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Subject: Re: [boost] [interest] underlying type library
From: Julian Gonggrijp (j.gonggrijp_at_[hidden])
Date: 2011-08-24 07:30:22


Dave Abrahams wrote:

> on Tue Aug 23 2011, Julian Gonggrijp <j.gonggrijp-AT-gmail.com> wrote:
>
>> Ion Gaztañaga wrote:
>>
>>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1377.htm
>>
>> That's interesting, but after reading that page I think raw_move is
>> not the same as destructive move. The difference: destructive move
>> acts as a destructor and hence makes the source object unavailable for
>> further use.
>
> Well, technically, it makes the source object's storage usable as raw
> memory.
>
>> In move_raw on the other hand, the source object is left
>> in an invalid but non-destructed state which it can only be rescued
>> from by raw moving another value into it (if it is non-POD).
>
> How is an "invalid but non-destructed state" practically different from
> raw memory? Are you saying I can't default-construct another value of
> the same type in that memory?

I must admit that you do a good job at making these moves look more
similar. :-) And yes, to be fair I must say that you can probably
default-construct a new object of the same type in a raw-moved-from
object.

But no, they are still not really the same. After a destructive move
the source object is in a destructed state (raw memory, as you say),
so in principle you should not need to worry about it anymore; i.e. it
won't be destructed again at the end of scope. You're not really
expected to store a new value of the moved-from type in the object; if
you do want to do so you'll first have to run a constructor again.

The source of a raw move on the other hand is a bomb waiting to
explode, because technically speaking it is still storing a value of
the moved-from type and it *will* be destructed at the end of scope.
Hence you are forced to give it a new, valid value.

It follows that they also have different use cases. A destructive move
seems useful for returning local variables from functions. A raw move
on the other hand can only be used if you are reordering values within
the same scope.

-Julian


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