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Subject: Re: [boost] [unique_ptr] So how is it used differently?
From: Michael Marcin (mike.marcin_at_[hidden])
Date: 2008-12-22 22:56:23

Howard Hinnant wrote:
> On Dec 22, 2008, at 9:10 PM, Michael Marcin wrote:
>> Mathias Gaunard wrote:
>>> Sid Sacek wrote:
>>>> 1) When would I choose unique_ptr<> over auto_ptr<> ?
>>> Always.
>>> auto_ptr is deprecated and is dangerous.
>>> unique_ptr, for example, perfectly works with containers.
>> With containers written to support unique_ptr or with any Container as
>> defined by the standard's concept?
> It will work with containers written to support unique_ptr (or move-only
> types). std-defined C++0X containers will support move-only types.
> std-defined C++03 containers do not.
> The basic advance of unique_ptr over auto_ptr is that with unique_ptr if
> things compile, they will work. With auto_ptr things might compile but
> produce run time errors. So if your container<unique_ptr<A>> compiles,
> then you can use it with confidence. But it will likely not compile if
> it hasn't been designed to work with move-only types.
> This state of affairs is not nearly as mysterious as it sounds. I'd
> like to lift the veil a little. The reason auto_ptr is prone to produce
> run time errors is because generic code is often written which looks like:
> template <class T>
> void
> foo(T t)
> {
> T t2 = t;
> assert(t2 == t);
> }
> The generic code may not (probably does not) have the explicit assert as
> shown above. But its logic may assume that when it makes a copy, either
> via construction or assignment, that the source and target are
> equivalent after the copy. When T turns out to be an auto_ptr, then
> this assumption is silently broken, and thus the logic of the generic
> algorithm is broken. When T turns out to be a unique_ptr, the copy
> won't compile. Thus the breakage with unique_ptr happens at compile
> time, whereas the breakage with auto_ptr occurs at run time.
> A generic algorithm designed to work with move-only types will avoid
> logic that demands copies. If it needs to, it might /move/ objects
> around instead of copy them around:
> template <class T>
> void
> foo(T t)
> {
> T t2 = std::move(t);
> // no assumption here on the value of t
> }

Sure makes sense but since we don't yet have a generic implementation of
move to use no generic algorithms can really rely on it, right?

This leaves me to believe the only real uses of unique_ptr right now in
C++03 are

1. a more descriptive name than auto_ptr or scoped_ptr for some use cases
2. custom deleter support

Michael Marcin

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