Hi, I have 2 fusion containers: a small vector (my_vec) and a bigger set (my_set). I want to overwrite the elements of the set with elements from the vector whenever they have the same type. Sounds simple: struct update { update(my_set_type& to_overwrite_):to_overwrite(to_overwrite_){} template<typename T> void operator()(T& elt) /*const?*/ { boost::fusion::at_key<T>(to_overwrite)=elt; } my_set_type& to_overwrite; }; boost::fusion::for_each(my_vec,update(my_set)); Except that this won't compile because the operator() is required to be const, which is sad, because it was very simple. This leaves me with the unattractive option of inverting the set and the vector inside update. Now, I iterate on the set's elements and overwrite them when found in the vector. This is unattractive because 1) the code gets really ugly 2) it could be slower (the set is bigger) and 3) it has a much bigger compile-time cost (I resorted to an enable_if of fusion::result_of::find and I make it short, it is actually much worse). I thought of using transform, but this will be slower as I will need to transform every element of the set, even for a vector of size 1. Am I missing something? If not, would it be possible to remove this constraint? Thanks, Christophe