Re: [boost] [mpl] multiset

Subject: Re: [boost] [mpl] multiset
From: Ganesh Prasad (sir.gnsp_at_[hidden])
Date: 2015-02-26 07:33:32

Well, now somehow, I think I need a better solution for this random access
thing. Looks like I am actually polluting the global namespace in this
pursuit. Also macros can break easily on extension.

Best wishes
Ganesh Prasad

On 26 February 2015 at 17:55, Ganesh Prasad <sir.gnsp_at_[hidden]> wrote:

> On 02/25/2015 11:02 AM, pfultz2 wrote:
> > There is no such thing as random access in TMP.
>
>
> But we can surely imitate a Random Access Container in TMP. Just this
> morning it came to my mind that we can make a Compile time random access
> container using templates and preprocessor macros. Here is the code, pretty
> roughly and quickly written.
>
>
>   1  #define STATIC_ASSERT(expr) { char assertion_stage[((expr) ? 0:-1)];
> }
>   2
>
>   3  #define RANDOM_ACCESS_CONTAINER_META(KEY_TYPE, val_type, name) \
>
>   4     template<KEY_TYPE KEY> \
>
>   5     struct meta_container_##name##_{ \
>
>   6         typedef val_type value_type; \
>
>   7         typedef KEY_TYPE key_type; \
>
>   8         static const value_type value = static_cast<value_type>(0); \
>
>   9     }; \
>
>  10     typedef KEY_TYPE meta_container_##name##_key_type_; \
>
>  11     typedef val_type meta_container_##name##_val_type_;
>
>  12
>
>  13  #define RANDOM_ACCESS_INSERT(name, key, val) \
>
>  14     template<> struct meta_container_##name##_<key>{ \
>
>  15         typedef meta_container_##name##_val_type_ value_type; \
>
>  16         typedef meta_container_##name##_key_type_ key_type; \
>
>  17         static const value_type value = val; \
>
>  18     };
>
>  19
>
>  20  #define RANDOM_ACCESS_GET(name, key)
> meta_container_##name##_<key>::value
>  21
>
>  22  RANDOM_ACCESS_CONTAINER_META(int, int, TMP_INT_MAP);
>
>  23
>
>  24  RANDOM_ACCESS_INSERT(TMP_INT_MAP, 5, 15);
>
>  25  RANDOM_ACCESS_INSERT(TMP_INT_MAP, 6, 18);
>
>  26  RANDOM_ACCESS_INSERT(TMP_INT_MAP, 7, 21);
>
>  27
>
>  28
>
>  29  int main(){
>
>  30
>
>  31     STATIC_ASSERT ( RANDOM_ACCESS_GET(TMP_INT_MAP, 5) == 15 );
>
>  32     STATIC_ASSERT ( RANDOM_ACCESS_GET(TMP_INT_MAP, 6) == 18 );
>
>  33     STATIC_ASSERT ( RANDOM_ACCESS_GET(TMP_INT_MAP, 7) == 21 );
>
>  34     STATIC_ASSERT ( RANDOM_ACCESS_GET(TMP_INT_MAP, 1) == 0 );
>
>  35
>
>  36     return 0;
>
>  37  }
>
>
> I might be wrong and this might not be the general case, but it's
> definitely possible to imitate. After all TMP is turing complete, hence,
> random access containers should not be non-existent.
>
>
> Best wishes
> Ganesh Prasad
>
>
>
> On 26 February 2015 at 05:20, Larry Evans <cppljevans_at_[hidden]>
> wrote:
>
>> On 02/25/2015 11:02 AM, pfultz2 wrote:
>> >> IMO the fact it mimics the standard library is very positive, for it
>> makes
>> >> transition much easier for someone used to the STL first experimenting
>> >> with
>> >> metaprogramming.
>> >
>> > Actually, that is what has helped me when I started using MPL. However,
>> > iterator-based approach for metaprogramming has a huge impact on
>> > performance.
>> > There is no such thing as random access in TMP.
>>
>> Not now, but maybe in the future:
>>
>> http://atpp.irrequietus.eu/atpp-c79f4b7.pdf
>>
>> -regards,
>> Larry
>>
>>
>>
>>
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>
>