Скачать или смотреть How to Pass Function Arguments to a New Thread and Handle Return Values in C+ +

Discover how to pass function arguments like pointers and references to threads in C+ + , and learn how to retrieve return values efficiently using `std::future`.
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How to Pass Function Arguments to a New Thread and Handle Return Values in C+ +

In the world of multithreading, efficiently passing function arguments to a new thread while also managing to get a return value can be a bit tricky. This becomes particularly challenging when dealing with class member functions. A common scenario might involve wanting to pass a pointer to a class instance from one member function to another in a separate thread. If you've encountered the error related to capturing variables in a lambda expression when trying to do this, you're not alone.

The Problem

Imagine you have a class AI that needs to utilize the state of another class tictactoe to make decisions in a game. You want to launch a separate thread to perform some calculations (like finding available spots on the game board), but when doing so, you stumble upon an error stating that the variable cannot be implicitly captured in a lambda. This is a classic multithreading issue where you must manage the scope of variables properly.

Sample Code That Causes Errors

Here’s a closer look at the original code that leads to this issue:

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In this snippet, the thread is trying to call findOptions(game), but without appropriate capturing of the game pointer, you end up with an error. The variable game is not available within the thread's lambda context.

The Solution

Fortunately, the solution lies in utilizing std::future and std::async, which are part of the standard library in C+ + . This approach not only allows you to pass pointers and references but also enables you to get return values from your threaded functions.

Step-by-Step Implementation

Here’s how you can effectively pass a pointer and retrieve the return value using std::async:

Include Necessary Headers: Make sure to include the required headers for futures and threads.

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Modify the pickSpot Function: Instead of creating a thread explicitly, leverage std::async. This will allow for easy capturing of the this pointer and any other needed variables.

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Implement the findOptions Method: This method remains unchanged but is effectively called in a thread now.

Key Benefits

Simplified Syntax: Using std::async helps avoid the complexity of managing thread lifecycles manually.

Automatic Handling of Return Values: std::future makes it easy to wait for and retrieve results from the asynchronous call.

Error Reduction: Capturing the relevant variables is more straightforward, reducing potential errors related to scope.

Conclusion

By utilizing std::async and std::future, you can effectively pass function arguments such as pointers and references to a new thread while also handling the return value seamlessly. This approach simplifies multithreading in C+ + , making your programs both efficient and easier to maintain. So, next time you're faced with the challenge of threading in your C+ + applications, remember these techniques to simplify your code.