From: Martin Hansson
Date: December 15 2010 12:38pm
Subject: Re: bzr commit into mysql-trunk-bugfixing branch (martin.hansson:3260)
 Bug#58918
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Tor Didriksen skrev 2010-12-15 12.42:
> On Tue, Dec 14, 2010 at 1:50 PM, Martin Hansson
> <martin.hansson@stripped>wrote:
>
>> #At file:///data0/martin/bzrroot/refactoring-Field_clone/n-mr-o-t/ based on
>> revid:tor.didriksen@stripped
>>
>>   3260 Martin Hansson    2010-12-14
>>       Bug#58918: Add a clone member function to the Field class for better
>>       duplication
>>
>>       The patch removes all copying of Field objects using memcpy() and
>> replaces it
>>       with virtual clone() and clone(MEM_ROOT*) member functions that rely
>> on the
>>       copy constructor. An operator new (MEM_ROOT*) is also implemented. The
>> sole
>>       purpose of the Field::size_of() member function was to support the
>> memcpy and
>>       it is now removed.
>>      @ sql/field.cc
>>         Removal of memcpy() based duplication.
>>      @ sql/field.h
>>         New implementation of clone() member functions.
>>         Removal of size_of() member functions
>>      @ sql/item.cc
>>         Removal of memcpy() based duplication.
>> @@ -84,8 +79,12 @@ class Field
>>    Field(const Item&);                         /* Prevent use of these */
>>    void operator=(Field&);
>>   public:
>> +  /* To do: inherit Sql_alloc and get these for free */
>>
>>
> Why not go all the way :-)
> This works like a charm:
>
> class Field : public Sql_alloc
> {
>    Field(const Item&);                /* Prevent use of these */
>    void operator=(Field&);
> public:
>    uchar        *ptr;            // Position to field in record
> ...
> ...
I am planning to. In case you didn't notice there's a comment about it. 
The reason I don't want to do it is because it would work by 
coincidence, not by design. The class Sql_alloc is -very wrongfully - 
defined in sql_list.h. We slip that include file in sideways by 
including mysqld.h which in turn includes sql_list.h. I plan as my next 
refactoring to create a file sql_alloc.h and then make every class that 
implements an identical 'new' operator (quite a few of those actually) 
inherit Sql_alloc instead. Only then will I have gone all the way :-)

If you insist, of course I can do it!

/Martin