在c++中,

为什么布尔值是1字节而不是1位? 为什么没有4位或2位整数类型?

在为CPU编写模拟器时,我忽略了上述内容


当前回答

考虑一下如何在模拟器级别实现这一点……

bool a[10] = {false};

bool &rbool = a[3];
bool *pbool = a + 3;

assert(pbool == &rbool);
rbool = true;
assert(*pbool);
*pbool = false;
assert(!rbool);

其他回答

Back in the old days when I had to walk to school in a raging blizzard, uphill both ways, and lunch was whatever animal we could track down in the woods behind the school and kill with our bare hands, computers had much less memory available than today. The first computer I ever used had 6K of RAM. Not 6 megabytes, not 6 gigabytes, 6 kilobytes. In that environment, it made a lot of sense to pack as many booleans into an int as you could, and so we would regularly use operations to take them out and put them in.

今天,当人们嘲笑你只有1gb的RAM,而你唯一能找到小于200gb的硬盘的地方是在古董店,它只是不值得麻烦打包比特。

因为在一般情况下,CPU以1字节作为基本单位分配内存,尽管一些CPU如MIPS使用4字节字。

但是vector以一种特殊的方式处理bool类型,vector<bool>为每个bool类型分配一个位。

因为字节是语言中最小的可寻址单位。

但是你可以让bool值为1位,如果你有很多 如。在结构体中,像这样:

struct A
{
  bool a:1, b:1, c:1, d:1, e:1;
};

考虑一下如何在模拟器级别实现这一点……

bool a[10] = {false};

bool &rbool = a[3];
bool *pbool = a + 3;

assert(pbool == &rbool);
rbool = true;
assert(*pbool);
*pbool = false;
assert(!rbool);

bool can be one byte -- the smallest addressable size of CPU, or can be bigger. It's not unusual to have bool to be the size of int for performance purposes. If for specific purposes (say hardware simulation) you need a type with N bits, you can find a library for that (e.g. GBL library has BitSet<N> class). If you are concerned with size of bool (you probably have a big container,) then you can pack bits yourself, or use std::vector<bool> that will do it for you (be careful with the latter, as it doesn't satisfy container requirments).