[C++知识库][学习笔记]c++指针和内存总结

内存空间

1. 代码区
   存放程序代码。

2. 保留区
   0地址开始，存放c的库代码

3. 全局区/静态区
   程序中所有全局变量和静态变量都存放在全局区/静态区，程序结束后由操作系统回收此部分内存。

4. 堆内存
   堆同样是内存区域，但是由程序员分配和释放。程序结束后由操作系统回收此部分内存。C语言的malloc、C++语言的new调用后得到的就是堆内存。

5. 栈内存
   栈存放的变量由编译器自动分配释放，存取速度比寄存器稍低。

6. 命令行参数/环境变量

7. 内核空间
   地址为高地址。

#include <iostream>

using namespace std;

int g1;
int g2 = 0;
int g3 = 1;
static int sg1 = 3;

int main(int argc, char *argv[]) {
  static int sl1 = 5;
  // 代码区
  cout << "main =" << main << endl;
  // 全局变量区/静态变量区
  cout << "g1 = " << &g1 << endl;
  cout << "g2 = " << &g2 << endl;
  cout << "(static global) g3 = " << &g3 << endl;
  cout << "(static local) sl1=" << &sl1 << endl;

  int *p1 = new int;
  int *p2 = new int;
  // 堆内存区
  cout << "heap p1=" << p1 << endl;
  cout << "heap p2=" << p2 << endl;
  cout << "stack pointer variable p1=" << &p1 << endl;
  cout << "stack pointer variable p2=" << &p2 << endl;
  int i1 = 100;
  int i2 = 101;
  // 栈内存区
  cout << "stack i1=" << &i1 << endl;
  cout << "stack i2=" << &i2 << endl;
  return 0;
}
/**
main =1
g1 = 0x555555558154
g2 = 0x555555558158
(static global) g3 = 0x555555558010
(static local) sl1=0x555555558018
heap p1=0x55555556b2c0
heap p2=0x55555556b2e0
stack pointer variable p1=0x7fffffffde78
stack pointer variable p2=0x7fffffffde80
stack i1=0x7fffffffde70
stack i2=0x7fffffffde74
**/

指针及指针变量

1. 指针类型
   a. 定义指针类型（存放在堆）：int *p1 = new int;
   b. delete以后，指针依然存放值，用if判断仍然为true。
   c. delete同一个指针变量两次会出错。
   d.可以把删除后的指针赋值为nullptr/NULL，这样delete指针变量两次不会出错。

#include <iostream>

int main(int argc, char *argv[]) {
  // 指针类型，存在栈中
  // new int存在堆中
  int *p1 = new int;
  // *间接符号
  *p1 = 101;
  // 存在栈中
  int i = 10;
  int *p2 = &i;
  *p2 = 102;
  std::cout << "p1 =" << p1 << std::endl;
  std::cout << "sizeof(p1) =" << sizeof(p1) << std::endl;
  std::cout << "sizeof(*p1) = " << sizeof(*p1) << std::endl;
  delete p1;
  std::cout << "after delete p1 = " << p1 << std::endl;
  // delete操作后p1仍然有
  if (p1) {
    std::cout << "p1 still exists." << std::endl;
  }
  p1 = nullptr;
  if (!p1) {
    std::cout << "p1 not exists." << std::endl;
  }
  delete p1;
  return 0;
}
/**result
p1 =0x55555556b2c0
sizeof(p1) =8
sizeof(*p1) = 4
after delete p1 = 0x55555556b2c0
p1 still exists.
p1 not exists.
**/

2. 数组类型
   a. 定义在堆空间的数组没有begin(),end()，因此不能通过foreach语法遍历数组。
   b. new出来的数组使用delete[]释放内存

#include <cstring>
#include <iostream>
using namespace std;

int main(int argc, char *argv[]) {
  // stack
  int arr1[10];
  memset(arr1, 0, sizeof(arr1)); // get mem size

  char str[] = "test";
  for (auto s : str) {
    cout << s << '-';
  }
  cout << endl;
  cout << "val of str =" << *str << endl;
  cout << "sizeof arr1= " << sizeof(arr1) << endl;
  cout << "arr1[0] addr= " << &arr1[0] << endl;
  cout << "arr1[1] addr= " << &arr1[1] << endl;

  // heap
  int psize = 10;
  int *parr2 = new int[psize];
  memset(&parr2, 0, psize * sizeof(int));
  cout << "val of parr2 =" << parr2 << endl;
  cout << "parr2 addr =" << &parr2 << endl;
  cout << "sizeof parr2= " << sizeof(parr2) << endl;

  // for(auto& s:parr2) doesn't exist begin()\end()

  delete[] parr2;
  parr2 = nullptr;
  return 0;
}
/**
t-e-s-t--
val of str =t
sizeof arr1= 40
arr1[0] addr= 0x7fffffffde60
arr1[1] addr= 0x7fffffffde64
val of parr2 =0
parr2 addr =0x7fffffffde40
sizeof parr2= 8
 */