Re: [boost] [ptr_container] Questionable strong guarantee of ptr_map::insert()

Subject: Re: [boost] [ptr_container] Questionable strong guarantee of ptr_map::insert()
From: Kazutoshi Satoda (k_satoda_at_[hidden])
Date: 2009-08-19 11:59:43

Thanks for the follow ups.

Now I noticed that the problem is not only in ptr_map::insert(). The
same problem seems wide spread in the whole ptr_container library.

I'll make the following 2 tickets in boost trac if nobody objects nor
gives better suggestion.

1. (Type: Bugs)
  False strong guarantee in ptr_container documentation

2. (Type: Feature Requests)
  Provide strong guaranteed overloads in ptr_container

Steven Watanabe wrote:
> David Abrahams wrote:
>> I think the implementation of ptr_map::insert is probably as correct as
>> can be in doing that deletion (i.e. the docs are wrong, not the
>> implementation), but if I had written ptr_map I'd have given it an
>> auto_ptr parameter instead.
> 
> There is an overload that takes an auto_ptr.

Yes, but unfortunately, it doesn't provide strong guarantee, as same as
the raw pointer version.

And also, the current auto_ptr pass-by-value version can be a
error-prone.

An casual user of ptr_map will likely write a code at first like this:

  map.insert(some_key(...), new value(...));

This doesn't compile by design. Then the user will look for the docs to
find alternative ways, find the auto_ptr version, and modify the code to
shut off the strange compile error, like this:

  map.insert(some_key(...), std::auto_ptr<value>(new value));

This does compile, with the risk of subtle memory leaks. But the user
can hardly notice the problem, and would think, "Aha, I can guess, the
use of auto_ptr is the fix for exception-safety problem which the FAQ
mentions. The library taught me that with the error. So nice."

-- 
k_satoda