Determining exception type after the exception is

问题:

Is there a way to determine the exception type even know you caught the exception with a catch all?

Example:

try
{
   SomeBigFunction();
}
catch(...)
{
   //Determine exception type here
}

回答1:

You can actully determine type inside a catch(...), but it is not very useful:

#include <iostream>
#include <exception>

    class E1 : public std::exception {};
    class E2 : public std::exception {};

    int main() {
        try {
            throw E2();
        }
        catch( ... ) {
            try {
                throw;
            }
            catch( const E1 & e ) {
                std::cout << "E1\n";
            }
            catch( const E2 & e ) {
                std::cout << "E2\n";
            }
        }
    }

回答2:

Short Answer: No.

Long Answer:

If you derive all your exceptions from a common base type (say std::exception) and catch this explicitly then you can use this to get type information from your exception.

But you should be using the feature of catch to catch as specific type of exception and then working from there.

The only real use for catch(...) is:

• Catch: and throw away exception (stop exception escaping destructor).
• Catch: Log an unknwon exception happend and re-throw.

Edited: You can extract type information via dynamic_cast<>() or via typid() Though as stated above this is not somthing I recomend. Use the case statements.

#include <stdexcept>
#include <iostream>

class X: public std::runtime_error  // I use runtime_error a lot
{                                   // its derived from std::exception
    public:                         // And has an implementation of what()
        X(std::string const& msg):
            runtime_error(msg)
        {}
};

int main()
{
    try
    {
        throw X("Test");
    }
    catch(std::exception const& e)
    {
        std::cout << "Message: " << e.what() << "\n";

        /*
         * Note this is platform/compiler specific
         * Your milage may very
         */
        std::cout << "Type:    " << typeid(e).name() << "\n";
    }
}

回答3:

There is no standard, portable way to do this. Here's a non-portable way to do it on GCC and clang

#include <iostream>
#include <cxxabi.h>

const char* currentExceptionTypeName()
{
    int status;
    return abi::__cxa_demangle(abi::__cxa_current_exception_type()->name(), 0, 0, &status);
}

int main()
{
    try {
        throw std::string();
    } catch (...) {
        std::cout<<"Type of caught exception is "<<currentExceptionTypeName()<<std::endl;
    }

    return 0;
}

Output:

Type of caught exception is std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >

回答4:

If you need to handle exceptions differently based on what they are, you should be catching specific exceptions. If there are groups of exceptions that all need to be handled identically, deriving them from a common base class and catching the base class would be the way to go. Leverage the power and paradigms of the language, don't fight against them!

回答5:

No.

Doing so would at the very least require you to be able to access the current exception. I do not believe there is a standard way of doing this.

Once you had the exception instance, you would have to use a type inspection algorithm. C++ doesn't have inherent support for this. At best you would have to have a big if/elseif statement with dynamic_cast's to check the type.

回答6:

I've tried various ways; this works for me:

Begin by subclassing runtime_error :

/*----------------------------------------------------------------------*/    
/* subclass runtime_error for safe exceptions in try/throw/catch        */

 #include <stdexcept>
/* a little preprocessor magic here -- makes a subclass of runtime_error*/

#define NEWERROR( NE )  class NE  : public runtime_error {              \
        public:  NE ( string const& error ) : runtime_error(error) {}  }


NEWERROR( FileError      );
NEWERROR( NetworkError   );
NEWERROR( StringError    );
NEWERROR( CofeeError     );

/*----------------------------------------------------------------------*/

Then you may create some instances of your exceptions.

/*----------------------------------------------------------------------*/
/* some example pre-defined exceptions  */

FileError     ReadOnly                ( "ReadOnly"             );
FileError     FileNotFound            ( "FileNotFound"         );
NetworkError  TimeOutExceeded         ( "TimeOutExceeded"      );
NetworkError  HostNotFound            ( "HostNotFound"         );
CoffeeError   OutOfCoffee             ( "OutOfCoffee"          );

/*----------------------------------------------------------------------*/

Explicitly notify the compiler that your function may throw an exception or the program will probably terminate at the point thrown, and data could be lost or corrupted if resources are in use at the time.

"Make sure you can and do catch anything that you can throw."

(I use the generic runtime_error because throwing and catching it covers all of my exceptions plus the systems' ones as well.)

/*----------------------------------------------------------------------*/
/* example function that may throw an exception */

#include <fstream>

ifstream& getFileStream (string fname) throw (runtime_error)
 {

    if ( fname == "" ) 
      throw StringError( "<getFileStream> fname:empty string" );
      // processing stops here if thrown

    try 
      {
       ifstream Inputfstream;  

       ifstream& ifsref = Inputfstream;

       // ifstream has its own <legacy> exception
       // mechanisms and procedures 
       ifsref.exceptions ( ifstream::failbit | ifstream::badbit );

       ifsref.open (fname , ifstream::in);  // could fail ==> ifstream::failure exception
      }
    catch (ifstream::failure e) 
      {
       throw FileError( fname + string(e.what() ) ); 
      }

    return ifsref;
 }

/*----------------------------------------------------------------------*/

then in your try/catch

/*----------------------------------------------------------------------*/
catch (FileNotFound fnf) //catch a specific error
 {
  if (DEBUG) cerr << "[File Not Found Error: " << fnf.what() << "]" << endl;
  ... (handle it) ... 
 }  
catch (FileError fe) //catch a specific type
 {
  if (DEBUG) cerr << "[File Error: " << fe.what() << "]" << endl;
  ... (handle it) ... 
 }  
catch (runtime_error re ) // catch a generic type
 {
   if (DEBUG) cerr << "[Runtime error: " << re.what() << "]" << endl;          

    // determine type by string comparison
   if ( re.what() == string("ResourceNotavailable") )  ...
   if ( re.what() == string("NetWorkError")         )  ...   

  ...

}
catch ( ... )  // catch everything else 
 { ... exit, rethrow, or ignore ... }

/*----------------------------------------------------------------------*/

The runtime-error class has good support in the c++ standard libraries, and compilers know about it internally, and how to optimize memory and dispatch, so you can use them over different code bases safely and confidently. The code is portable and compatible with many different compilers and architectures.

It may be preferable and somewhat faster to catch each error separately in a catch clause, from more specific to more generic,if you feel a series of string matches is a terrible waste of cpu and memory (the compiler optimizes these though ).

<stdexcept> gives you several kinds of exceptions in 2 groups:

• Logic errors:

  logic_error
  domain_error
  invalid_argument
  length_error
  out_of_range
  

• Runtime errors:

  runtime_error
  range_error
  overflow_error
  underflow_error
  

usage syntax is slightly different for some of them.

Conventional Wisdom in C++ says that your exceptions should be relatively "flat", meaning that large hierarchies of specific categories of exceptions should be eschewed in favor of short generic but informative ones for general programming tasks. Domain specific tasks like network system logic, higher maths, etc. may benefit from specificity, but that can be achieved handily by making intelligent error strings with generic runtime/logic exceptions.

Lastly, My Point is: You can achieve all of this by throwing and catching only runtime_error.

You don't have to create a whole trick-bag of highly specific exceptions (like java does) for each class, each handling one specific error.

回答7:

This question was asked some time ago and I'm offering this answer as a companion to the accepted answer from 9 years ago. I'd have to concur with that respondent that that answer, "... is not very useful." Further, it opens the door to an exception which was once handled being unhandled. To illustrate, let me build upon the respondent's answer

#include <iostream>
#include <exception>

class E1 : public std::exception {};
class E2 : public std::exception {};
class E3 : public std::exception {};

int main() {
    try {
        throw E3();
    }
    catch( ... ) {
        try {
            // OOOPS!!! E3 is now unhandled!!!!!!
            throw;
        }
        catch( const E1 & e ) {
            std::cout << "E1\n";
        }
        catch( const E2 & e ) {
            std::cout << "E2\n";
        }
    }
}

An alternative to this approach would be the following:

#include <iostream>
#include <exception>

class E1 : public std::exception {};
class E2 : public std::exception {};
class E3 : public std::exception {};

int main() {
    try {
        throw E3();
    }
    catch( const E1 & e ) {
        std::cout << "E1\n";
    }
    catch( const E2 & e ) {
        std::cout << "E2\n";
    }
    catch( ... ) {
        std::cout << "Catch-all...";
    }
}

This second approach seems to be tantamount to the first and has the advantage of specifically handling E1 and E2 and then catching everything else. This is offered only as an alternative.

Please note that, according to C++ draft of 2011-02-28, paragraph 15.3, bullet item 5, "If present, a ... handler shall be the last handler for its try block."

回答8:

provided that c++11 available,

bool throwing_func()
{
    // something is wrong...
    throw char('5');

    // ...

    return true;
}

void exception_handler(std::exception_ptr _Eptr)
{
    try 
    {
        if (_Eptr) {std::rethrow_exception(_Eptr);}
    }


    catch(int _Xi)
    {
        std::cout << "int\n";
    }

    catch(char _Xc)
    {
        std::cout << "char\n";
    }

    catch(const std::exception& _Xe)
    {
       std::cout << "std::exception " << _Xe.what() << "\n";
    }

    catch (...)
    {// develop more catch cases above to avoid what follows
        std::cout << "unhandled exception\n";
        // grande problema
    }
}

int main()
{

    try
    {
        throwing_func();
    }

    catch(...)
    {
        //Determine exception type here

         exception_handler(std::current_exception());
    }

    return 0;
}

回答9:

If you're using Visual C++ (managed), you can use the GetType() method to get the type of exception and handle it from there.

E.g.

try
    {
        // Run the application
        Application::Run(mainForm);
    }
    catch (Exception^ e)
    {
        String^ exception_type = e->GetType()->ToString();
        throw;
    }

The string will contain something like "System.ArgumentOutOfRangeException".