2011/8/31 Vicente J. Botet Escriba <vicente.botet@wanadoo.fr>
Hi,
during the Boost.Conversion review and pre-review it has been argued that as the user must provide specializations of the conversion function for other UDT's, it could promotr ODR violations as two independent libraries can provide the same specialization making them incompatible for an end user.
This issue is present for any library for which the user could provide an overload or could specialize a class. The standard library protects itself from this issue saying that is undefined behavior any specializations of standard classes (except maybe numeric_traits that has a single parameters and it is intendeed to be specialized by the library providing the UDT).
My question is: Should Boost (all the Boost libraries) document explicitly that any class specialization or overload of free fuctions could result in undefined behavior? Is this already implicitly acknowledged?
This problem arises only with functions taking arguments of different UDT types. It's not a problem with operator+ or swap: they logically form the UDT's interface and hence are "owned" by the author of the UDT. It's also not a problem with something like bool parse(const string&, udt*), even though it has two different UDTs as parameters. It is, however, a problem with conversion functions because it's not clear who "owns" them. Can I, as author of class foo, provide conversion from footo bar? What about the reverse? Roman Perepelitsa.