Boost C++ Libraries

Lazy Evaluation

Unlike MPL, Fusion algorithms are lazy and non sequence-type preserving. What does that mean? It means that when you operate on a sequence through a Fusion algorithm that returns a sequence, the sequence returned may not be of the same class as the original. This is by design. Runtime efficiency is given a high priority. Like MPL, and unlike STL, fusion algorithms are functional in nature such that algorithms are non mutating (no side effects). However, due to the high cost of returning full sequences such as vectors and lists, Views are returned from Fusion algorithms instead. For example, the transform algorithm does not actually return a transformed version of the original sequence. transform returns a transform_view. This view holds a reference to the original sequence plus the transform function. Iteration over the transform_view will apply the transform function over the sequence elements on demand. This lazy evaluation scheme allows us to chain as many algorithms as we want without incurring a high runtime penalty.

Sequence Extension

The lazy evaluation scheme where Algorithms return Views also allows operations such as push_back to be totally generic. In Fusion, push_back is actually a generic algorithm that works on all sequences. Given an input sequence s and a value x, Fusion's push_back algorithm simply returns a joint_view: a view that holds a reference to the original sequence s and the value x. Functions that were once sequence specific and need to be implemented N times over N different sequences are now implemented only once. That is to say that Fusion sequences are cheaply extensible. However, an important caveat is that the result of a sequence extending operation like push_back does not retain the properties of the original sequence such as associativity of set(s). To regain the original sequence, Conversion functions are provided. You may use one of the Conversion functions to convert back to the original sequence type.

Header

#include <boost/fusion/algorithm.hpp>
#include <boost/fusion/include/algorithm.hpp>