(原創) 为什么在header file中，要使用fully qualified library name? (C/C++)

The content of a header are copied into our program text by the preprocessor. When we #include a file, it is as if the exact header, it is equivalent to placing the same using declaration in every program that includes the header whether that program wants the using declaration or not. In general, is is good pratice for for headers do define only what is strictly necessary.

Reference
C++ Primer 4th P.80