Larry Evans wrote:
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.
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