smart pointer - what if constructor throws?

Question

I have a class that connects to a USB device in the constructor. If the device isn't present or some other situation fails then the constructor throws an exception and the calling code deals with it.

Something akin to:

CDevice* pDevice = NULL;
try
{
    pDevice = new CDevice();
}

and so on. I would like to replace this call with an auto_ptr but am not clear how to trap the exception while maintaining the correct scope of the object.

If you're using C++11, `auto_ptr` is deprecated. If C++03, there's still TR1 and Boost. — chris, Apr 18 '13 at 20:07

Captain Obvlious · Accepted Answer · 2013-04-18T20:44:19.207

6

First, I recommend you don't use auto_ptr, it's somewhat broken and has been deprecated in C++11. Prefer either Boost or C++11 SBRM classes like std::unique_ptr. You can do this without much modification to your example.

std::unique_ptr<CDevice> pDevice;
try
{
    pDevice.reset(new CDevice());
}
catch(...)
{
    //....
}

If new or the constructor of CDevice throws then pDevice will remain empty. Using auto_ptr isn't much different, just not recommended given the available alternatives.

std::auto_ptr<CDevice> pDevice;

try
{
    pDevice.reset(new CDevice());

    //pDevice = std::auto_ptr<CDevice>(new CDevice());
    // ^^ historical masochism. 
}
catch(...)
{
    //....
}

edited Apr 18 '13 at 20:44

answered Apr 18 '13 at 20:07

Captain Obvlious

19,754
5
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Seems like I will have an unhandled exception in the first creation of the pointer now. – ethrbunny Apr 18 '13 at 20:09
@ethrbunny: the default constructor is explicitly noexcept, meaning it will never ever throw. – rubenvb Apr 18 '13 at 20:11
Hmm. I'd love to try this but the recommended '-std=c++0x' option isn't recognized on my system. (CentOS 6.2) – ethrbunny Apr 18 '13 at 20:28
1

@ethrbunny I've included an example for `auto_ptr` that should get be what you're looking for. – Captain Obvlious Apr 18 '13 at 20:39
1

Your auto_ptr example could use the `reset()` method as well. – Dave S Apr 18 '13 at 20:40
`std::auto_ptr` is a perfectly good class, just understand what it does and don't misuse it! – Jack Aidley Apr 18 '13 at 20:42
@ethrbunny catches himself abusing `auto_ptr` already. Dang. That didn't take long. – ethrbunny Apr 18 '13 at 20:48

smart pointer - what if constructor throws?

1 Answers1