[boost] Re: Small tuple extension

From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-04-27 11:54:14

On 04/27/2005 11:16 AM, Giovanni P. Deretta wrote:
> ... replying to myself
> 
> Giovanni P. Deretta wrote:
> 
>> Larry Evans wrote:
>>
>>>
>>> But it wouldn't work for repeated types, unless, as you
>>> noted previously, some sort of "tag" were "associated" with
[snip]
> I couldn't parse this paragraph correctly the first time, the second 
> time i *thought* i did get it. The third time i *did* get it, but i had 
> already sent my reply... :)

OK. Sorry for not being clearer :(  Other's in this list have stumbled
over my wording also.

> 
>> I think i failed to expain this, from your next example i think you 
>> thought that my extended_tuple where some sort of mpl-like container.
> 
> 
> It was me that didn't understand. You are right, a map is better.
OK.
> 
>>
>> Well, in my extended tuple example tags are _not_ associated with a 
>> tuple.  Different tuples can reuse the same tag set or part of it.

True, but that's not type safe.  For example:

   enum indices{i0,i1};
   int ivec1[1];
   int ivec2[2];
   ivec2[i1];//OK
   ivec1[i1];//BAD

Although c vectors are used, I think you can see the analogy.  the same
"tag set" is used for both "tuple"'s ivec1 and ivec2; yet, one element
of the tag set, i.e. i1, is not valid for the "tuple" ivec1.

>> This is very useful for generic programming, for example, i may have a 
>> function that accept a tuple having a some_tag slot.  Doing this with 
>> enum is clumsy and not type safe.  I.e. a tuple might not map the 
>> whole enum set enum {a, b, c}, but get<a>(tuple_foo) would still 
>> compile even if 'a' is not logically part of tuple_foo. Also you are 

If you've defined the enum and map together, e.g. in the enum_map<i>
structure, and used them together in enum_mapped, as shown here:

http://boost-sandbox.sourceforge.net/vault/index.php?&direction=0&order=&directory=cppljevans

in tuple_nested_enum_test.cpp, then 'a' (or 'f_0' in the vault file), is
logically part of the tuple in the same way that 'a' is logically part
of:

   struct abc
   { int a; float b; char c;};

and the:

   tuple_mapped<enum_map<i> >::get<index>

for some i in 0..2, will only compile for some index in
enum_map<i>::field_names, just as:

   abc::index

will only compile for some index in a,b,c.

>> forced to use the enum ordering.  Anyway, i don't think you proposed 
>> to reuse the same enums, only the names, but then you are forced to 
>> write enums for every tuple.

True, just as, for every c or c++ struct the programmer is forced to
write names for every member variable.

>> Anyway, I think i will try fusion now :)
>>
> This still stand.
> 

I'd be curious to see how it turns out.