NULL不是关键字！

写了一个头文件，忘记包含iostream，结果就出现了错误，说NULL是未定义的标识符。今后要记住，NULL is not a keyword. It's an identifier defined in some standard headers. You can include #include <cstddef> .

iostream里面包含istream和ostream，而istream和ostream都包含ios，cstddef在ios第47行有包含（GNU）：

include <cstdio> // For SEEK_SET, SEEK_CUR, SEEK_END