[boost] [range] [extensions] Default values for range access adaptors?

Subject: [boost] [range] [extensions] Default values for range access adaptors?
From: Viktor Sehr (viktor.sehr_at_[hidden])
Date: 2013-06-05 08:50:34

I'd like to suggest the addition of a default value to the range access
adaptors. Either by adding it as an extra, non-necessary parameter for the
functions, or by duplicating the functions as "front_or_default(T default
value)".

For example it would be useful for finding objects which I am not sure
exists in a range.

std::vector<int> vec = boost::assign::list_of(2)(4)(6);
auto first_odd = vec | filtered(is_odd) | front(); // undefined?
auto first_odd = vec | filtered(is_odd) | front_or_default(-1);  //
everything is fine!

Another option would be a function which returns the adress, or nullptr if
the value does not exist?

/Viktor