Singleton with read-only and write access

Question

I have a class T and want a single global object of that class given by accessor functions like T const& read_singleton() and T& modify_singleton(). The object should be instantiated at first use of one of these functions by the default constructor of T and the usage of the functions should be thread-safe. What is a (good) design pattern to achieve this?

Answer 1

It's easy enough to write these functions:

T& modify_singleton()
{
    T static t{};
    return t;
}

T const& read_singleton()
{
    return modify_singleton();
}

This is the Meyers singleton pattern and can also be implemented as static functions of T, in case you have control over its implementation and want to make its constructor/destructor and such private.

The initialisation of variable t will be thread-safe. However, what happens when multiple threads obtain a T& is outside the control of modify_singleton. You will have to coordinate modifications to the object, either with a global mutex or by designing its class to be thread-safe. If you cannot modify class T, you could also provide a thread-safe interface through a wrapper class.