Related
I have class A, which has constructor A(string filepath)
class A {
public:
A(string filePath);
private:
A();
}
I need to initialize A, and use it later. Constructor A throws exceptions.
int main() {
A a;
try {
a = A("./my/file/path");
} catch (exception e) {
return 1;
}
// Use a
return 0;
}
What would be the most elegant solution? (I dont want to have something like init function, because it makes it possible to have empty A object.)
The most elegant solution is probably to move all your logic which uses a into a function
int use_a(A &a);
int main() {
try {
A a("./my/file/path");
// if we reach here, no exception was thrown
// hence a is safe to use
return use_a(a);
} catch (exception e) {
// if we reach here a never really existed
return 1;
}
}
You're correct to prefer avoiding two-phase initialization - this way use_a can implicitly rely on a being safe to use.
The most elegant solution is:
int main() {
try {
A a("...");
// use a
} catch (const std::exception& e) {
// log e
return 1;
}
return 0;
}
If you don't want empty A objects consider deleting the default constructor. Note that this makes using some stl containers more difficult, while ensuring no invalid object exists.
In addition to that i think using throw in constructors very much allowed. If you don't want to do that consider the noexcept keyword. Note that throws in destructors can cause problems and are usually avoided.
As alternative, you might turn the exception in "optional/checkable" construction with std::optional or std::unique_ptr:
std::optional<A> MakeA(const std::filesystem::path& path)
{
try {
return A{path};
} catch (std::exception& e) {
return std::nullopt;
}
}
int main() {
auto a = MakeA("./my/file/path");
if (!a) {
return 1;
}
// Use *a
return 0;
}
I read several discussion on the use of exception and error code and when each of them is suitable.
I believe in my case exceptions are better, but I have one specific problem:
if I throw a std::runtime_error("this is an error") and then I catch it, I can not find what was the error, I just understand that there was an error.
What I am looking is an exception that I can throw and add an error code to it so later I can check the error code when I catch it, some thing such as this:
enum errorCodes
{
error_1,
error_2,
error_3
}
try
{
throw std::runtime_error(error_1,"can not do the job!");
}
catch (std::runtime_error & err)
{
switch (err.errorCode)
{
case error_1:
// I can not contunue, so re throw it
rethrow err;
case error_2:
// it is not important for me, so I can continue
break;
case error_3:
//Oh, I need to do something before continue
re_init();
break;
default:
rethrow err;
}
I can write such exception, but before doing this, I am wondering if such type of exception exist in STL or BOOST?
Is there any better way of doing this ( I am not interested to throw different type of exception, so catching different type of exception is not a solution from my point of view.
Yes, there's std::system_error. It's derived from std::runtime_error. It's pretty Unix-land-oriented but it does support error codes in general, and I suggest that you use it that way.
The following code demonstrates how to define one's own error category for application specific error codes:
#include <string>
#include <system_error>
#include <typeinfo>
namespace my{
struct Error_code
{
enum Enum
{
error_1 = 101,
error_2 = 102,
error_3 = 103
};
};
class App_error_category
: public std::error_category
{
using Base = std::error_category;
public:
auto name() const noexcept
-> char const*
override
{ return "App error"; }
auto default_error_condition( int const code ) const noexcept
-> std::error_condition
override
{ (void) code; return {}; }
auto equivalent( int const code, std::error_condition const& condition ) const noexcept
-> bool
override
{ (void) code; (void) condition; return false; }
// The intended functionality of this func is pretty unclear.
// It apparently can't do its job (properly) in the general case.
auto equivalent( std::error_code const& code, int const condition ) const noexcept
-> bool
override
{ return Base::equivalent( code, condition ); }
auto message( int const condition ) const
-> std::string
override
{ return "An application error occurred, code = " + std::to_string( condition ); }
constexpr
App_error_category(): Base{} {}
};
auto app_error_category()
-> App_error_category const&
{
static App_error_category the_instance;
return the_instance;
}
class App_error
: public std::system_error
{
using Base = std::system_error;
public:
auto app_error_code() const
-> Error_code::Enum
{ return static_cast<Error_code::Enum>( code().value() ); }
App_error( Error_code::Enum const code )
: Base{ code, app_error_category() }
{}
App_error( Error_code::Enum const code, std::string const& description )
: Base{ code, app_error_category(), description }
{}
};
} // namespace my
void foo()
{
try
{
throw my::App_error( my::Error_code::error_1, "can not do the job!" );
}
catch( my::App_error const& x )
{
switch( x.app_error_code() )
{
case my::Error_code::error_1:
// I can not contunue, so re throw it
throw;
case my::Error_code::error_2:
// it is not important for me, so I can continue
break;
case my::Error_code::error_3:
//Oh, I need to do something before continue
//re_init();
break;
}
}
}
#include <iostream>
#include <stdlib.h> // EXIT_SUCCESS, EXIT_FAILURE
using namespace std;
auto main()
-> int
{
try
{
foo();
return EXIT_SUCCESS;
}
catch( exception const& x )
{
cerr << "!" << x.what() << endl;
}
return EXIT_FAILURE;
}
With both Visual C++ 2015 and MingW g++ 6.4.0 this produces the output
!can not do the job!: An application error occurred, code = 101
In general it's more practical to define specific exception classes than to use error codes. However, for system error codes it's more practical to just pass these codes up with the exception. And for that the complexity shown above can be avoided, since the std::system_category is then eminently suitable.
In short, the complexity here stems from your requirement to not go with the flow, but in a direction somewhat against the current.
Consider using std::system_error out of the box.
Note that using simply throw; without an argument is a far better way of re-throwing your exception (you seem to be using throw err;) since the former will throw the exception by reference rather than by value: the latter can cause object slicing.
I have a short question. Given a function which returns an object of a class as result, what should I return if there is no result (say because an index is out of range)? I could return a new "empty" object but how can I point out that there was no successful calculation?
I suppose there is a common approach.
The common approach in C++ is either to throw an exception or to use some wrapper like boost::optional.
An exception should be thrown if it is some kind of error, the boost::optional-approach is more appropriate if it is a valid use-case of your function to return an empty result. One example that comes to mind is SQL's NULL. boost::optional turned out quite handy in our codebase.
Going by the philosophy of the vector::at method throw out_of_range exception if possible.
If we are talking about erroneous situation, throwing an exception is proper solution.
#include<exception>
Object * GenerateObject(int i)
{
if (i < 0)
throw std::out_of_range("i");
return new Object(i);
}
int main(int argc, char * argv[])
{
try
{
Object * obj = GenerateObject(-1);
// Succeeded
return 0;
}
catch (std::exception & e)
{
// Failed, exiting with error value
return 1;
}
}
If an empty value is allowed, you can specify a specific value for this class, eg.
class Rectangle
{
private:
int left, top, width, height;
public:
Rectangle(l, t, w, h)
{
left = l;
top = t;
width = w;
height = h;
}
public static Rectangle empty;
}
Rectangle Rectangle::empty = Rectangle(0, 0, -1, -1);
// ...
Rectangle DoSth(int i)
{
// i < 0 is NOT considered an error here
if (i < 0)
return Rectangle::empty;
// Further processing
}
It depends on what the semantics of the operation is.
If an error occurred, you should definitely throw an exception:
#include <stdexcept> // Necessary for standard exceptions
X foo()
{
...
if (/* something goes wrong... */)
{
// There may be a more appropriate exception class. You could
// also derive your own exception class from std::exception...
throw std::logic_error("Whatever!");
}
...
}
...
try
{
X x = foo();
// Work with x...
}
catch (std::logic_error const& e) // Catch what is appropriate...
{
std::cout << e.what();
}
If returning a no-value does not denote an error condition, you could use Boost.Optional. Alternatively, provided you can create an "empty" object of type X, you could think of returning a pair whose second member is a bool flag that tells whether the first member is a valid object or not, as below:
std::pair<X, bool> foo();
...
bool valid;
X x;
std::tie(x, valid) = foo();
if (valid)
{
// Use x...
}
You can throw an exception when values don't match expected results.
A tutorial can be found at http://www.cplusplus.com/doc/tutorial/exceptions
An exception works with a try and catch principle.
A program "tries" to execute code.
If something unexpected happens the executed code "throws" an object, variable or whatever and this will be caught.
In the catch statement you can place code what should happen if the unexpected has happened.
Just follow the tutorial.
You can pair an enumeration with the object type being returned. If the returned enumeration is a certain value, the object is valid, otherwise the object is in an invalid state.
// This is a working C++11 example.
#include <utility>
#include <memory>
enum result
{
ok,
out_of_range,
some_other_error
};
class object
{
public:
object() {}
};
typedef std::shared_ptr< object > object_ptr;
typedef std::pair< result, object_ptr > return_type;
return_type some_function( int index )
{
if ( index > 5 )
{
return return_type{ result::out_of_range, nullptr };
}
return return_type{ result::ok, object_ptr{ new object() } };
}
int main()
{
return_type res = some_function( 10 );
if ( res.first == result::ok )
{
// Do something with res.second
}
else
{
// Handle the error
}
}
I would probably just throw an exception instead.
I'd like to implement an Exception class in C++ that mimics the one from .NET framework (and Java has something similar too), for the following purposes:
Exception chaining: I'd like to implement the concept of "exception translation", when exceptions caught at higher levels wrap and "translate" the lower level exceptions, also preserving these lower-level exceptions somehow (in the InnerException member, in this case). For this, there should be some mechanism to store inner exceptions along with each exception thrown at the upper level. InnerException member provides this in the implementation below.
Exception inheritance: there should be possible to derive IoException from Exception, and SerialPortException from IoException, for example. While this seems trivial, there should be ability to identify the type of caught exceptions dynamically (e.g. for logging purposes, or to display to user), preferably without the overhead of RTTI and typeid.
This is the sample exception handling logic I'd like to make possible:
try
{
try
{
try
{
throw ThirdException(L"this should be ThirdException");
}
catch(Exception &ex)
{
throw SubException(L"this should be SubException", ex);
}
}
catch(Exception &ex)
{
throw SubException(L"this should be SubException again", ex);
}
}
catch(Exception &ex)
{
throw Exception(L"and this should be Exception", ex);
}
and when catching the "outer-most" exception in the upmost layer I'd like to be able to parse and format whole exception chain through the InnerException member, to display something like this:
I've come up with the following implementation so far:
Small note: CString is Microsoft-specific string class (just for the people not familiar with Visual C++ stuff).
class Exception
{
protected:
Exception(const Exception&) {};
Exception& operator= (const Exception&) {};
public:
Exception(const CString &message) : InnerException(0), Message(message) {}
Exception(const CString &message, const Exception &innerException) : InnerException(innerException.Clone()), Message(message) {}
virtual CString GetExceptionName() const { return L"Exception"; }
virtual Exception *Clone() const
{
Exception *ex = new Exception(this->Message);
ex->InnerException = this->InnerException ? this->InnerException->Clone() : 0;
return ex;
}
public:
virtual ~Exception() { if (InnerException) delete InnerException; }
CString Message;
const Exception *InnerException;
};
Now what do we have here. Copy constructor and assignment operator are made protected to prevent copying. Each object will "own" its inner exception object (and delete it in destructor), so default shallow-copying would be unacceptable. Then we have two pretty standard-looking constructors and virtual destructor that deletes the InnerException object. Clone() virtual method is responsible for deep-copying the objects, primarily for storing the inner exception object (see the second constructor). And finally GetExceptionName() virtual method provides the cheap alternative to RTTI for identification of exception class names (I don't think this looks cool but I couldn't come up with better solution; for comparison: in .NET one could simply use someException.GetType().Name).
Now this does the job. But... I don't like this solution for one particular reason: the amount of coding needed for each derived class. Consider I have to derive SubException class, which provides absolutely zero additions to the base class functionality, it just provides the custom name ("SubException", which might be "IoException", "ProjectException", ...) to differentiate it for its usage scenario. I have to provide almost same amount of code for each of such exception class. Here it is:
class SubException : public Exception
{
protected:
SubException(const SubException& source) : Exception(source) {};
SubException& operator= (const SubException&) {};
public:
SubException(const CString &message) : Exception(message) {};
SubException(const CString &message, const Exception &innerException) : Exception(message, innerException) {};
virtual CString GetExceptionName() const { return L"SubException"; }
virtual Exception *Clone() const
{
SubException *ex = new SubException(this->Message);
ex->InnerException = this->InnerException ? this->InnerException->Clone() : 0;
return ex;
}
};
I don't like the fact that I have to provide protected copy constructor and assignment operator each time, I don't like the fact that I have to clone the Clone method each time, duplicating even the code of copying the base members (InnerException...), simply... I don't think this is the elegant solution. But I was unable to think of better one. Do you have any ideas how to implement this concept "properly"? Or maybe this is the best implementation of this concept that is possible in C++? Or maybe I'm doing this completely wrong?
P.S.: I know there exist some mechanisms in C++11 (also in Boost) for this purpose (exception chaining) with some new exception classes, but I'm primarily interested in custom, "old-C++-compatible" ways. But it would be good, in addition, if someone could provide any code in C++11 that accomplishes the same.
C++11 already has nested_exception. There was a talk about exceptions in C++03 and C++11 at Boostcon/C++Next 2012. Videos are on youtube:
http://www.youtube.com/watch?v=N9bR0ztmmEQ&feature=plcp
http://www.youtube.com/watch?v=UiZfODgB-Oc&feature=plcp
There is a lot of extra code, but the good thing is it's really EASY extra code that doesn't change at all from class to class, so it's possible to preprocessor macro it.
#define SUB_EXCEPTION(ClassName, BaseName) \
class ClassName : public BaseName\
{\
protected:\
\
ClassName(const ClassName& source) : BaseName(source) {};\
ClassName& operator= (const ClassName&) {};\
\
public:\
\
ClassName(const CString &message) : BaseName(message) {};\
ClassName(const CString &message, const BaseName &innerException) : BaseName(message, innerException) {};\
\
virtual CString GetExceptionName() const { return L"ClassName"; }\
\
virtual BaseName *Clone() const\
{\
ClassName *ex = new ClassName(this->Message);\
ex->InnerException = this->InnerException ? this->InnerException->Clone() : 0;\
return ex;\
}\
};
Then you can define various utility exceptions by just doing:
SUB_EXCEPTION(IoException, Exception);
SUB_EXCEPTION(SerialPortException, IoException);
Please don't follow boost::exception approach. Boost::exception is for different use case - in particular it's usefull when you want to collect precise exception context scatered over call stack. Consider the following example:
#include "TSTException.hpp"
struct DerivedException: TST::Exception {};
int main() try
{
try
{
try
{
try
{
throw std::runtime_error("initial exception");
}
catch(...)
{
throw TST::Exception("chaining without context info");
}
}
catch(...)
{
TST_THROW("hello world" << '!');
}
}
catch(...)
{
TST_THROW_EX(DerivedException, "another exception");
}
}
catch(const TST::Exception& ex)
{
cout << "diagnostics():\n" << ex;
}
catch(const std::exception& ex)
{
cout << "what(): " << ex.what() << endl;
}
The "exception chaining" solution as I understand it should produce output similar to this:
$ ./test
diagnostics():
Exception: another exception raised from [function: int main() at main.cpp:220]
Exception: hello world! raised from [function: int main() at main.cpp:215]
Exception: chaining without context info raised from [function: unknown_function at unknown_file:0]
Exception: initial exception
As you see there are exceptions chained to each other and diagnostic output contains all exceptions with context information and optional stack trace (not shown here, because it's compiler/platform dependent).
"Exception chaining" can be naturally achieved using new C++11 error handling features (std::current_exception or std::nested_exception). Here is implementation of TSTException.hpp (please bear with more source code):
#include <iostream>
#include <sstream>
#include <stdexcept>
#include <exception>
#include <vector>
#include <string>
#include <memory>
#include <boost/current_function.hpp>
#include <boost/foreach.hpp>
using namespace std;
namespace TST
{
class Exception: virtual public std::exception
{
public:
class Context
{
public:
Context():
file_("unknown_file"),
line_(0),
function_("unknown_function")
{}
Context(const char* file, int line, const char* function):
file_(file? file: "unknown_file"),
line_(line),
function_(function? function: "unknown_function")
{}
const char* file() const { return file_; }
int line() const { return line_; }
const char* function() const { return function_; }
private:
const char* file_;
int line_;
const char* function_;
};
typedef std::vector<std::string> Stacktrace;
//...
Exception()
{
initStacktraceAndNestedException();
}
explicit Exception(const std::string& message, const Context&& context = Context()):
message_(message),
context_(context)
{
message.c_str();
initStacktraceAndNestedException();
}
~Exception() throw() {}
//...
void setContext(const Context& context) { context_ = context; }
void setMessage(const std::string& message) { (message_ = message).c_str(); }
const char* what() const throw () { return message_.c_str(); }
void diagnostics(std::ostream& os) const;
protected:
const Context& context() const { return context_; }
const std::exception_ptr& nested() const { return nested_; }
const std::shared_ptr<Stacktrace>& stacktrace() const { return stacktrace_; }
const std::string& message() const { return message_; }
private:
void initStacktraceAndNestedException();
void printStacktrace(std::ostream& os) const;
std::string message_;
Context context_;
std::shared_ptr<Stacktrace> stacktrace_;
std::exception_ptr nested_;
};
std::ostream& operator<<(std::ostream& os, const Exception& ex)
{
ex.diagnostics(os);
return os;
}
std::ostream& operator<<(std::ostream& os, const Exception::Context& context)
{
return os << "[function: " << context.function()
<< " at " << context.file() << ':' << context.line() << ']';
}
void Exception::diagnostics(std::ostream& os) const
{
os << "Exception: " << what() << " raised from " << context_ << '\n';
if (const bool haveNestedException = nested_ != std::exception_ptr())
{
try
{
std::rethrow_exception(nested_);
}
catch(const TST::Exception& ex)
{
if(stacktrace_ && !ex.stacktrace())//if nested exception doesn't have stacktrace then we print what we have here
printStacktrace(os);
os << ex;
}
catch(const std::exception& ex)
{
if(stacktrace_)
printStacktrace(os);
os << "Exception: " << ex.what() << '\n';
}
catch(...)
{
if(stacktrace_)
printStacktrace(os);
os << "Unknown exception\n";
}
}
else if(stacktrace_)
{
printStacktrace(os);
}
}
void Exception::printStacktrace(std::ostream& os) const
{
if(!stacktrace_)
{
os << "No stack trace\n";
return;
}
os << "Stack trace:";
BOOST_FOREACH(const auto& frame, *stacktrace_)
{
os << '\n' << frame;
}
os << '\n';
}
void Exception::initStacktraceAndNestedException()
{
nested_ = std::current_exception();
if(const bool haveNestedException = nested_ != std::exception_ptr())
{
try
{
throw;
}
catch(const TST::Exception& ex)
{
if(ex.stacktrace())
{
stacktrace_ = ex.stacktrace();
return;
}
}
catch(...) {}
}
/*TODO: setStacktrace(...); */
}
}//namespace TST
#ifdef TST_THROW_EX_WITH_CONTEXT
#error "TST_THROW_EX_WITH_CONTEXT is already defined. Consider changing its name"
#endif /*TST_THROW_EX_WITH_CONTEXT*/
#define TST_THROW_EX_WITH_CONTEXT( \
CTX_FILE, CTX_LINE, CTX_FUNCTION, EXCEPTION, MESSAGE) \
do \
{ \
EXCEPTION newEx; \
{ \
std::ostringstream strm; \
strm << MESSAGE; \
newEx.setMessage(strm.str()); \
} \
newEx.setContext( \
TST::Exception::Context( \
CTX_FILE, CTX_LINE, CTX_FUNCTION)); \
throw newEx; \
} \
while(0)
#ifdef TST_THROW_EX
#error "TST_THROW_EX is already defined. Consider changing its name"
#endif /*TST_THROW_EX*/
#define TST_THROW_EX(EXCEPTION, MESSAGE) \
TST_THROW_EX_WITH_CONTEXT(__FILE__, __LINE__, BOOST_CURRENT_FUNCTION, EXCEPTION, MESSAGE)
#ifdef TST_THROW
#error "TST_THROW is already defined. Consider changing its name"
#endif /*TST_THROW*/
#define TST_THROW(MESSAGE) \
TST_THROW_EX(TST::Exception, MESSAGE)
I use compiler with partial C++11 support (gcc 4.4.7) so you can see some old style peaces of code here. Just for reference you can use the following compilation parameters to build this example (-rdynamic is for stack trace):
g++ main.cpp TSTException.hpp -rdynamic -o test -std=c++0x
Few years ago I wrote this: Unchaining Chained Exceptions in C++
Basically, the exceptions are not nested inside each other, because it would be difficult to catch the original one, but another mechanism keeps track of all the functions visited by the exception while it travels to its catch point.
A revisited version of that can be found in the library Imebra on Bitbucket, here and here.
Now I would rewrite that with some improvements (e.g. use local thread storage to keep the stack trace).
Using this approach allows you to catch the original exception that was thrown, but to still have the stack trace and possibly other information added by the functions visited by the exception while it travels back to the catch statement.
I'm a student in my first C++ programming class, and I'm working on a project where we have to create multiple custom exception classes, and then in one of our event handlers, use a try/catch block to handle them appropriately.
My question is: How do I catch my multiple custom exceptions in my try/catch block? GetMessage() is a custom method in my exception classes that returns the exception explanation as a std::string. Below I've included all the relevant code from my project.
Thanks for your help!
try/catch block
// 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() Method
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!");
}
Just one of my custom exception classes, the others have the same structure as this one
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;
};
If you have multiple exception types, and assuming there's a hierarchy of exceptions (and all derived publicly from some subclass of std::exception,) start from the most specific and continue to more general:
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
}
On the other hand, if you are interested in just the error message - throw same exception, say std::runtime_error with different messages, and then catch that:
try
{
// code throws some subclass of std::exception
}
catch ( const std::exception& e )
{
std::cerr << "ERROR: " << e.what() << std::endl;
}
Also remember - throw by value, catch by [const] reference.
You should create a base exception class and have all of your specific exceptions derive from it:
class BaseException { };
class HourOutOfRangeException : public BaseException { };
class MinuteOutOfRangeException : public BaseException { };
You can then catch all of them in a single catch block:
catch (const BaseException& e) { }
If you want to be able to call GetMessage, you'll need to either:
place that logic into BaseException, or
make GetMessage a virtual member function in BaseException and override it in each of the derived exception classes.
You might also consider having your exceptions derive from one of the standard library exceptions, like std::runtime_error and use the idiomatic what() member function instead of GetMessage().
Another way to solve this problem when you don't have control on the class hierarchy of the exceptions and that you can't duplicate the contents of the catch block is using dynamic_cast Like this:
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
}
When templates can't, macros save the day.
The solution is taken from Boost. It boils to 7 lines of code.
/// #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
Derive all of your exceptions from a common base class BaseException that has a virtual method GetMessage().
Then catch(const BaseException& e).
I had a similar problem today, but it turned out I didn't need my solution to solve my problem. Honestly, I couldn't think of real use cases (logging?), and I didn't find much use for it in my code.
Anyway, this is an approach with type lists (requires C++11). I think the advantage of this approach is that there's no need to have a common base class for custom exceptions (except for std::exception, maybe?). In other words, it is not intrusive to your exception hierarchy.
There might be some subtle errors that I am not aware of.
#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.
}
}
I run into the same problem and here is what I ended up with:
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;
}
In my case the function returns when it doesn't get an exception. So I don't actually have to do anything inside the catch but can do the work outside the try.
#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;
}`