Re: Exception in the constructor can't be caught?

From:

mlimber <mlimber@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 31 Jul 2008 11:49:44 -0700 (PDT)

Message-ID:

<2ecd6cb3-f2d9-4605-aa9d-e8c74fafa556@k13g2000hse.googlegroups.com>

On Jul 31, 1:47 pm, Peter Jansson <webmas...@jansson.net> wrote:

Dear newsgroup,

In the following code, it looks as though the exception thrown in the
constructor is not caught properly. I get this output:

---- standard output ----
Base() constructor.
Exception in Base(): "Going down (B can't be
construced properly)!"
     (the exception has now been dealt with).
main(): We got an exception while constructing B
     that already should have been dealt with?
-------------------------

Here is the code:

---- Code ----
#include <iostream>
#include <stdexcept>
class Base
{
    public:
       Base()
          try
          {
             std::cout << "Base() constructor.\n";
             throw std::runtime_error("Going down (B can't =

be construced "

                   "properly)!");
          }
       catch(const std::exception& ex)
       {
          std::cout << "Exception in Base(): \""
             << ex.what()
                << "\"\n (the exception has now be=

en dealt with).\n";

       }
       virtual ~Base()
       {
          std::cout << "~Base()\n";
       }

};

int main()
{
    try
    {
       Base B;
    }
    catch(const std::exception& ex)
    {
       std::cout << "main(): "
          <<"We got an exception while constructing B\n"
             " that already should have been dealt w=

ith?\n";

}
return 0;}

---- End-of-Code ----

Anybody have an explanation to why the exception looks to have been
re-thrown?

It is caught and rethrown, as mandated by the standard (cf. the GOTW
article mentioned above). If you don't want it to be rethrown, put
your try-catch block *in* the constructor body rather than using a
function-try-block *as* the body:

Base::Base()
{ // Note this brace
   try
   {
     std::cout << "Base() constructor.\n";
     throw std::runtime_error("Going down (B can't be construced "
       "properly)!");
   }
   catch(const std::exception& ex)
   {
     std::cout << "Exception in Base(): \""
               << ex.what()
               << "\"\n (the exception has now been dealt with).
\n";
    }
} // Note this brace

Cheers! --M