为什么程序在堆栈展开后无法到达正确的 return 指令？

Question

编译器：g++ 9.2.0
操作系统：Windows 10
g++ 调用：

g++ -E main.cpp -v -o main.i
g++ -c main.cpp -v -o main.o 
g++ main.o -v -o main.exe
main.exe

main.cpp:

#include <chrono>
#include <iostream>
#include <string>
#include <exception>
#include <iostream>
//#include <thread>
#include "mingw.thread.h"
struct Object{
    struct Exception : public std::exception{
        std::string error_;
        Exception(std::string str){
            this->error_ = str;
        }
        ~Exception() {
        }
        std::string get(){
            return error_;
        }
    };
    void DoSomeWork() {
        try {
        std::thread AnotherTh(&Object::GenerateException ,this);
        AnotherTh.detach ();
        while(true);
    }
        catch (...) {
            throw ;
        }
    }
    void GenerateException(){
        std::this_thread::sleep_for (std::chrono::seconds(5));
        throw Object::Exception ("Some error");
    }
};
int main(){
    try{
        Object instance;
        std::thread th(&Object::DoSomeWork,std::ref(instance));
        th.join ();
    }
    catch (Object::Exception &ex ) {
        std::cout << ex.get ();
    }
    catch (std::exception &ex ){
        std::cout << ex.what ();
    }
    catch (...){
    }
    std::cout << "never reach this";
    return 0;
}

输出：

terminate called after throwing an instance of 'Object::Exception'
  what():  std::exception

我正在启动 主线程 和 新线程 (th) 并等待它，在 th 中 启动 另一个线程 将抛出异常。所以，当它出现时，开始堆栈展开（从 Object::GenerateException 到 Object::DoSomeWork 因为没有更多的调用是 Object::GenerateException 的堆栈）并且管理被传递给 Object::DoSomeWork's try-catch ，main 的 try-catch 调用链与从 main 调用的 Object::DoSomeWork "knows" 相同。

我不明白为什么它不能处理异常并将其传递给 main 的 try-catch。

Answer 1

Why program cannot reach proper return instruction after stack unwinding c++?

因为您的代码创建了多个线程，而您没有在实际抛出异常的线程中捕获异常。即使您调用 std::thread 的 join() 成员函数，异常也不会跨线程传播。

Try blocks 被定义为堆栈的动态构造。 try 块捕获代码抛出的异常，代码通过调用从其内容动态访问。

当你创建一个新线程时，你创建了一个全新的堆栈，它根本不是 try 块的动态上下文的一部分，即使调用 pthread_create 或构造 join-able std::thread()在里面试试。

要捕获源自线程 X 的异常，您必须在线程 X 中使用 try-catch 子句（例如，围绕线程函数中的所有内容，类似于您已经在 main 中执行的操作）。

有关相关问题，请参阅 How can I propagate exceptions between threads?。

一个例子：

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


struct Object {

    void DoSomeWork() 
    {
        std::cout << "DoSomeWork Thread ID: " << std::this_thread::get_id() << std::endl;
        try {
            std::thread thread(&Object::GenerateException, this);
            thread.detach();
            while(true);
        }
        catch (...) {
            std::cout << "Caught exception: " << std::this_thread::get_id() << std::endl;
            throw ;
        }
    }
    void GenerateException(void)
    {
        std::cout << "GenerateException Thread ID: " << std::this_thread::get_id() << std::endl;
        try {
            std::this_thread::sleep_for (std::chrono::seconds(5));
            throw std::runtime_error("Some error");
        } catch (...) {
            std::cout << "Caught exception: " << std::this_thread::get_id() << std::endl;
            throw;
        }
    }
};
int main()
{
    std::cout << "Main Thread ID: " << std::this_thread::get_id() << std::endl;
    try {
        Object instance;
        std::thread th(&Object::DoSomeWork,std::ref(instance));
        th.join();
    }
    catch (const std::exception &ex) {
        std::cout << ex.what() << std::endl;
        std::cout << "Exception caught at: " << std::this_thread::get_id() << std::endl;
    }
    std::cout << "never reach this" << std::endl;
    return 0;
}

输出：

Main Thread ID: 140596684195648
DoSomeWork Thread ID: 140596665124608
GenerateException Thread ID: 140596656670464
Caught exception: 140596656670464
terminate called after throwing an instance of 'std::runtime_error'
  what():  Some error
Aborted (core dumped)

Answer 2

来自this std::thread reference：

... if it terminates by throwing an exception, std::terminate is called.

如果在线程中抛出未捕获的异常，则程序将被强制终止。

为什么程序在堆栈展开后无法到达正确的 return 指令？

Why program cannot reach proper return instruction after stack unwinding?

c++

stack

g++

exception

stack-unwinding