In c++ why ( # ) is used in header file

Answer 1

The hash symbol ( # ) at first non blank character on a line begin a preprocessor command for the compiler to execute at parsing source code.
Most common usage is for the 'include' command, but you may see others, especially in header files, where could be needed to define some preprocessors variables, or others actions in order to prepare to compil...

Answer 2

Complete #include is the syntax to include header files in ur program.