include the header file

A header file declares a variable B is as defined in the header file, a file header is not B;

The first reference point B in a header file and then A.CPP not declare an error in the header file references A, otherwise it will be reported.

For file header contains double quotes, first find the file path order:

The current head of the file directory

Include path provided compiler (Compiler can be used to explicitly specify search paths -I)

System Variables CPLUS_INCLUDE_PATH / C_INCLUDE_PATH specified header file path

For file header included angle brackets, to find the path to the file header order:

Include path provided compiler (Compiler can be used to explicitly specify search paths -I)

System Variables CPLUS_INCLUDE_PATH / C_INCLUDE_PATH specified header file path