Re: regarding exception_ptr

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2021-09-30 13:35:13

On 9/30/21 4:33 PM, Andrey Semashev wrote:
> On 9/30/21 1:12 PM, rajesh daggumati via Boost wrote:
>> Hi Team,
>> PFB code snippet:
>> try
>>         {
>>             BOOST_THROW_EXCEPTION(std::invalid_argument("Blah"));
>>         }
>>         catch(...)
>>         {
>>                boost::exception_ptr exception = boost::current_exception();
>>                const std::invalid_arguement* p;
>>                 try{ boost :: rethrow_exception(exceptionptr)  }
>>                catch(std::invalid_argument &e)
>>               {
>>                 p = dynamic_cast<const std::invalid_argument*>(&e)
>>                 }
>>                const std::invalid_argument* actualStdPtr =
>> ExceptionHelper::ExtractConstPtr<std::invalid_argument>(exceptionptr);
>>                EXPECT_TRUE(((void*)p) == ((void*)actualStdPtr)) << "Same
>> Object";   //this condition is getting failed.
>>           }}
>>       static const t* ExtractConstPtr(boost::exception_ptr ep)
>>       {
>>           try {
>>                 boost::rethrow_excepton(ep);
>>            }
>>           catch(T &e){
>>                      const T* exceptType = dynamic_cast<const T*>(&e);
>>                       return exceptType;
>>           }
>>           catch (std::exception & e){
>>               const T* exceptType = dynamic_cast<const T*>(&e);
>>                      return exceptType;
>>             }
>>          }
>> //EXPECT_TRUE(((void*)p) == ((void*)actualStdPtr)) -->what will be the
>> reason to get failed as I do same thing in both cases.
> 
> Throwing an exception (which is part of rethrow_excepton) creates a copy
> of the exception (http://eel.is/c++draft/except.throw#3). So the object
> referred to by exception_ptr and the object you receive in the catch
> block are different objects. Same with different catch blocks - every
> time you catch an exception, you catch a new object.

To be more accurate - every time you throw an exception, you catch a new
object. Which is what happens in your example.

> Also, the pointer to the caught exception (i.e. the one you receive in
> the catch block) is dangling as soon as you leave the catch block, as
> the exception is destroyed at this point. It may compare equal between
> different catch blocks by accident, but in general it is invalid to use
> it like you do.
>