Declaration and definition of C + + global variables __c++

1. Declare global variables in Header1.h, header1.cpp define global variables

Header1.h
#ifndef header1_h_
#define HEADER1_H_

extern int g_n;//header file must have extern declaration global variable, no value
//int g_ o; error
//int g_o = 10; error
//extern int g_o; error

#endif

Header1.cpp
#include "header1.h"

int g_n = 100;//definition can never be added extern, do not give G_n assignment is automatically assigned 0//int the g_n;//
is also correct, g_ N is 0

2. Define global variables in HEADER2.HPP

HEADER2.HPP
#ifndef header2_hpp_
#define HEADER2_HPP_

int g_m = 1;//correct, global variable defined in HPP file//int
m;//correct, automatically initialized to 0
//extern int g_m = 1;//correct

//extern int g_m;//error, which is only declared undefined and needs to be int in a CPP file G_m =?; The definition is correct

#endif