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Note that it is impossible to implement m() to deliver either the basic or strong exception safety guarantees if the behaviour in the presence of exceptions of the methods it calls isn't known. This is particularly relevant when the client of m() (that is l()) supplies the methods to be called either as callbacks or as implementations of virtual member methods. In such cases the only recourse is to document the constraints on them.
If we assume a design with fully encapsulated data then each method need only be held directly responsible for aspects of the object of which it is a member. For the rest, the code in each function must rely on the functions it calls to behave as documented. (We have to rely on documentation in this case, since in both Java and C++ there is no way to express these constraints in the code.)