Question

So I want to access a singleton class from multiple threads. Conceptually I d think that calling non-const methods on this singleton instance would be not thread-safe. I ve been looking online and no one seems to address this possible issue. Is there an actual problem with this, is the only issue with Singleton s thread-safety, the initialization of the instance variable?

Answer 1

You are correct, calling a non-const methods or methods that depend on instance data that could be modified by other threads must be syncronized.

Answer 2

A singleton instance has the same thread safety issues as any other instance, so calls to its methods or access to its members should be synchronized.

The initialization of the singleton itself is another issue...in gcc static initialization is threadsafe, but probably not so much on other platforms.

Also take a look at this paper addressing some threading singleton issues by Andrei Alexandrescu. His Modern C++ Design book also addresses singleton issues.

Answer 3

Apart from the thread-safety issue with the initialization of instance variable, the singleton objects should be treated as any other normal objects with respect to thread-safety for the method calls.

ie, Any methods ( you need synchronization for const method as well, if we are trying to read the value of a variable, which will get updated in another method by another thread) of the singleton object accessed by multiple threads and involves in shared data readwrite must be synchronized.

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