std::call_once is a c++ function which can be called by multiple threads. It was designed to simplify the task of ensuring that the function you call is called only once. This is a bit of a trickier task in a multithreaded environment. The key to a successful call_once is to ensure that the function is called only once and to ensure that the correct thread gets the opportunity to invoke the function. Having a lock in place is essential to achieving this goal.
The std::once_flag is a helpful structure which allows coordination of multiple calls to std::call_once. Its state can be atomic or double-checked. It can also be used to display the appropriate symbol to the user. If a function is called with a std::once_flag, the resulting state will be an instance of the same struct.
The std::once_flag may be used as an input to std::cout. This function calls the function fn with a list of args and then modifies the flag. It is important to note that std::cout is not an optimisation over std::call_once. A call to std::cout will fail with a system error.
std::cout is one of several optimisations in the UE4 library. This library includes a multithreading subset which monitors code execution and tracks object dependencies. It also implements a suite of low-level optimizations that are specific to the platform in question. It also has a number of container implementations, which makes it a good fit for multithreaded applications.
std::call_once can be used to show a std::once_flag instance to the user, which is the same struct but the user gets a list of all calls to the struct. It is also useful in selecting which of the two possible std::call_once methods is the most appropriate for a particular function. This can help prevent a call to std::once_flag from being omitted from a compiled program.
The std::once_flag can also be used to create a function which can perform a number of functions in conjunction with std::call_once. These functions include the aforementioned cout, but also a function to perform the most obvious of all function calls. The std::cout function is not a gimmick, but instead does a good job of creating a decay copy of rvalues and a call to fn. It is also worth noting that std::cout uses a crude spinlock to ensure that only one thread initializes the mutex. This can help to prevent deadlock in greenthread-esque programming frameworks.
The std::once_flag and std::cout are the best examples of a pair of related functions that can be used to simplify the initialization of a singleton. However, there are many other ways to perform the same task, such as std::shared_lock, std::scoped_lock and std::unique_lock. Choosing the std::once_flag for the right purpose is a great way to simplify a singleton and avoid unnecessary complexity.
The std::once_flag has a number of other advantages as well. It may be the best choice for implementing a multithreaded program, as its recursive nature means that a call to std::once_flag will not be repeated in other functions.