llvm dev - Jul 2015 - [LLVMdev] StringMap question

Hello everyone!

I'm a newcomer for the great LLVM project. I've started to explore the
source code of LLVM project to become more familiar with it, and I've found
some strange usage of move semantics in constructor of StringMapImpl(
StringMapImpl &&RHS) {...} class in include/llvm/ADT/StringMap.h line
56.
Could anyone explain me the purpose of zeroing of all fields of RHS in the
body of this constructor if the declaration of the constructor uses move
semantics? Here is the consturctor code below:

*StringMapImpl(StringMapImpl &&RHS)*
*      : TheTable(RHS.TheTable), NumBuckets(RHS.NumBuckets),*
*        NumItems(RHS.NumItems), NumTombstones(RHS.NumTombstones),*
*        ItemSize(RHS.ItemSize) {*
*    RHS.TheTable = nullptr;*
*    RHS.NumBuckets = 0;*
*    RHS.NumItems = 0;*
*    RHS.NumTombstones = 0;*
*  }*

Thank you very much!

BR,
Valeriy
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On Sat, Jul 11, 2015 at 5:41 AM, Valery Pushkar <pollnossa at gmail.com>
wrote:
> Hello everyone!
>
> I'm a newcomer for the great LLVM project. I've started to explore
the
> source code of LLVM project to become more familiar with it, and I've
found
> some strange usage of move semantics in constructor of
> StringMapImpl(StringMapImpl &&RHS) {...} class in
> include/llvm/ADT/StringMap.h line 56. Could anyone explain me the purpose
of
> zeroing of all fields of RHS in the body of this constructor if the
> declaration of the constructor uses move semantics? Here is the consturctor
> code below:
I'm not a C++ expert, but this looks like a basic
move-constructor'ism.  We know that RHS is not going to be used after
the move constructor has run, so we steal the memory (TheTable and
related metadata) into *this.  Zeroing out the fields ensures that
when the destructor of RHS runs, it won't try to delete memory that
*this will use from now on.

Does that answer your question?

-- Sanjoy

> On 2015-Jul-11, at 12:03, Sanjoy Das <sanjoy at
playingwithpointers.com> wrote:
> 
> On Sat, Jul 11, 2015 at 5:41 AM, Valery Pushkar <pollnossa at
gmail.com> wrote:
>> Hello everyone!
>> 
>> I'm a newcomer for the great LLVM project. I've started to
explore the
>> source code of LLVM project to become more familiar with it, and
I've found
>> some strange usage of move semantics in constructor of
>> StringMapImpl(StringMapImpl &&RHS) {...} class in
>> include/llvm/ADT/StringMap.h line 56. Could anyone explain me the
purpose of
>> zeroing of all fields of RHS in the body of this constructor if the
>> declaration of the constructor uses move semantics? Here is the
consturctor
>> code below:
> 
> I'm not a C++ expert, but this looks like a basic
> move-constructor'ism.  We know that RHS is not going to be used after
> the move constructor has run, so we steal the memory (TheTable and
> related metadata) into *this.  Zeroing out the fields ensures that
> when the destructor of RHS runs, it won't try to delete memory that
> *this will use from now on.
> 
> Does that answer your question?
> 
> -- Sanjoy
Just to add to this, the contract of move constructors/assignment is
that, after copying/moving resources, the original should be left in
a consistent state.  If we didn't zero out `RHS`'s fields, then when
its destructor eventually ran, it'd delete the contents of `this`
since it would still be pointing there.