主题：用g++编译的c++程序怎样引用外部类的static 变量？ -- 金口玉言

C++ How to Program, 5th edition, page 98, Section 3.8.

" How Header Files Are Located

Notice that the name of the GradeBook.h header file in line 7 of Fig. 3.10 is enclosed in quotes (" ") rather than angle brackets (< >). Normally, a program's source-code files and user-defined header files are placed in the same directory. When the preprocessor encounters a header file name in quotes (e.g., "GradeBook.h"), the preprocessor attempts to locate the header file in the same directory as the file in which the #include directive appears. If the preprocessor cannot find the header file in that directory, it searches for it in the same location(s) as the C++ Standard Library header files. When the preprocessor encounters a header file name in angle brackets (e.g., <iostream>), it assumes that the header is part of the C++ Standard Library and does not look in the directory of the program that is being preprocessed. "

"Error-Prevention Tip 3.3

To ensure that the preprocessor can locate header files correctly, #include preprocessor directives should place the names of user-defined header files in quotes (e.g., "GradeBook.h") and place the names of C++ Standard Library header files in angle brackets (e.g., <iostream>)."