How to throw a C exception

Question

I have a very poor understanding of exception handling i e   how to customize throw  try  catch statements for my own purposes    For example  I have defined a function as follows  int compare int a  int b        I d like the function to throw an exception with some message when either a or b is negative   How should I approach this in the definition of the function

User · Answer

Wanted to ADD to the other answers described here an additional note  in the case of custom exceptions    In the case where you create your own custom exception  that derives from std  exception  when you catch  all possible  exceptions types  you should always start the catch clauses with the  most derived  exception type that may be caught  See the example  of what NOT to do     include  lt iostream gt   include  lt string gt   using namespace std   class MyException   public exception   public      MyException const string amp  msg    m msg msg                cout  lt  lt   MyException  MyException - set m msg to    lt  lt  m msg  lt  lt  endl             MyException                cout  lt  lt   MyException   MyException   lt  lt  endl           virtual const char  what   const throw                  cout  lt  lt   MyException - what   lt  lt  endl          return m msg c str             const string m msg      void throwDerivedException         cout  lt  lt   throwDerivedException - thrown a derived exception   lt  lt  endl      string execptionMessage  MyException thrown        throw  MyException execptionMessage       void illustrateDerivedExceptionCatch         cout  lt  lt   illustrateDerivedExceptionsCatch - start   lt  lt  endl      try                throwDerivedException              catch  const exception amp  e                cout  lt  lt   illustrateDerivedExceptionsCatch - caught an std  exception  e what    lt  lt  e what    lt  lt  endl             some additional code due to the fact that std  exception was thrown              catch const MyException amp  e                cout  lt  lt   illustrateDerivedExceptionsCatch - caught an MyException  e what     lt  lt  e what    lt  lt  endl             some additional code due to the fact that MyException was thrown               cout  lt  lt   illustrateDerivedExceptionsCatch - end   lt  lt  endl     int main int argc  char   argv        cout  lt  lt   main - start   lt  lt  endl      illustrateDerivedExceptionCatch        cout  lt  lt   main - end   lt  lt  endl      return 0      NOTE   0  The proper order should be vice-versa  i e - first you catch  const MyException amp  e  which is followed by catch  const std  exception amp  e    1  As you can see  when you run the program as is  the first catch clause will be executed  which is probably what you did NOT wanted in the first place    2  Even though the type caught in the first catch clause is of type std  exception  the  proper  version of what   will be called - cause it is caught by reference  change at least the caught argument std  exception type to be by value - and you will experience the  object slicing  phenomena in action    3  In case that the  some code due to the fact that  XXX exception was thrown     does important stuff WITH RESPECT to the exception type  there is misbehavior of your code here   4  This is also relevant if the caught objects were  normal  object like  class Base    and class Derived   public Base        5  g   7 3 0 on Ubuntu 18 04 1 produces a warning that indicates the mentioned issue      In function    void illustrateDerivedExceptionCatch         item12Linux cpp 48 2  warning  exception of type    MyException    will be caught     catch const MyException amp  e                 item12Linux cpp 43 2  warning     by earlier handler for    std  exception        catch  const exception amp  e              Again  I will say  that this answer is only to ADD to the other answers described here  I thought this point is worth mention  yet could not depict it within a comment

User · Answer

Though this question is rather old and has already been answered  I just want to add a note on how to do proper exception handling in C  11   Use std  nested exception and std  throw with nested  It is described on StackOverflow here and here  how you can get a backtrace on your exceptions inside your code without need for a debugger or cumbersome logging  by simply writing a proper exception handler which will rethrow nested exceptions   Since you can do this with any derived exception class  you can add a lot of information to such a backtrace  You may also take a look at my MWE on GitHub  where a backtrace would look something like this   Library API  Exception caught in function  api function  Backtrace    Git mwe-cpp-exception src detail Library cpp 17   library function failed   Git mwe-cpp-exception src detail Library cpp 13   could not open file  nonexistent txt

User · Answer

You could define a message to throw when a certain error occurs    throw std  invalid argument   received negative value       or you could define it like this    std  runtime error greatScott  Great Scott               double getEnergySync int year                                if  year    1955    year    1885  throw greatScott       return 1 21e9                                                                                                   Typically  you would have a try     catch block like this   try      do something that causes an exception  catch  std  exception amp  e   std  cerr  lt  lt   exception     lt  lt  e what    lt  lt  std  endl

User · Answer

Simple    include  lt stdexcept gt   int compare  int a  int b         if   a  lt  0    b  lt  0             throw std  invalid argument   received negative value               The Standard Library comes with a nice collection of built-in exception objects you can throw   Keep in mind that you should always throw by value and catch by reference   try       compare  -1  3      catch  const std  invalid argument amp  e            do stuff with exception         You can have multiple catch   statements after each try  so you can handle different exception types separately if you want     You can also re-throw exceptions   catch  const std  invalid argument amp  e            do something         let someone higher up the call stack handle it if they want     throw      And to catch exceptions regardless of type   catch

User · Answer

Just add throw where needed  and try block to the caller that handles the error  By convention you should only throw things that derive from std  exception  so include  lt stdexcept gt  first   int compare int a  int b        if  a  lt  0    b  lt  0            throw std  invalid argument  a or b negative             void foo         try           compare -1  0         catch  const std  invalid argument amp  e                             Also  look into Boost Exception

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