How to catch this error? [C++]

Question

Hi, I'm trying to catch dividing by zero attempt:

int _tmain(int argc, _TCHAR* argv[])
{
int a = 5;
try
{
int b = a / 0;
}
catch(const exception& e)
{
 cerr << e.what();
}
catch(...)
{
 cerr << "Unknown error.";
}
 cin.get();
return 0;
}

and basically it doesn't work. Any advice why? Thank you. P.S. Any chance that in the future code can be placed between [code][/code] tags instead of four spaces?

Answer 1

Divide by zero does not raise any exceptions in Standard C++, instead it gives you undefined behaviour. Typically, if you want to raise an exception you need to do it yourself:

int divide( int a, int b ) {
   if ( b == 0 ) {
      throw DivideByZero;    // or whatever
   }
   return a / b;
}

Answer 2

The best way would be to check if the divisor equals 0.

C++ doesn't check divide-by-zero. A brief discussion can be found here.

Answer 3

You appear to be currently writing code for Windows (evidence: _tmain and _TCHAR are not portable C++).

If you would like a solution which only works on Windows, then you can use so-called "structured exception handling" (SEH):

• Enable structured exceptions by compiling with /EHa: http://msdn.microsoft.com/en-us/library/1deeycx5.aspx
• Use __try, __except and an exception filter to only catch EXCEPTION_INT_DIVIDE_BY_ZERO: http://msdn.microsoft.com/en-us/library/s58ftw19.aspx, http://msdn.microsoft.com/en-us/library/aa363082.aspx

However, this is a MS-only feature. If you rely on an exception being thrown to deal with this error, then it will take considerable effort to change your code later if you want it to work on other platforms.

It's better to write your code with an awareness of when the divisor might be 0 (just as when you do mathematics, and divide by a quantity, you must consider whether it might be 0). Neil's approach is a good low-impact solution for this: you can use his divide function whenever you aren't sure.