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C++ async await

Many programming languages provide a syntactic property called async/await that allows an asynchronous or non-blocking procedure to be organized similarly to a typical synchronous method in computer programmes. An easy way to write asynchronous code is to use async and await.

For instance, running some calculations and obtaining some data via I/O. Asynchronous programming improves responsiveness and is characterized as a close approach to writing multithreading programs.


  • This keyword designates an asynchronous function.
  • When a function is asynchronous, it operates in the background, freeing up the caller code to carry out its operations without waiting for the function to finish.


  • An asynchronous function can be made to wait for a specified asynchronous operation to finish by using the await keyword.
  • It permits you to await the outcome of the process without obstructing the primary thread.


1. Policy:

The launching policy is indicated by this bitmask value.

  • launch::async:
    • This asynchronous method creates a new thread to call the function, treating it as though it had been created with functions and arguments, and uses the returned future to access the shared state.
  • launch::deferred:
    • It is postponed, and the function call is postponed until the shared state of the returning future is retrieved using either get or wait. The function is then considered to be called in such case, and it is no longer considered delayed. The shared state of the returning future is prepared at the moment this specific call is returned.
  • launch::async|launch::deferred:
    • It is an automatic process in which the function chooses the policy on its own at a certain moment. It depends on how the library is implemented and how the system works, which often optimizes for the system's current concurrent availability.

2. fn:

  • It is a reference to any kind of move-constructible function object, member, or function whose class implements the operator() that takes closures and functional objects into account.
  • In this case, the function uses the parameter's decay copy. As the shared state to be attained by the future object that async returns, the fn return value is preserved.

3. Args:

  • The args and fn are regarded as the template arguments. In other words, they will be the argument's proper lvalue/rvalue reference types if it is implicitly inferred.
  • In this case, a future object with a shared state that is getting ready after the fn execution is finished will be the return value. The value returned by function fn (if any) will be the value achieved by the future:: get member. Even if the shared state has never been visited if launch::async is selected, the future that is returned is linked to the end of the created thread. In this case, the return of fn synchronizes with the return of fn. Therefore, even though the function fn returns void, the return value won't be disregarded for its asynchronous behaviour at that point.
  • When a function is invoked with parameters of the kinds in Args, it returns a type result_of::type.


Let's take an example to determine the use of async and await in C++.


C++ async await


In C++, using async and await has the following advantages:

1. Better Readability:

Asynchronous code becomes easier to comprehend and maintain since it is more readable and resembles synchronous code.

2. Concurrency Control:

Await helps to manage concurrency and prevent data races by making sure an asynchronous function doesn't start until the task it is waiting on is finished.

3. Easier Error addressing:

Since exceptions thrown in asynchronous functions can be handled by the calling code, addressing errors in asynchronous code is simpler.

4. Improved Debugging:

It is easier to debug because asynchronous code has a more dependable and synchronous-like flow.

5. Improved Resource Utilisation:

Asynchronous code can boost performance in multi-threaded programs by making greater use of system resources.

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