

It is the point. Singletons can do stuff during *construction*. In
this case the two singletons whilst not referencing each other reference
a third object during construction namely std::cout.

You seem to think that a program can behave differently depending on the
order its source files are built; this is an interesting approach to
software engineering.

/Leigh