我正在用以下代码编写一个自定义异常类:
#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)
块中捕获自己的异常。它只被catch (...)
块捕获。
由于MyException
是从std::exception
继承的,我认为它会被第一个catch
块捕获。。。为什么没有发生这种事?
此处的原始代码链接
这并不能直接回答问题,但它非常重要
该功能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
的临时字符串内的c字符串。
当您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。
这里有一个安全的替代品:
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,当你再次投掷时,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
}
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
的异常,但
不要这样做:
catch (const std::exception* exc) // bad practice
有关更多详细信息,请参阅我应该投掷/接住什么?
或
Alexandrescu/Sutter's,C++编码标准:101规则,规则73:
按值抛出,按参考捕获
这:
throw new MyException("test", "This is a test");
应为:
throw MyException("test", "This is a test");
否则,您需要通过指针进行捕捉,这不是标准做法。您当前的catch
by const引用是惯用的和正确的——您只需要直接抛出异常,而不是动态分配。