#include <iostream>
int main(){
  unsigned char a = 4,b = 255;
  int g = (unsigned char)a + (unsigned char)b;
  std::cout << g << std::endl;
  return 0;
}

结果：

259

为什么结果是259,而不是3？如果添加了两个unsigned char变量,应该有溢出,结果应该是3,然后它应该从unsigned char 3转换为int 3.