Regarding arguments to phoenix expressions there are two possibilities. The first is: Arguments are mutable. The current phoenix is implemented to be able to this. Example: int i( 1 ); ( ++phoenix::arg_names::arg1 )( i ); The result of ++phoenix::arg_names::arg1 will be 2, i will have the value 2 after the expression is evaluated. The other approach would follow a more functional programming style as the data will not be mutable int i( 1 ); [1] ( ++phoenix::arg_names::arg1 )( i ); [2] ( ++phoenix::arg_names::arg1 + phoenix::arg_names::arg1 )( i ); The result of [1] will be 2, however i does not get changed (it remains to be 1). The result of [2] will be 3. i still unchanged. This example would be working "out-of-the-box" with the current prototypes. To get the mutability back into phoenix i propose the use of phoenix::ref. I favor the second approach. However, this would break API compatibility between the existing implementation. The question i want to ask the current and prospective users if it feasible to break your existing code? Is there a lot of existing code depending on this behaviour? Does it make sense to break code in such a way? Regards, Thomas