Proto for Boost mailing list page: [proto] The proper way to compose function returning expressions

Subject: [proto] The proper way to compose function returning expressions
From: Joel Falcou (joel.falcou_at_[hidden])
Date: 2012-04-23 16:01:43

Let's say we have a bunch of functions like sum and sqr defined on a 
proto domain to return
expression of tag sum_ and sqr_ in this domain. One day we want to make 
a norm2(x) function
which is basically sum(sqr(x)).

My feeling is that I should be able to write it using sqr and sum 
expressions.
Alas it seems this results in dandling reference, crash and some sad pandas.

Then I remember about proto::deep_copy but I have a worries. x is 
usually a terminal
holding a huge matrix like value and I just don't want this huge matrix 
to be copied.

What's the correct way to handle such a problem ? How can I build new 
function returning
expressions built from expression composition without incurring a huge 
amount of copy ?