Exception Handling Differences
The major difference between structured exception handling and C++ exception handling is that the C++ exception handling model deals in types, while the C structured exception handling model deals with exceptions of one type — specifically, unsigned int. That is, C exceptions are identified by an unsigned integer value, whereas C++ exceptions are identified by data type. When an exception is raised in C, each possible handler executes a filter that examines the C exception context and determines whether to accept the exception, pass it to some other handler, or ignore it. When an exception is thrown in C++, it may be of any type.
A second difference is that the C structured exception handling model is referred to as “asynchronous” in that exceptions occur secondary to the normal flow of control. The C++ exception handling mechanism is fully “synchronous,” which means that exceptions occur only when they are thrown.
If a C exception is raised in a C++ program, it can be handled by a structured exception handler with its associated filter or by a C++ catch handler, whichever is dynamically nearer to the exception context. For example, the following C++ program raises a C exception inside a C++ try context:
#include <iostream.h>
void SEHFunc( void );
int main()
{
try
{
SEHFunc();
}
catch( ... )
{
cout << "Caught a C exception."<< endl;
}
return 0;
}
void SEHFunc()
{
__try
{
int x, y = 0;
x = 5 / y;
}
__finally
{
cout << "In finally." << endl;
}
}
This is the output from the preceding example:
In finally.
Caught a C exception.
C Exception Wrapper Class
In a simple example like the above, the C exception can be caught only by an ellipsis (...) catch handler. No information about the type or nature of the exception is communicated to the handler. While this method works, in some cases you may need to define a transformation between the two exception handling models so that each C exception is associated with a specific class. To do this, you can define a C exception "wrapper" class, which can be used or derived from in order to attribute a specific class type to a C exception. By doing so, each C exception can be handled by a C++ catch handler more separately than in the preceding example.
Your wrapper class might have an interface consisting of some member functions that determine the value of the exception, and that access the extended exception context information provided by the C exception model. You might also want to define a default constructor and a constructor that accepts an unsigned int argument (to provide for the underlying C exception representation), and a bitwise copy constructor. The following is a possible implementation of a C exception wrapper class:
class SE_Exception
{
private:
SE_Exception() {}
SE_Exception( SE_Exception& ) {}
unsigned int nSE;
public:
SE_Exception( unsigned int n ) : nSE( n ) {}
~SE_Exception() {}
unsigned int getSeNumber() { return nSE; }
};
To use this class, you install a custom C exception translation function that is called by the internal exception handling mechanism each time a C exception is thrown. Within your translation function, you can throw any typed exception (perhaps an SE_Exception type, or a class type derived from SE_Exception) that can be caught by an appropriate matching C++ catch handler. The translation function can simply return, which indicates that it did not handle the exception. If the translation function itself raises a C exception, terminate is called.
To specify a custom translation function, call the _set_se_translator function with the name of your translation function as its single argument. The translation function that you write is called once for each function invocation on the stack that has try blocks. There is no default translation function; if you do not specify one by calling _set_se_translator, the C exception can only be caught by an ellipsis catch handler.
For example, the following code installs a custom translation function, then raises a C exception that is wrapped by the SE_Exception class:
#include <stdio.h>
#include <eh.h>
#include <windows.h>
class SE_Exception {
private:
SE_Exception() {}
SE_Exception( SE_Exception& ) {}
unsigned int nSE;
public:
SE_Exception(unsigned int n) : nSE(n) {}
~SE_Exception() {}
unsigned int getSeNumber() { return nSE; }
};
void SEFunc(void);
void trans_func( unsigned, _EXCEPTION_POINTERS*);
int main()
{
_set_se_translator( trans_func );
try
{
SEFunc();
}
catch( SE_Exception e )
{
printf( "Caught a __try exception with SE_Exception.\n" );
printf( "nSE = 0x%x\n", e.getSeNumber() );
}
return 0;
}
void SEFunc()
{
__try
{
int x, y=0;
x = 5 / y;
}
__finally
{
printf( "In finally\n" );
}
}
void trans_func( unsigned int u, _EXCEPTION_POINTERS* pExp )
{
printf( "In trans_func.\n" );
throw SE_Exception( u );
}