Question

Is there a way to implement a singleton object in C++ that is:

1. Lazily constructed in a thread-safe manner (two threads might simultaneously be the first user of the singleton – it should still only be constructed once).
2. Doesn’t rely on static variables being constructed beforehand (so the singleton object is itself safe to use during the construction of static variables).

(I don’t know my C++ well enough, but is it the case that integral and constant static variables are initialized before any code is executed (ie, even before static constructors are executed – their values may already be "initialized" in the program image)? If so – perhaps this can be exploited to implement a singleton mutex – which can in turn be used to guard the creation of the real singleton..)

Answer 1

Basically, you’re asking for synchronized creation of a singleton, without using any synchronization (previously-constructed variables). In general, no, this is not possible. You need something available for synchronization.

As for your other question, yes, static variables which can be statically initialized (i.e. no runtime code necessary) are guaranteed to be initialized before other code is executed. This makes it possible to use a statically-initialized mutex to synchronize creation of the singleton.

From the 2003 revision of the C++ standard:

Objects with static storage duration (3.7.1) shall be zero-initialized (8.5) before any other initialization takes place. Zero-initialization and initialization with a constant expression are collectively called static initialization; all other initialization is dynamic initialization. Objects of POD types (3.9) with static storage duration initialized with constant expressions (5.19) shall be initialized before any dynamic initialization takes place. Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit.

If you know that you will be using this singleton during the initialization of other static objects, I think you’ll find that synchronization is a non-issue. To the best of my knowledge, all major compilers initialize static objects in a single thread, so thread-safety during static initialization. You can declare your singleton pointer to be NULL, and then check to see if it’s been initialized before you use it.

However, this assumes that you know that you’ll use this singleton during static initialization. This is also not guaranteed by the standard, so if you want to be completely safe, use a statically-initialized mutex.

Edit: Chris’s suggestion to use an atomic compare-and-swap would certainly work. If portability is not an issue (and creating additional temporary singletons is not a problem), then it is a slightly lower overhead solution.