Re: [boost] [mpl]suggested addition invault/cppljevans/mpl/child_i_depth_j.zip

From: Larry Evans (cppljevans_at_[hidden])
Date: 2005-08-03 22:28:14

On 08/03/2005 08:51 PM, Jonathan Turkanis wrote:
> Larry Evans wrote:
> 
>>child_i_depth_j allows the traversal of a tree of applications.
>>It's useful for accessing, for example, the i-th superclass of
>>the results of mpl/fold, as demonstrated by the test file:
>>
>>   libs/mpl/test/child_i_depth_j_test.cpp
>>
>>included in the zip file mentioned in the subject line.
>>
>>Could this be added to mpl?  Is there something I could do
>>to facilitate this addition?
> 
> 
> Writing some documentation might help. For instance, what is an application? You

 >>accessing, for example, the i-th superclass of
 >>the results of mpl/fold,

For example, I've used fold to calculate the size of both a tuple and
variant.  In order to access the offset of the j-th element of the
tuple, I can use child_i_depth_j.  I've also used it to access the
j-th acceptor method in a class derived the variant.  For a variant
of n elements, there are n acceptor methods, one for each element.
They are generated also with fold.  Given that the value of the
variant's tag is a run-time value, I need to create a vector
of methods and access the appropriate acceptor method.... Oh it's
getting to complicated to explain. I'll just upload it to the vault.

> say it's defined in application_parts.hpp, but I don't see any definition there.

The comment under child_i_depth_j.zip in:

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

says it *uses* application_parts.  It's defined in child_i_depth_j.hpp.
The only reason I mentioned application_parts was that the last time
I proposed application_parts I didn't have a real good application for
it.  child_i_depth_j is such an application.