未捕获从 std::exception 派生的 C++ 自定义异常

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，但我无法从 catch (const std::exception &exc) 块中捕获我自己的异常。它只被catch (...) 块捕获。

由于MyException 是从std::exception 继承的，我想它会被第一个catch 块捕获...为什么没有发生？

原码链接here

Answer 1

这并不能直接回答问题，但它非常重要

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() 正在返回名为 whatString 的临时字符串中的 c 字符串。

当你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();
}

另外，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!
}

改为这样做：

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

这个：

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

应该是：

throw MyException("test", "This is a test");

否则您需要通过指针来捕捉，这不是标准做法。您当前的 catch by const-reference 是惯用且正确的——您只需要直接抛出异常而不是动态分配。

Answer 3

按值抛出：

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

从技术上讲，您可以通过指针捕获 new'ed 异常，但
不要这样做：

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

---

更多详情请见What should I throw/catch?

或

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

按值抛出，按引用捕获