The way to access global variables in multi-source file mode (extern usage)

One . Example of the problem of global variables in multi file mode

Example

When I want to pass a variable not only in the source file 1 To visit , And I'm still accessing source file 2 , I want to define a header file , Then the source file 1 Contains the header file , Source file 2 This header file is also included , Then define a variable in the header file .
I want to access this variable through File Inclusion , Get the overall effect .
but , The facts are not what people want , There was a mistake .

Two . Use global variables in multi file mode extern Way

To solve such problems , utilize extern How to solve , Multi file global variable problem .
extern, Declare a variable , And then define variables , This variable defines a global scope .

I declare variables in the header file a, utilize extern Statement a, Then in the main function a Define as global variable , At this time a It can be accessed in other files , At this time a No matter in which file , In the case of multiple source files , Other source files to access this a, I just need to include the header file , You can successfully access a

Output results 11

3、 ... and . The convenience benefits obtained – Fewer functions pass more parameters ( Creation and display of linked list )

main.cpp

#include <iostream>
#include " header .h"
using namespace std;

PNode head = (PNode)malloc(sizeof(Node));

int main()
{
    
	head->next = NULL;
	
	Create_Node();
	Display();
	return 0;
}

Function module

#include " header .h"
#include <iostream>
using namespace std;

void Create_Node()
{
    
	int i;
	PNode p = head;
	for (i = 0; i < 3; i++)
	{
    
		PNode q = (PNode)malloc(sizeof(Node));
		q->Node = i;

		q->next = p->next;
		p->next = q;
	}
}
void Display()
{
    
	PNode p = head->next;
	while (p != NULL)
	{
    
		cout << p->Node << " ";
		p = p->next;
	}
}

The header file

#pragma once
typedef struct Node
{
    
	int Node;
	struct Node* next;
}Node, *PNode;

extern PNode head;
void Create_Node();
void Display();

Output results
2 1 0

utilize extern Set the pointer to the head node of the linked list to global , The advantage of this is , When the insertion module of the linked list node is in other source files , If I execute the function of inserting new nodes , Then I have to find the pointer to the header node of the table , Then we can find the pointer of the header and insert , Because the pointer of the head node is global , So I can use it directly through the source file of the function module , There is no need to use the parameters of the function to find the head node of the table , It mainly reduces the number of parameters passed by the function .

About extern Specific other usage , Reference resources ：https://www.cnblogs.com/mch0dm1n/p/5727667.html