bool变量如何不等于True和False？ [英] How can bool variable be not equal to both True and False?

问题描述

根据该问题的可接受答案
如果…else如果带有else子句的构造终止，有什么好处？

According to accepted answer of this question What is the benefit of terminating if … else if constructs with an else clause?

有一个腐败案（在嵌入式系统中） ）可能会导致 bool变量 （1位） 与True和False都不同，这表示

There is a corruption case (in embedded system) that can cause a bool variable (which is 1 bit) differ to both True and False, it means the else path in this code could be covered instead of be a dead code.

if (g_str.bool_variable == True) {
 ...
}
else if (g_str.bool_variable == False) {
 ...
}
else {
 //handle error
}

我尝试找出问题所在

有可能吗？
和
如何？

编辑：为了更清楚地说明，我将给出布尔变量，例如：

For more clearly, I will give the declaration of the bool variable like:

struct {
   unsigned char bool_variable : 1;

} g_str;

并定义：

#define True 1
#define False 0

推荐答案

unsigned char bool_variable：1 不是布尔变量。它是一个1位整数位字段。 _Bool bool_variable 是布尔变量。

unsigned char bool_variable : 1 is not a boolean variable. It is a 1 bit integer bit-field. _Bool bool_variable is a boolean variable.

位字段应具有类型是 _Bool ， signed int ， unsigned int 或其他一些实现定义的类型。是否允许原子类型由实现定义。 > C11dr§6.7.2.1

A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type. It is implementation-defined whether atomic types are permitted. > C11dr §6.7.2.1

因此立即 unsigned char bool_variable：1

如果这样的实现处理了 unsigned char 位字段（例如 int 位字段），因为 unsigned char 范围可以适合 int 范围，然后出现1位 int 位域的问题。由实现定义的是1位 int 位字段采用的值是 0、1 还是 0，-1 。这会导致此 if（）块的 // handle错误子句。

If such an implementation treated unsigned char bit-fields like int bit-fields, as unsigned char range can fit in int range, then troubles occur with 1-bit int bit-fields. It is implementation defined if a 1-bit int bit-field takes on the values of 0, 1 or 0, -1. This leads to the //handle error clause of this if() block.

if (g_str.bool_variable == True) { // same as if (g_str.bool_variable == 1)
 ...
}
else if (g_str.bool_variable == False) { // same as if (g_str.bool_variable == 0)
 ...
}
else {
 //handle error
}

解决方案是简化 if（）测试：

if (g_str.bool_variable) {
 ...
}
else  {
 ...
}

---

具有位域，它是C中的一个角，其中 unsigned int ， signed int 是不同的，但是 int 的位域小于 int 的全宽被视为 signed int 或 unsigned int 。对于位域，最好是显式的，并使用 _Bool ， signed int ， unsigned int 。注意：使用 unsigned 与 unsigned int 是同义的。

---

With bit-fields, it is a corner in C where unsigned int, signed int are different, but int bit-fields less than the full width of an int may be treated as signed int or unsigned int. With bit-fields, it is best to be explicit and use _Bool, signed int, unsigned int. Note: using unsigned is synonymous with unsigned int.

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