Hi there, While trying to use a stateful unary function objects, I stumbled over and old discussion from 2007 [1]. From the current Fusion documentation, I understand that the functor passed to the for_each algorithm must satisfy the Regular Callable Object concept. I presume that regularity, which I understand as replacing inputs with equal objects results in equal outputs, here implies that operator()(T& x) must be const. In other words, mutable function objects are not supported. As pointed out in [1], it is of course possible to bypass this restriction by defining the state that needs to changed as mutable. I was wondering if this is still the correct way to deal with mutable function objects, as illustrated in the small example below: #include <iostream> #include <string> #include <boost/bind.hpp> #include <boost/fusion/include/algorithm.hpp> #include <boost/fusion/include/container.hpp> struct container { struct printer { template <typename T> void operator()(T& x) const { std::cout << ++i << ": " << x << std::endl; } mutable unsigned i; }; template <typename T> void write(const T& row) { boost::fusion::for_each(row, printer()); } }; int main() { int i = 42; unsigned j = 7; std::string s = "foo"; container c; c.write(boost::fusion::vector_tie(i, j, s)); return 0; } Matthias [1] http://lists.boost.org/boost-users/2007/03/26338.php -- Matthias Vallentin vallentin@icsi.berkeley.edu http://www.icir.org/matthias