概述

本文通过c++示例代码演示指针的加减法运算及对 “指针 = 地址 + 偏移量” 的理解。

研究示例

1. 首先来检查各种变量类型所占的内存大小

#include <iostream>
using namespace std;

int main(){
	cout << sizeof(char) << endl;      // 1 Byte
	cout << sizeof(short) << endl;     // 2 Byte
	cout << sizeof(int) << endl;       // 4 Byte
	cout << sizeof(long long) << endl; // 8 Byte
	return 0;
}

2. 各类型的指针进行加减运算

各类型的指针进行加减运算是以元素为单位进行加减，而非地址的最小单位(1个Byte)。以下演示各类型指针指向的地址：

#include <iostream>
using namespace std;

int main(){
	char *p_char = new char('a');
	cout << "*p_char = " << *p_char << endl; 
	cout << "p_char = " << (void*)p_char << endl;    // char指针在使用cout输出会直接打印变量而非地址，要加(void*)
	cout << "p_char + 1 = " << (void*)(p_char + 1) << endl; 
	cout << "p_char + 2 = " << (void*)(p_char + 2) << endl; 
	cout << "p_char - 1 = " << (void*)(p_char - 1) << endl; 

	short *p_short = new short(456);
	cout << "*p_short = " << *p_short << endl; 
	cout << "p_short = " << p_short << endl; 
	cout << "p_short + 1 = " << p_short + 1 << endl; 
	cout << "p_short + 2 = " << p_short + 2 << endl; 
	cout << "p_short - 1 = " << p_short - 1 << endl; 

	int *p_int = new int(123);
	cout << "*p_int = " << *p_int << endl; 
	cout << "p_int = " << p_int << endl; 
	cout << "p_int + 1 = " << p_int + 1 << endl; 
	cout << "p_int + 2 = " << p_int + 2 << endl; 
	cout << "p_int - 1 = " << p_int - 1 << endl; 

	long long *p_long_long = new long long(456789);
	cout << "*p_long_long = " << *p_long_long << endl; 
	cout << "p_long_long = " << p_long_long << endl; 
	cout << "p_long_long + 1 = " << p_long_long + 1 << endl; 
	cout << "p_long_long + 2 = " << p_long_long + 2 << endl; 
	cout << "p_long_long - 1 = " << p_long_long - 1 << endl; 

	return 0;
}

输出：（char, short, int, long long的指针的最小移动单位分别是1, 2, 4, 8 Byte，刚好是对应的单个元素的类型所占内存大小）

*p_char = a
p_char = 0xe41600
p_char + 1 = 0xe41601
p_char + 2 = 0xe41602
p_char - 1 = 0xe415ff
*p_short = 456
p_short = 0xe41620
p_short + 1 = 0xe41622
p_short + 2 = 0xe41624
p_short - 1 = 0xe4161e
*p_int = 123
p_int = 0xe41640
p_int + 1 = 0xe41644
p_int + 2 = 0xe41648
p_int - 1 = 0xe4163c
*p_long_long = 456789
p_long_long = 0xe41660
p_long_long + 1 = 0xe41668
p_long_long + 2 = 0xe41670
p_long_long - 1 = 0xe41658

说明：指针 = 地址 + 偏移量。即指针除了包含地址信息之外，还包含解析这个地址的方式（从该地址开始向后读取多少个Byte），因此“指针就是地址”的说法是不准确的，一个int* p1和char* p2 指向的地址可能是相同的，但是解析这个地址的方式是不同的。

3. 强制转换指针类型对地址进行解析

可以强行指定指针的类型对同一块内存的数据进行不同方式的解析。例如：

#include <iostream>
using namespace std;

int main(){
	char arr[10] = {0,1,2,3,4,5,6,7,8,9};
	unsigned int l = sizeof(arr)/sizeof(arr[0]);

	char* p_char = arr;
	cout << "*p_char = " << (int)*p_char << endl; 
	cout << "*(p_char + 1) = " << (int) *(p_char + 1) << endl; 
	cout << "*(p_char + 2) = " << (int)*(p_char + 2) << endl; 
	cout << "*(p_char + 3) = " << (int)*(p_char + 3) << endl; 
	cout << "*(p_char + 9) = " << (int)*(p_char + 9) << endl; 
		
	short* p_short = (short*)arr;
	cout << hex << "*p_short = " << *p_short << endl; 
	cout << hex << "*(p_short + 1) = " << *(p_short + 1) << endl; 
	cout << hex << "*(p_short + 2) = " << *(p_short + 2) << endl; 
	
	int *p_int = (int*)arr;
	cout << hex << "*p_int = " << *p_int << endl; 
	cout << hex << "*(p_int + 1) = " << *(p_int + 1) << endl; 
	cout << hex << "*(p_int + 2) = " << *(p_int + 2) << endl; 

	return 0;
}

输出：

*p_char = 0
*(p_char + 1) = 1
*(p_char + 2) = 2
*(p_char + 3) = 3
*(p_char + 9) = 9
*p_short = 100
*(p_short + 1) = 302
*(p_short + 2) = 504
*p_int = 3020100
*(p_int + 1) = 7060504
*(p_int + 2) = fdf60908

解释如下图：arr数组在内存中占10个Byte，分别定义了三种类型的指针char*, short*, int*并且都指向首地址arr[0]，则三种指针的差异体现在两个方面：1、解引用（解析地址）时的偏移量分别为1,2,4个Byte；2、加减时分别以1,2,4个Byte为单位移动地址。