在C++中，当一个线程出现错误时，可以通过捕获异常并重新启动线程来实现自动重启线程的功能。以下是一个简单的例子，展示了如何实现这一功能：

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

void threadFunction() {
    static int counter = 0;
    counter++;
    std::cout << "Thread started, attempt " << counter << std::endl;

    if (counter < 2) {
        // 模拟异常
        throw std::runtime_error("Thread encountered an error");
    }

    std::cout << "Thread completed successfully" << std::endl;
}

void startThread() {
    while (true) {
        bool success = false;
        try {
            std::thread t([](){
                try {
                    threadFunction();
                } catch (const std::exception& e) {
                    std::cerr << "Exception in thread: " << e.what() << std::endl;
                    std::this_thread::sleep_for(std::chrono::seconds(1));
                    threadFunction();
                }
            });
            t.join(); // 等待线程完成
            success = true; // 如果线程成功完成，则设置成功标志
        } catch (const std::exception& e) {
            std::cerr << "Exception caught: " << e.what() << std::endl;
            std::cerr << "Restarting thread..." << std::endl;
            std::this_thread::sleep_for(std::chrono::seconds(1)); // 等待一秒钟再重新启动线程
        }

        if (success) {
            break; // 如果线程成功完成，则跳出循环
        }
    }
}

int main() {
    startThread();
    std::cout << "Main thread finished" << std::endl;
    return 0;
}