C++的struct类型的内存问题

最新推荐文章于 2025-03-03 12:48:35 发布

原创最新推荐文章于 2025-03-03 12:48:35 发布 · 844 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#struct #内存分配 #char #内存对齐

本文通过实验展示了结构体中不同数据类型的内存分配情况，并深入探讨了内存对齐问题及其与操作系统分配内存单元大小的关系。进一步，通过添加多个char型数据，验证了内存对齐是否影响数据在结构体内的布局。

在用struct定义一个结构体的时候，突发想象：struct变量中的不同类型是怎么样分配内存的？

于是，用VC做了个小实验

struct中定义了char、int、float等类型数据

先来看看只有一个char类型数据，一个int类型数据，一个float类型数据，一个double类型数据的struct类型

struct Structure1 {
  char c1;
  int i;
  float f;
  double d;
};

#include <iostream>
using namespace std;

//typedef struct Structure1 struction;

int main() {
  
  struct Structure1 s1, s2;
  
  int size = sizeof s1;
  cout<<"Size of char is "<<sizeof(char)<<endl;
  cout<<"Size of int is "<<sizeof(int)<<endl;
  cout<<"Size of float is "<<sizeof(float)<<endl;
  cout<<"Size of double is "<<sizeof(double)<<endl;
  cout<<"Size of struct is "<<size<<endl;
  s1.c1 = 'a'; // Select an element using a '.'
  s1.i = 1;
  s1.f = 3.14;
  s1.d = 0.00093;
  cout<<"Size of struct is "<<size<<endl;
  cout<<"Address of s1 is "<<&s1<<endl;
  if((&s1) == (struct Structure1*)((void*)&(s1.c1)))
  {
	  cout<<"s1 share the same address with s1.c1!"<<endl;
  }
  else
  {
	  cout<<"s1 didn't share the same address with s1.c1!"<<endl;
  }
  cout<<"Address of s1.c1 is "<<&(s1.c1)<<endl;
  cout<<"Address of s1.i is "<<&(s1.i)<<endl;
  cout<<"Address of s1.f is "<<&(s1.f)<<endl;
  cout<<"Address of s1.d is "<<&(s1.d)<<endl;
  cout<<"Size of s1.d is "<<sizeof s1.d<<endl;
  cout<<"Size of s1.c1 is "<<sizeof s1.c1<<endl;
  s2.c1 = 'a';
  s2.i = 1;
  s2.f = 3.14;
  s2.d = 0.00093;
}

运行结果

看到的是本来char型只有一个字节，但是从int地址和s1的起始地址来看，char型应该是占据了四个字节。这其实是一个内存对齐的问题，主要跟操作系统分配内存的单元大小有关。

所以下面，我就多添加几个char型

struct Structure1 {
  char c1;
  int i;
  float f;
  
  char c2;//让c2在定义的时候不跟在c1的后面，也就是想看看有没有内存对齐的问题
  char c3;
  char c4;
  double d;
};

#include <iostream>
using namespace std;

int main() {
  
  struct Structure1 s1, s2;
  
  int size = sizeof s1;
  cout<<"Size of char is "<<sizeof(char)<<endl;
  cout<<"Size of int is "<<sizeof(int)<<endl;
  cout<<"Size of float is "<<sizeof(float)<<endl;
  cout<<"Size of double is "<<sizeof(double)<<endl;
  cout<<"Size of struct is "<<size<<endl;
  s1.c1 = 'a'; // 
  s1.i = 1;
  s1.f = 3.14;
  s1.d = 0.00093;
  cout<<"Size of struct is "<<size<<endl;
  cout<<"Address of s1 is "<<&s1<<endl;
  if((&s1) == (struct Structure1*)((void*)&(s1.c1)))//判断有没有c1的地址是和struct变量的起始地址是一样的
  {
	  cout<<"s1 share the same address with s1.c1!"<<endl;
  }
  else
  {
	  cout<<"s1 didn't share the same address with s1.c1!"<<endl;
  }
  cout<<"Address of s1.c1 is "<<&(s1.c1)<<endl;
  cout<<"Address of s1.c2 is "<<&(s1.c2)<<endl;
  cout<<"Address of s1.c3 is "<<&(s1.c3)<<endl;
  cout<<"Address of s1.i is "<<&(s1.i)<<endl;
  cout<<"Address of s1.f is "<<&(s1.f)<<endl;
  cout<<"Address of s1.d is "<<&(s1.d)<<endl;
  cout<<"Size of s1.d is "<<sizeof s1.d<<endl;
  cout<<"Size of s1.c1 is "<<sizeof s1.c1<<endl;
  cout<<"Size of s1.c2 is "<<sizeof s1.c2<<endl;
  cout<<"Size of s1.c3 is "<<sizeof s1.c3<<endl;
}

运行的结果如下：