std::exception_ptr

Type: std::exception_ptr in C++

The std::exception_ptr is a powerful tool introduced in C++11 that allows you to capture and store exceptions, and then rethrow them later, potentially in a different context, such as another thread or function. It is especially useful in asynchronous programming, where exceptions need to be propagated between different parts of the code, for instance, from a worker thread back to the main thread.

Key Concepts

• Capturing an Exception: You can capture an exception using std::current_exception() which returns a std::exception_ptr to the current exception.
• Storing an Exception: The std::exception_ptr can store any type of exception and can be safely passed around or stored.
• Rethrowing an Exception: You can rethrow the stored exception using std::rethrow_exception().

Example Scenario: Capturing and Rethrowing Exceptions Across Threads

Here's an example where we capture an exception in one thread, store it using std::exception_ptr, and then rethrow it in the main thread.

#include <iostream>
#include <thread>
#include <exception>

std::exception_ptr globalExceptionPtr = nullptr;

void faultyFunction() {
    try {
        // Simulate an exception being thrown
        throw std::runtime_error("Something went wrong in the thread!");
    } catch (...) {
        // Capture the exception and store it in the global exception pointer
        globalExceptionPtr = std::current_exception();
    }
}

int main() {
    // Start a thread that may throw an exception
    std::thread t(faultyFunction);
    t.join();

    // Check if an exception was captured
    if (globalExceptionPtr) {
        try {
            // Rethrow the exception in the main thread
            std::rethrow_exception(globalExceptionPtr);
        } catch (const std::exception& e) {
            // Handle the exception here
            std::cerr << "Caught exception: " << e.what() << std::endl;
        }
    } else {
        std::cout << "No exception caught in the thread." << std::endl;
    }

    return 0;
}

Explanation

• std::exception_ptr globalExceptionPtr = nullptr;: We declare a global std::exception_ptr to store the exception that might be thrown in the thread.
• try-catch block in faultyFunction(): Within the thread, an exception is thrown and immediately caught. The std::current_exception() function is used to capture the current exception and store it in the globalExceptionPtr.
• std::rethrow_exception(globalExceptionPtr);: In the main thread, after the worker thread has completed, we check if an exception was captured. If so, we rethrow it using std::rethrow_exception() and handle it in the main thread’s catch block.

Output

Caught exception: Something went wrong in the thread!

When to Use std::exception_ptr

• Multithreaded Environments: Particularly useful in scenarios where exceptions need to be passed between threads.
• Asynchronous Programming: Capturing and propagating exceptions in asynchronous operations or callbacks.
• Complex Exception Handling: Useful in complex exception handling scenarios where exceptions need to be captured, stored, and later analyzed or rethrown.

Benefits of std::exception_ptr

• Type Safety: std::exception_ptr is type-safe and can store any type of exception.
• Flexibility: It allows exceptions to be captured and rethrown, enabling exception propagation across different parts of a program.
• Multithreading: It is particularly useful in multithreaded environments where exceptions need to be communicated between threads.

Conclusion

std::exception_ptr is a versatile tool in modern C++ that enhances the robustness of exception handling, especially in multithreaded or asynchronous applications. It allows developers to safely capture, store, and rethrow exceptions across different contexts, ensuring that exceptions are handled appropriately no matter where they occur.