[英]C++ Custom exception derived from std::exception not being caught
我正在使用以下代碼編寫自定義異常類:
#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;
}
}
它編譯得很好,但我無法從catch (const std::exception &exc)
塊中捕獲我自己的異常。 它只能被catch (...)
塊catch (...)
。
由於MyException
是從std::exception
繼承的,我認為它會被第一個catch
塊catch
...為什么不發生?
原始代碼鏈接在這里
按價值投擲:
void test()
{
throw MyException("test", "This is a test");
}
從技術上講,你可以通過指針捕獲new
'ed異常,但是
不要這樣做 :
catch (const std::exception* exc) // bad practice
有關更多詳細信息,請參閱我應該扔什么/捕獲?
要么
Alexandrescu / Sutter, C ++編碼標准:101規則...... ,規則73:
按價值投擲,以參考方式捕獲
這並沒有直接回答這個問題,但這非常重要
這個函數是一個等待發生的不安全崩潰:
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()
返回名為whatString
的臨時字符串中的c-string。
編寫這樣的異常類時,必須將完整的錯誤消息存儲在異常中 - 在構造函數中構建它。
這是一個安全的替代品:
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,當你重新投擲時,不要這樣做:
catch (const std::exception &exc)
{
std::cerr << "Exception detected:" << std::endl;
std::cerr << exc.what();
throw exc; // <--- this is bad - you're potentially slicing!
}
改為:
catch (const std::exception &exc)
{
std::cerr << "Exception detected:" << std::endl;
std::cerr << exc.what();
throw; // <--- ok, compiler will now rethrow the complete object
}
這個:
throw new MyException("test", "This is a test");
應該:
throw MyException("test", "This is a test");
否則你需要通過指針捕獲,這不是標准做法。 const-reference的當前catch
是慣用的和正確的 - 你只需要直接拋出異常而不是動態分配。
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