布尔在内存中如何表示?
正如已经在 文档中讨论过的，一个 bool 数据类型至少占用一个字节的内存.之前在 SO 上提出了类似的问题(How a bool类型变量是否存储在内存中?(C++))，但是这个讨论和文档似乎只讨论布尔数据类型占用的空间量，而不是实际发生的情况在内存中当我这样做时:
As already discussed in the docs, a bool data type occupies at least a byte of memory. A similar question was asked on SO before (How a bool type variable is stored in memory? (C++)), but this discussion and the documentation only seem to discuss the amount of space occupied by a boolean data type, not what actually happens in memory when I do this:
bool b = true;
那么内存中到底发生了什么?未用于存储此信息的 7 位会发生什么?标准是否为此规定了行为?
So what does actually happen in memory? What happens to the 7 bits that are not used in storing this information? Does the standard prescribe behavior for this?
它们是未定义的吗?还是 C++ 总部的某个人这样做了:
Are they undefined? Or did someone at C++ headquarters just do this:
enum bool : char
{
false = 0,
true = 1
};
推荐答案
标准规定 bool 值表现为整数类型，但没有指定它们在内存中的具体表示:
Standard states that bool values behave as integral types, yet it doesn't specify their concrete representation in memory:
"bool 类型的值为 true 或 false.如下所述，bool值表现为整数类型.bool 类型的值参与整数提升" ~ C++03 3.9.1 §6
"Values of type bool are either true or false. As described below, bool values behave as integral types. Values of type bool participate in integral promotions" ~ C++03 3.9.1 §6
整数类型的同义词是整数类型.整数类型的表示应使用纯二进制计数系统定义值" ~ C++03 3.9.1 §7
然而它定义了从 bool 到 int 的整体提升:
Yet it defines the integral promotion from bool to int:
"bool 类型的右值可以转换为 int 类型的右值，false 变为零和 true 合而为一.这些转换称为积分提升." ~ C++03 4.5 §4-5
"An rvalue of type bool can be converted to an rvalue of type int, with false becoming zero and true becoming one. These conversions are called integral promotions." ~ C++03 4.5 §4-5
以及从其他类型到bool的转换:
as well as conversion from other types to bool:
零值、空指针值或空成员指针值转换为false；任何其他值转换为true." ~ C++03 4.12 §1
"A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true." ~ C++03 4.12 §1
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