#define is is pre processor directive, 
no memory is allocated and it is not terminated with ";"
const is allocated identifier size of memory in read only portion.

Header file:- 

Header files help in reduction of repetitive code

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Header files cannot have definitions of functions

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A header file may contain declarations of functions, macros


Structure memory allocation
When a structure variable is declared inside a block of function,
memory is allocated for the elements of the structure