[LLVMdev] StringMap question

[Duncan P. N. Exon Smith]

> 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.