[linking][weak symbol] link resource

https://en.wikipedia.org/wiki/Weak_symbol

http://ofekshilon.com/2014/02/10/linker-weak-symbols/

http://stackoverflow.com/questions/8460327/why-are-anonymous-namespaces-not-a-sufficient-replacement-for-namespace-static

http://stackoverflow.com/questions/4181059/linkage-of-symbols-within-anonymous-namespace-within-a-regular-namespace