deriving from std::exception

Hi everyone,

I want to derive some exception classes from std::exception, which serve as bug-hunting devices with specific messages. But the screen output seems really awkward to me. An example:

Code:

class VectorException: public exception
{
 public:
 VectorException() : message( (string)"Vector: ") {};
 VectorException( string const& mess) : message( (string)"Vector: " + mess) {};
  
  virtual const char* what() const throw()
  {
    return message.c_str();
  }
  
 private:
  string const& message;
};

int main()
{
  try
    {
      v[ 3]; //throws the exception
  catch( exception& e)
    {
      cout<< "Exception running testVector: "<< e.what()<< endl;
    }
      
  return 0;
}

This result in the following screen output:

Code:

Exception running testVector: �d��P�ͷ�l�����d��P�ͷ

Might it be the string-member function 'c_str()' which causes the problem?

> message.c_str();
message goes out of scope, and your c_str() pointer is dangling in mid-air with nothing to support it (perhaps).

Does it work with true string constants which have a lifetime of the entire program?

>> string const& message;
"message" needs to be a real object and not a reference.

>> : message( (string)"Vector: " + mess)
So "message" is a reference to the temp object returned by "string + string". Make "message" a real object so it can store the results of that "string + string".

gg

But why didn't my compiler (g++-4.3) give me any warnings on that? Weird, normally it does.

I modified the code:

Code:

class VectorException: public std::exception
{ // line 6 where error occurs
 public:
 VectorException( std::string& mess) 
   : message( (std::string)"Vector: " + mess) {};
  
  virtual const char* what() const throw()
  {
    return message.c_str();
  }
  
 private:
  std::string message;
};

But now the compiler generates the following message:

Exceptions.h:6: error: looser throw specifier for ‘virtual VectorException::~VectorException()’
/usr/include/c++/4.1.3/exception:58: error: overriding ‘virtual std::exception::~exception() throw ()’
make: *** [Exceptions.o] Error 1

Do I really need to override the (virtual) destructor? It didn't cause problems with the 'const&' .

Here's a good explanation:
http://groups.google.com/group/comp....2764a68e6e419/

gg

Thanks!