Asynchronous programming allows a program to execute tasks concurrently, enabling the efficient use of system resources. C++ provides several features to support asynchronous programming, including std::function and std::future.
std::function is a type-erased function wrapper that can store and invoke any callable object, including function pointers, lambda functions, and functors. It can be used to create a callback mechanism that allows one part of a program to register a function to be called later by another part of the program.
For example, suppose we have a function void perform_async_task(std::function<void()> task) that takes a std::function object representing a task to be performed asynchronously. We can call this function and pass a lambda function that prints a message as follows:
perform_async_task([](){
std::cout << "Performing task asynchronouslyn";
});std::future is a synchronization primitive that represents a value that may not be available yet. It provides a way for a thread to wait for a value to be computed asynchronously by another thread.
For example, suppose we have a function int compute_async_value() that performs a long-running computation and returns a result asynchronously. We can call this function using std::async and obtain a std::future object that represents the result as follows:
std::future<int> future_result = std::async(std::launch::async, [](){
// long-running computation
return 42;
});We can then wait for the result using the std::future::get() method, which blocks until the result is available:
int result = future_result.get();std::future can also be used to implement continuations, which are functions that are executed when the asynchronous operation completes. For example, we can use std::future::then() to execute a lambda function when the result of a computation becomes available:
std::future<int> future_result = std::async(std::launch::async, [](){
// long-running computation
return 42;
});
std::future<void> continuation = future_result.then([](int result){
std::cout << "Result is " << result << std::endl;
});
In this example, the lambda function passed to std::future::then() takes the result of the computation as its argument and prints it to the console. The std::future<void> object returned by std::future::then() represents the continuation and can be used to wait for its completion or to chain further continuations.