The main problem of unnamed namespaces in headers is the possibility of ODR violation. Has anyone seen any compiler complain about this? Not this case specifically, although I've seen complaints about more obvious violations of the ODR, e.g. when you try to link against two versions of the CRT in Visual C++. (Typically because you specify one and a static library you include has another in its dependencies.) Now, I don't know if this is an appropriate topic here, but I would question the usefullness of the ODR itself. The ODR is a logical necessity, not a feature that may be useful or not. If the compiler generates two differing definitions of a symbol, how can
Arkadiy Vertleyb wrote: the linker possibly know which one to use? And if it doesn't know which one to use, how can the program have defined behaviour? A better question would be whether to relax the ODR such that "equivalent" (under specific rules) multiple definitions do not cause the ODR to be violated, similar to what is already done for template instantiations.
My main problem with it -- it contradicts to quite useful, IMO, idiom, where a library author defines a main template in his/her library as a customization point, and the users provide specializations of this template (similar to virtual functions in runtime world).
What does this have to do with the ODR? Sebastian Redl