Re: [Boost-users] boost::multi_index

Subject: Re: [Boost-users] boost::multi_index
From: Priyank Patel (ppatel_at_[hidden])
Date: 2011-04-07 09:27:00

On 04/07/2011 01:22 AM, joaquin_at_[hidden] wrote:
> ppatel_at_[hidden] escribió:
>    
>> Thanks for your answer Igor. Unfortunately that is out of my control.
>> This is the type of structure we used across multiple  applications. I
>> was checking key extractors and I think that might work but haven't
>> tried out yet.
>>
>>      
> Hello Priyank,
>
> Please don't top-post:
> http://www.boost.org/community/policy.html#quoting
>
> The rest of my answer below.
>
>    

I will keep that in mind from this email and onwards.

>>>> I have a question regarding creating composite key/specifying
>>>> length for
>>>> arrays while using multi_index container based on following
>>>> structure.
>>>>
>>>>
>>>>
>>>> struct TestStruct {
>>>>
>>>>                  char firstKey[50];
>>>>
>>>>                  char secondKeyPart[3];
>>>>
>>>>                  uint32_t thirdKeyPart;
>>>>
>>>>                  ……Some other information….
>>>>
>>>> };
>>>>
>>>>
>>>>
>>>> Do I really read to specify this lengths for character arrays? If
>>>> yes, how
>>>> do I do that while specifying key parts as below?
>>>>
>>>> typedef multi_index_container<
>>>>
>>>> TestStruct,
>>>>
>>>>    indexed_by<
>>>>
>>>>      //non-unique as some subscribers might have more than one number
>>>>
>>>>      hashed_unique<
>>>>
>>>>        composite_key<
>>>>
>>>>          TestStruct,
>>>>
>>>>          member<TestStruct, char*,&TestStruct::firstKeyPart>,
>>>>
>>>>          member<TestStruct, char*,&TestStruct::secondKeyPart>,
>>>>
>>>>          member<TestStruct, uint32_t,&TestStruct::thirdKeyPart>,
>>>>
>>>>        >
>>>>
>>>>      >
>>>>
>>>>    >
>>>>
>>>>
>>>>          
>>>>> TS;
>>>>>
>>>>>            
>>> The above keys will hash the pointer values, which is probably not
>>> what you want.
>>> If you change plain C arrays to std::string, it will work as you
>>> expect.
>>>
>>>        
> Leaving aside the fact that the syntax for member that you've used us
> incorrect (see the
> following example for the proper usage), Igor correctly points out that
> you'd be hashing
> pointers rather than the strings these arrays contain. The way to do
> this is by providing
> user-defined hashers and equality comparers as explained in
> http://www.boost.org/libs/multi_index/doc/tutorial/key_extraction.html#composite_keys_hash
>
> So, in your particular example we can have something like this:
>
>    struct CStrHasher // (C) Ion Gaztañaga
>    {
>       std::size_t operator()(const char *str)const
>       {
>          std::size_t seed = 0;
>          for(;*str;++str)boost::hash_combine(seed,*str);
>          return seed;
>       }
>    };
>
>    struct CStrEqualTo
>    {
>       bool operator()(const char *str1,const char *str2)const
>       {
>         return std::strcmp(str1,str2)==0;
>       }
>    };
>
>    typedef multi_index_container<
>      TestStruct,
>      indexed_by<
>        hashed_unique<
>          composite_key<
>            TestStruct,
>            member<TestStruct,char[50],&TestStruct::firstKeyPart>,
>            member<TestStruct,char[3],&TestStruct::secondKeyPart>,
>            member<TestStruct,uint32_t,&TestStruct::thirdKeyPart>
>          >,
>          composite_key_hash<
>            CStrHasher,
>            CStrHasher,
>            boost::hash<uint32_t>
>          >,
>          composite_key_equal_to<
>            CStrEqualTo,
>            CStrEqualTo,
>            std::equal_to<uint32_t>
>          >
>        >
>      >
>    >  TS;
>
> Hope this helps,
>
>    
Yes. For sure. I will try this out and get back to you in case of any 
questions. Thanks a lot for quick reply.
> Joaquín M López Muñoz
> Telefónica, Investigación y Desarrollo
>
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