Error with catching std::runtime_error as std::exception

Question

we have a funny problem with try catch and std::runtime_error. Can someone explain to me why this is returning "Unknown error" as output ? Thanks very much for helping me !

#include "stdafx.h"
#include <iostream>
#include <stdexcept>

int magicCode()
{
    throw std::runtime_error("FunnyError");
}

int funnyCatch()
{
    try{
        magicCode();
    } catch (std::exception& e) {
        throw e;
    }

}

int _tmain(int argc, _TCHAR* argv[])
{
    try
    {
        funnyCatch();
    }
    catch (std::exception& e)
    {
        std::cout << e.what();
    }
 return 0;
}

It prints "FunnyError" for me, using Visual C++ 2010. Which compiler are you using? — Doug
– Doug, Commented Sep 23, 2010 at 13:36

Lara Bailey · Accepted Answer · 2010-09-23 13:36:30Z

24

The problem is with this line. Because throw with an expression uses the static type of that expression to determine the exception thrown, this slices the exception object constructing a new std::exception object copying only the base object part of the std::runtime_error that e is a reference to.

throw e;

To re-throw the caught exception you should always use throw without an expression.

throw;

answered Sep 23, 2010 at 13:36

Lara Bailey

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7 Comments

Björn Pollex Over a year ago

18 seconds faster ... how does he do it ?!

Doug Over a year ago

Ah, exception's copy constructor doesn't have to preserve what(). I didn't realise that.

BlueTrin Over a year ago

Charles, I thought that throw e; would call the copy constructor std::exception(const std::exception e), which would create a correct copy of the exception ? Is it not the case ?

Lara Bailey Over a year ago

Yes, it uses std::exception's copy constructor. No, this won't necessarily result in a 'correct' copy of the exception as std::exception is only the base class of the exception that e actually refers to.

BlueTrin Over a year ago

Thank you for the complete answer, I have been taught to always rethrow using throw, while investigating an issue, I was curious about why the message returned by what() was not preserved. Thanks to you I know the reason, Charles !

|

Community · Accepted Answer · 2017-05-23 12:23:40Z

1

I found a perfect response to this problem:

C++ makes an explicit distinction between reference and value copy. Use

catch (const std::exception& e)

to catch by reference instead of value.

Go give upvotes for the original responder here

edited May 23, 2017 at 12:23

CommunityBot

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answered Oct 15, 2013 at 11:52

umlimo

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1 Comment

BlueTrin Over a year ago

I didn't catch by value copy.

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