源自std :: exception的C++自定义异常未被捕获

Question

我正在使用以下代码编写自定义异常类:

#include <iostream>
#include <string>
#include <memory>
#include <sstream>
#include <iomanip>
#include <algorithm>    


class MyException : public std::exception {

public:
    MyException();
    explicit MyException(std::string message);
    MyException(std::string source, std::string message);
    MyException(int code, std::string source, std::string message);
    const char *what() const throw();


private:
    int exceptionCode;
    std::string exceptionSource;
    std::string exceptionMessage;
};


MyException::MyException() :
        exceptionCode(0),
        exceptionSource ("No source."),
        exceptionMessage ("No message.") {}

MyException::MyException(std::string message) :
        exceptionCode(0),
        exceptionSource ("No source."),
        exceptionMessage (std::move(message)) {}

MyException::MyException(std::string source, std::string message) :
        exceptionCode(0),
        exceptionSource (std::move(source)),
        exceptionMessage (std::move(message)) {}

MyException::MyException(int code, std::string source, std::string message) :
        exceptionCode(code),
        exceptionSource (source),
        exceptionMessage (message) {}

const char *MyException::what() const throw()
{
    std::cout << "What:" << exceptionMessage << std::endl;

    std::stringstream s;
    s << "MyException Data:" << std::endl;
    s << "Code    : " << exceptionCode << std::endl;
    s << "Source  : " << exceptionSource << std::endl;
    s << "Message : " << exceptionMessage << std::endl;

    std::string whatString = s.str();
    return whatString.c_str();
}


void test()
{
    throw new MyException("test", "This is a test");
}


int main()
{
    try
    {
        test();
    }
    catch (const std::exception &exc)
    {
        std::cerr << "Exception detected:" << std::endl;
        std::cerr << exc.what();
        throw exc;
    }
    catch (...)
    {
        std::cerr << "An unknown exception was called." << std::endl;
        throw;
    }
}

它编译得很好,但是我无法从catch (const std::exception &exc)块中捕获自己的异常.它只能被catch (...)街区捕获.

由于MyException是继承了std::exception我以为它会被先抓catch块......这是为什么没有发生？

原始代码链接在这里

Answer 1

这并没有直接回答这个问题,但这非常重要

这个函数是一个等待发生的不安全崩溃:

const char *MyException::what() const throw()
{
    std::cout << "What:" << exceptionMessage << std::endl;

    std::stringstream s;
    s << "MyException Data:" << std::endl;
    s << "Code    : " << exceptionCode << std::endl;
    s << "Source  : " << exceptionSource << std::endl;
    s << "Message : " << exceptionMessage << std::endl;

    std::string whatString = s.str();
    return whatString.c_str();
}

string::c_str()在被调用的临时字符串中返回c-string whatString.

编写这样的异常类时,必须将完整的错误消息存储在异常中 - 在构造函数中构建它.

这是一个安全的替代品:

class MyException : public std::exception {

public:
    MyException();
    explicit MyException(const std::string& message);
    MyException(const std::string& source, const std::string& message);
    MyException(int code, const std::string& source, const std::string& message);
    const char *what() const throw();

private:
    // helper function
    static std::string make_message(int code, const std::string& source, const std::string& message);
    std::string message;
};


MyException::MyException() :
MyException(0, "No source.", "No message.") {}

MyException::MyException(const std::string& message) :
MyException(0, "No source.", std::move(message)) {}

MyException::MyException(const std::string& source, const std::string& message) :
MyException(0, std::move(source), std::move(message)) {}

MyException::MyException(int code, const std::string& source, const std::string& message) :
message(make_message(code, source, message))
{}

const char *MyException::what() const throw()
{
    // message is a class member, not a temporary
    return message.c_str();
}

std::string MyException::make_message(int code, const std::string& source, const std::string& message)
{
    std::stringstream s;
    s << "MyException Data:" << std::endl;
    s << "Code    : " << code << std::endl;
    s << "Source  : " << source << std::endl;
    s << "Message : " << message << std::endl;

    // takes a copy, returns a copy - safe!
    return s.str();
}

此外,当你重新投掷时,不要这样做:

catch (const std::exception &exc)
{
    std::cerr << "Exception detected:" << std::endl;
    std::cerr << exc.what();
    throw exc; // <--- this is bad - you're potentially slicing!
}

改为:

catch (const std::exception &exc)
{
    std::cerr << "Exception detected:" << std::endl;
    std::cerr << exc.what();
    throw;     // <--- ok, compiler will now rethrow the complete object
}

Answer 2

按价值投掷:

void test()
{
    throw MyException("test", "This is a test");
}

从技术上讲,您可以new通过指针捕获'ed异常,但
不要这样做:

catch (const std::exception* exc) // bad practice

有关更多详细信息,请参阅我应该扔什么/捕获？

要么

Alexandrescu/Sutter,C++编码标准:101规则......,规则73:

按价值投掷,以参考方式捕获