① Function declaration and definition ② Separate file compilation ③ Pointer definition and use ④ Memory space occupied by pointer

Function can be declared multiple times, but can only be defined once

【the reason】:

The declaration is just to tell the compiler that there is this function

If it is defined multiple times, the compiler does not know which one to run

#include <iostream>
using namespace std;

//函数可以声明多次，但是定义只能有一次
//【理由】：
//声明只是为了告诉编译器有这个函数，而如果定义多次，则让编译器不知道运行哪个

//声明：
int score(int num1, int num2);
int score(int num1, int num2);

//定义：
int score(int num1, int num2)
{
    
    
	int sum = num1 + num2;
	return sum;
}

int main()
{
    
    
	int a = 10;
	int b = 29;
	score(a, b);
	cout << score(a, b) << endl;
	return 0;
}

Sub-document preparation

1. Create a header file with the suffix .h
2. Create a source file with the extension .cpp
3. Write the function declaration in the header file
4. Write the definition of the function in the source file

Other file you want to use the file, only by adding a #include "swap.h" to

【Source File】:

#include "swap.h"
/*  分文件编写
*   1、创建后缀名为 .h 的头文件
*   2、创建后缀名为 .cpp 的源文件
*   3、在头文件中写函数的声明
*   4、在源文件中写函数的定义
* 
其他文件想使用该文件，则只用加一个#include "swap.h"即可
*/
int main()
{
    
    
	int a = 10;
	int b = 20;
	swap(a, b);
	cout << swap(a, b) << endl;
	return 0;
}

【head File】:

#include <iostream>
using namespace std;

int swap(int num1, int num2)
{
    
    
	int sum = num1 + num2;
	return sum;
}

Pointer definition and use

Pointer definition

Data type * pointer variable name;

Use of pointers

Add * in front of the pointer represents dereference , find the data in the memory space pointed to by the pointer

#include <iostream>
using namespace std;

//指针：记录地址编号
   
int main()
{
    
    
	//指针定义
	//数据类型 * 指针变量名；
	int* p;
	int a = 10;
	p = &a;
	cout << p << endl;//a的地址
	cout << *p << endl;//a的值

	//指针的使用
	//指针前面加*代表解引用，找到指针指向的内存空间的数据
	*p = 1000;
	cout << p << endl;//a的地址
	cout << *p << endl;//更新的值
	return 0;
}

Memory space occupied by pointers

A pointer is 4 bytes of memory space in a 32-bit operating system ( regardless of the data type )

#include <iostream>
using namespace std;

int main(){
    
    
    //指针在32位操作系统中是4个字节的内存空间（无论什么的数据类型）
	cout << sizeof(int*) << endl;
	cout << sizeof(float*) << endl;
	cout << sizeof(long*) << endl;
	return 0;
}

① Function declaration and definition ② Separate file compilation ③ Pointer definition and use ④ Memory space occupied by pointer

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