std call once in C++
Introduction
This tutorial aims to describe the usage of std::call_once, discussing its syntax, behavior, and practical applications.
Understanding std::call_once
The std::call_once function is defined in the <mutex> header. It enables the execution of a specified function only once, regardless of how many threads invoke it concurrently.
Every group of call_once invocations that gets the same std::once_flag object must meet the following conditions:
- One function is executed precisely once and is supplied as f to all of the group's invocations. Which function will be chosen to be executed is unknown. The call_once invocation to which the specified function was provided executes on the same thread.
- No invocation inside the group returns before the aforementioned execution of the chosen function is effectively finished, that is, before it exits via an exception.
- If the chosen function terminates via an exception, the caller is notified. A different function is chosen and run.
Syntax:
Its has the following syntax:
template<class Callable, class... Args>
void call_once(std::once_flag& flag, Callable&& f, Args&&... args);
Parameters:
- flag: It is a reference to an std::once_flag object, which is used to ensure the associated function is called only once.
- f: The callable object (function, lambda, functor) to be executed.
- args: Arguments to be passed to the callable object f.
Key Functionalities:
- One-Time Execution: It ensures that f is invoked only once, regardless of how many threads call std::call_once with the same std::once_flag object.
- Thread Safety: It provides synchronization to prevent race conditions during initialization, ensuring a consistent state across threads.
- Lazy Initialization: It allows delaying f's execution until it's truly needed, potentially improving performance.
Example:
Let us take an example to illustrate the std::call_once function in C++:
#include <iostream>
#include <thread>
#include <mutex>
std::once_flag flag;
void execute()
{
std::call_once(flag, [](){ std::cout << "It is called once" << std::endl; });
}
int main()
{
std::thread t1(execute);
std::thread t2(execute);
std::thread t3(execute);
std::thread t4(execute);
t1.join();
t2.join();
t3.join();
t4.join();
}
Output:
![std::call_once in C++](https://static.tutorialandexample.com/cpp/std-call-once-in-cpp1.png)
Behavior and Considerations
- Thread Safety: The std::call_once ensures thread safety by ensuring that the associated function is executed by only one thread, regardless of concurrent invocations.
- Lazy Initialization: Initialization occurs only when std::call_once is first invoked. Subsequent invocations have no effect.
- Exception Safety: If the callable object throws an exception, subsequent invocations of std::call_once with the same once_flag will still result in the function being executed.
- Callable Object: The std::call_once function accepts any callable object, including functions, lambdas, and functors.
- Arguments: Additional arguments can be passed to the callable object via std::call_once.
Practical Applications
The std::call_once function finds applications in various scenarios, including:
- Singleton Initialization: Ensuring that a singleton object is created only once, even in a multithreaded environment.
- Resource Initialization: Initializing shared resources such as databases, caches, or connection pools, ensuring initialization occurs only once.
- Lazy Initialization: Initializing objects or data structures lazily, i.e., on-demand, while guaranteeing thread safety.
Performance Optimization:
- Function-Local Statics: For thread-local initialization within functions, using function-local static objects might be a more efficient choice compared to std::call_once. It is because static variables are guaranteed to be initialized only once within the scope of the function.
- Lazy vs. Eager Initialization: The std::call_once function enables lazy initialization, delaying resource creation until it's truly needed. It can improve performance if the resource is used infrequently. However, in order to frequently use resources, eager initialization (creating the resource at program startup) might be preferable to avoid potential overhead during first use.
Comparison with Other Mechanisms:
- Static Initialization Order Fiasco (SIODF): The std::call_once function offers a safer alternative to SIODF, a potential pitfall in C++ when global variables with complex initialization logic are declared static. The std::call_once provides thread safety and avoids undefined behavior associated with SIODF.
- Mutexes: While the std::call_once function ensures one-time initialization, it doesn't provide ongoing data protection like a mutex. Mutexes are essential for protecting shared data structures accessed by multiple threads across their entire lifetime.
Conclusion:
In conclusion, the std::call_once function provides a robust and efficient mechanism for one-time initialization in multithreaded C++ applications. By leveraging a simple yet powerful API, developers can ensure thread-safe initialization with minimal overhead. Understanding its usage and behavior is essential for building reliable and scalable concurrent applications.nto multithreaded programs, you can produce more dependable and durable code.