C++:17---sizeof运算符

• 
  
• 功能：以字节位单位，返回一个表达式或一个数据类型所占的字节数
• 返回值类型：是size_t类型
• sizeof有无括号：

• sizeof不加括号，后面不可以直接跟数据类型
• sizeof加括号，后面既可以跟表达式也可以跟数据类型

• 注意事项

• 对引用类型执行sizeof运算得到被引用对象所占空间的大小

• 对指针执行sizeof运算得到指针本身所占空间大小

• 对解引用指针执行sizeof运算得到指针指向指向的对象所占空间的大小，指针不需有效

• 对数组执行sizeof运算得到整个数组所占空间的大小，等价于对数组中所有的元素各执行一次sizeof运算并将结果求和。注意，sizeof运算不会把数组转换指针来处理

• 对string对象或vector对象执行sizeof运算只返回该类型固定部分的大小，不会计算对象中的元素占用了多少空间

  
  

  
  

二话不说，来看代码：

#include <iostream>
#include <string>
using namespace std;
int main(){
  int a = 12;
  short int b = 12;
  float   c = 12.0f;
  double  d = 12.333;
  long  int e = 123;
        long long int  f = 1233;
  char ch = '0';
  
  cout << sizeof a << ","  << sizeof b << ","  << sizeof c << ","  << sizeof d << ","  << sizeof e << ","  << sizeof f << ","  << sizeof ch <<endl;


  uint8_t   ua = 1;
  uint16_t  ub = 2;
   uint32_t  uc = 3;
  uint64_t  ud = 4;


  cout << sizeof ua << ","  << sizeof ub << ","  << sizeof uc << ","  << sizeof ud <<endl;


  char* pa = &ch;
  int*  pb = &a;
  
  cout <<sizeof pa << ","  << sizeof pb <<endl;  
  uint32_t& p = uc;
  cout << sizeof p <<endl;
        
        int   array[10];
  cout << sizeof array <<endl;


  string  s = "hello world";
  string  s_empty; 
  cout << sizeof s << ","  << sizeof s_empty << endl;
  return 0;
}