[英]Catch Multiple Custom Exceptions? - C++
我是我的第一个 C++ 编程课程的学生,我正在从事一个项目,我们必须创建多个自定义异常类,然后在我们的一个事件处理程序中,使用try/catch
块来适当地处理它们。
我的问题是:如何在try/catch
块中捕获多个自定义异常? GetMessage()
是我的异常类中的一个自定义方法,它将异常解释作为std::string
。 下面我已经包含了我项目中的所有相关代码。
谢谢你的帮助!
尝试/捕获块
// This is in one of my event handlers, newEnd is a wxTextCtrl
try {
first.ValidateData();
newEndT = first.ComputeEndTime();
*newEnd << newEndT;
}
catch (// don't know what do to here) {
wxMessageBox(_(e.GetMessage()),
_("Something Went Wrong!"),
wxOK | wxICON_INFORMATION, this);;
}
ValidateData() 方法
void Time::ValidateData()
{
int startHours, startMins, endHours, endMins;
startHours = startTime / MINUTES_TO_HOURS;
startMins = startTime % MINUTES_TO_HOURS;
endHours = endTime / MINUTES_TO_HOURS;
endMins = endTime % MINUTES_TO_HOURS;
if (!(startHours <= HOURS_MAX && startHours >= HOURS_MIN))
throw new HourOutOfRangeException("Beginning Time Hour Out of Range!");
if (!(endHours <= HOURS_MAX && endHours >= HOURS_MIN))
throw new HourOutOfRangeException("Ending Time Hour Out of Range!");
if (!(startMins <= MINUTE_MAX && startMins >= MINUTE_MIN))
throw new MinuteOutOfRangeException("Starting Time Minute Out of Range!");
if (!(endMins <= MINUTE_MAX && endMins >= MINUTE_MIN))
throw new MinuteOutOfRangeException("Ending Time Minute Out of Range!");
if(!(timeDifference <= P_MAX && timeDifference >= P_MIN))
throw new PercentageOutOfRangeException("Percentage Change Out of Range!");
if (!(startTime < endTime))
throw new StartEndException("Start Time Cannot Be Less Than End Time!");
}
只是我的自定义异常类之一,其他与此具有相同的结构
class HourOutOfRangeException
{
public:
// param constructor
// initializes message to passed paramater
// preconditions - param will be a string
// postconditions - message will be initialized
// params a string
// no return type
HourOutOfRangeException(string pMessage) : message(pMessage) {}
// GetMessage is getter for var message
// params none
// preconditions - none
// postconditions - none
// returns string
string GetMessage() { return message; }
// destructor
~HourOutOfRangeException() {}
private:
string message;
};
如果您有多个异常类型,并且假设存在异常层次结构(并且所有异常都是从std::exception
某个子类公开派生的),则从最具体的开始并继续更一般:
try
{
// throws something
}
catch ( const MostSpecificException& e )
{
// handle custom exception
}
catch ( const LessSpecificException& e )
{
// handle custom exception
}
catch ( const std::exception& e )
{
// standard exceptions
}
catch ( ... )
{
// everything else
}
另一方面,如果您只对错误消息感兴趣 - throw
相同的异常,请说std::runtime_error
包含不同的消息,然后catch
:
try
{
// code throws some subclass of std::exception
}
catch ( const std::exception& e )
{
std::cerr << "ERROR: " << e.what() << std::endl;
}
还要记住 - 按值抛出,按[const]引用捕获。
您应该创建一个基本异常类,并从中获取所有特定异常:
class BaseException { };
class HourOutOfRangeException : public BaseException { };
class MinuteOutOfRangeException : public BaseException { };
然后,您可以在一个catch块中捕获所有这些:
catch (const BaseException& e) { }
如果您希望能够调用GetMessage
,则需要:
BaseException
,或 GetMessage
成为BaseException
的虚拟成员函数,并在每个派生的异常类中覆盖它。 您可能还会考虑将异常派生自标准库异常之一,例如std::runtime_error
并使用惯用的what()
成员函数而不是GetMessage()
。
从具有虚方法GetMessage()
的公共基类BaseException
派生所有异常。
然后catch(const BaseException& e)
。
我今天遇到了类似的问题,但事实证明我不需要我的解决方案来解决我的问题。 老实说,我想不出真正的用例(日志?),我在我的代码中没有找到太多用处。
无论如何,这是一种带有类型列表的方法(需要C ++ 11)。 我认为这种方法的优点是不需要为自定义异常提供公共基类(除了std :: exception,可能?)。 换句话说,它不会侵扰您的异常层次结构。
可能会有一些我不知道的微妙错误。
#include <type_traits>
#include <exception>
/// Helper class to handle multiple specific exception types
/// in cases when inheritance based approach would catch exceptions
/// that are not meant to be caught.
///
/// If the body of exception handling code is the same
/// for several exceptions,
/// these exceptions can be joined into one catch.
///
/// Only message data of the caught exception is provided.
///
/// @tparam T Exception types.
/// @tparam Ts At least one more exception type is required.
template <class T, class... Ts>
class MultiCatch;
/// Terminal case that holds the message.
/// ``void`` needs to be given as terminal explicitly.
template <>
class MultiCatch<void> {
protected:
explicit MultiCatch(const char* err_msg) : msg(err_msg) {}
const char* msg;
};
template <class T, class... Ts>
class MultiCatch : public MultiCatch<Ts...> {
static_assert(std::is_base_of<std::exception, T>::value, "Not an exception");
public:
using MultiCatch<Ts...>::MultiCatch;
/// Implicit conversion from the guest exception.
MultiCatch(const T& error) : MultiCatch<Ts...>(error.what()) {} // NOLINT
/// @returns The message of the original exception.
const char* what() const noexcept {
return MultiCatch<void>::msg;
}
};
/// To avoid explicit ``void`` in the type list.
template <class... Ts>
using OneOf = MultiCatch<Ts..., void>;
/// Contrived example.
void foo() {
try {
bar(); // May throw three or more sibling or unrelated exceptions.
} catch (const OneOf<IOError, OutOfMemory>& err) {
log() << "External failure: " << err.what();
throw; // Throw the original exception.
}
}
当模板不能时,宏保存一天。 解决方案来自Boost 。 它可以归结为7行代码。
/// @file multicatch.hpp
#include <boost/preprocessor/variadic/to_list.hpp>
#include <boost/preprocessor/list/for_each.hpp>
/// Callers must define CATCH_BODY(err) to handle the error,
/// they can redefine the CATCH itself, but it is not as convenient.
#define CATCH(R, _, T) \
catch (T & err) { \
CATCH_BODY(err) \
}
/// Generates catches for multiple exception types
/// with the same error handling body.
#define MULTICATCH(...) \
BOOST_PP_LIST_FOR_EACH(CATCH, _, BOOST_PP_VARIADIC_TO_LIST(__VA_ARGS__))
// end of file multicatch.hpp
/// @file app.cc
#include "multicatch.hpp"
// Contrived example.
/// Supply the error handling logic.
#define CATCH_BODY(err) \
log() << "External failure: " << err.what(); \
throw;
void foo() {
try {
bar(); // May throw three or more sibling or unrelated exceptions.
}
MULTICATCH(IOError, OutOfMemory)
}
#undef CATCH_BODY
我遇到了同样的问题,这就是我最终的结果:
std::shared_ptr<MappedImage> MappedImage::get(const std::string & image_dir,
const std::string & name,
const Packet::Checksum & checksum) {
try {
return std::shared_ptr<MappedImage>(images_.at(checksum));
} catch (std::out_of_range) {
} catch (std::bad_weak_ptr) {
}
std::shared_ptr<MappedImage> img =
std::make_shared<MappedImage>(image_dir, name, checksum);
images_[checksum_] = img;
return img;
}
在我的情况下,函数在没有异常时返回。 所以我实际上不需要在catch中做任何事情,但可以在try之外完成工作。
当您无法控制异常的类层次结构并且无法复制 catch 块的内容时,解决此问题的另一种方法是使用dynamic_cast
像这样:
try
{
...
}
catch (std::exception& e)
{
if( nullptr == dynamic_cast<exception_type_1*> (&e)
&& nullptr == dynamic_cast<exception_type_2*> (&e))
{
throw;
}
// here you process the expected exception types
}
#include <iostream>
void test(int x)`
{
try{
if(x==1)
throw (1);
else if(x==2)
throw (2.0);
}
catch(int a)
{
cout<<"It's Integer";
}
catch(double b)
{
cout<<"it's Double";
}
}
int main(){
cout<<" x=1";
test(1);
cout<<"X=2";
test(2.0);
return 0;
}`
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