Can you explain what std::exception_ptr is in C++ and how it is used for exception handling?

std::exception_ptr is a smart pointer in C++ that stores information about an exception, allowing it to be preserved and rethrown later. This is useful for passing exceptions between threads or for deferred processing.

Key points:

• You can obtain an exception_ptr using std::current_exception() inside a catch block.
• You can rethrow the exception using std::rethrow_exception(exception_ptr).

Example:

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

void threadFunc(std::exception_ptr& eptr) {
    try {
        throw std::runtime_error("Error in thread");
    } catch (...) {
        eptr = std::current_exception(); // save the exception
    }
}

int main() {
    std::exception_ptr eptr;
    std::thread t(threadFunc, std::ref(eptr));
    t.join();

    if (eptr) {
        try {
            std::rethrow_exception(eptr); // rethrow
        } catch (const std::exception& e) {
            std::cout << "Caught exception: " << e.what() << std::endl;
        }
    }
    return 0;
}

Thus, std::exception_ptr allows safe transfer of exceptions between contexts.