Thread: try-catch is not working

good evening.

Code:

 
#include <iostream> 
using std::cout; using std::cin; using std::endl;

#include <exception> 
using std::exception; 

int main() 
{ 
   int val1, val2, result; 
   val1 = val2 = result = 0;
   
   cout << "Please enter two values separated by spaces:\n"; 
   while(cin >> val1 >> val2) 
   { 
     try 
     { 
        result = val1 / val2; 
        cout << "Result: " << result << endl;
     } 
     catch(exception err) 
     { 
       cout << "Error: Second value must be higher than 0\n";
       cout << err.what() 
            << "\nTry again? Enter y or n" << endl; 
       char c; 
       cin >> c; 
       if(!cin || c == 'n') 
         break;      
     }              
   }                    
   return 0;
}

Code:

thames@semaht ~/C++/Exceptions $ g++ -g -Wall exc2.cc -std=c++11
thames@semaht ~/C++/Exceptions $ valgrind ./a.out
==4606== Memcheck, a memory error detector
==4606== Copyright (C) 2002-2011, and GNU GPL'd, by Julian Seward et al.
==4606== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==4606== Command: ./a.out
==4606== 
Please enter two values separated by spaces:
5 0
==4606== 
==4606== Process terminating with default action of signal 8 (SIGFPE)
==4606==  Integer divide by zero at address 0x402CB32CD
==4606==    at 0x400E07: main (exc2.cc:17)
==4606== 
==4606== HEAP SUMMARY:
==4606==     in use at exit: 0 bytes in 0 blocks
==4606==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==4606== 
==4606== All heap blocks were freed -- no leaks are possible
==4606== 
==4606== For counts of detected and suppressed errors, rerun with: -v
==4606== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 2 from 2)
Exceção de ponto flutuante

O_o

C++ doesn't really have native numeric exceptions. (Yes, some compilers offer it as an option, but it is not standard.)

Do the exception stuff manually if you want to handle such an error condition with an exception. (Possibly putting the numeric logic into a class.)

However, processing input for validity isn't and shouldn't be part of the logic of values. Do your job, and confirm the validity of input before you start using the input.

Soma

still, it's not wrong to throw an exception there, right? (I was studying the exception chapter)

Code:

 
#include <iostream> 
#include <exception> 

using std::cout; 
using std::cin; 
using std::endl;
using std::exception; 

int main() 
{ 
   double val1, val2, result; 
   val1 = val2 = result = 0;
   
   cout << "Please enter two values separated by spaces:\n"; 
   while(cin >> val1 >> val2) 
   { 
     try 
     { 
        if(val2 == 0) 
        { 
          throw exception();    
        }     
        result = val1 / val2; 
        cout << "Result: " << result << endl;
     } 
     catch(exception err) 
     { 
       cout << "Error: Second value must be higher or lower than 0\n";
       cout << err.what() 
            << "\nTry again? Enter y or n" << endl; 
       char c; 
       cin >> c; 
       if(!cin || c == 'n') 
         break;      
     }              
   }                    
   return 0;
}

Yes, it is wrong in the given context.

Don't get me wrong! There is absolutely nothing wrong with building a function to validate input and using exceptions to signal an error condition within that function. There is absolutely nothing wrong to transform a condition into an exception for the handling of errors when multiple paths may yield the same or similar errors requiring the same handling. (Actually, this second example is a perfect use of exceptions.)

However, here you are transforming a simple condition into an exception only to toss it to yourself locally for immediate processing. You are actually complicating logic because of these pointless steps.

Code:

if(val2)
{
    // do division
}
else
{
    // report error
}

That example is significantly clearer.

If you add multiple values, things start to change almost immediately (confirming the validity of two values) leaning heavily towards using exceptions because the fail branch for every validity test can be isolated into a single part of the function.

[Edit]
To be clear, your logic of confirming the validity of input should usually be part of the input process in any event making the use of exceptions pointless even for a lot of values.
[/Edit]

Using exceptions properly is a balancing act; don't let anybody tell you different.

Soma