将布尔值更改为 0 或 1 以外的值? [英] Changing a bool to a value other than 0 or 1?

问题描述

我有以下代码:

#include <stdio.h>

int main()
{
    bool b;

    // Set b to 123
    char *p = (char*)&b;
    *p = 123;

    // Check if b is considered to be true
    if (b == true) printf("b is true");

    return 0;
}

b 不被认为是 true，那么 true 究竟是什么意思，是 true 等于1?

b is not considered to be true, so what does true means exactly, is true equals to 1?

忘记说我使用的是 Visual C++.

Forgot to say that I'm using Visual C++.

推荐答案

bool b;

// Set b to 123
char *p = (char*)&b;
*p = 123;

C++ 标准未指定 bool 的存储方式...例如一个编译器使用一个字节而另一个编译器使用四个字节是完全合法的.此外，一个编译器可能存储 0 表示 false 和 all-bits-on 表示 true 和另一个 0 和1 或任何其他值.重要的是，当在布尔上下文中使用时，逻辑可以正常工作，并且在转换为 int 或从 int 转换时:

The C++ Standard leaves it unspecified how bool are stored... e.g. entirely legal for one compiler to use one byte and another to use four. Further, one compiler might store say 0 for false and all-bits-on for true and another 0 and 1 or whatever other values. All that mattered is that when used in a boolean context the logic works correctly, and that when converted to or from an int:

• int(b) 是 1 如果 b 是 true，并且 0 如果 b 是 false，并且

• int(b) is 1 if b is true, and 0 if b is false, and

bool(n) is true if-and-only-if n is not 0>.

bool(n) is true if-and-only-if n was not 0.

因此，在测试布尔值时可能会或可能不会咨询 bool 表示中最低内存地址处的字符:它可能是一个编译器生成的代码使用了四个-然后读取的字节 int 仅测试最低有效位或字节，根据字节顺序，*p = 123 可能未触及该字节.类似地，编译器可能会将该值读入一个有符号整数寄存器并测试是否为负(当 true 时期望一个全位值)，即使 *p = 123 也可能失败 已写入唯一或最高有效字节.

Because of this, the character at the lowest memory address in the bool representation may or may not be consulted when testing the value of the boolean: it could be one compiler's generated code uses a four-byte int that is read then has only the least significant bit or byte tested, which - depending on endianness - may not have been touched by the *p = 123. Similarly, the compiler might read the value into a signed integer register and test for negativeness (expecting an all-bits-on value when true), which could also fail even if *p = 123 had written to the only or most significant byte.

因此 - 即使没有其他东西在起作用 - 下面的行可能不会报告b 为真"...

Consequently - even if there was nothing else at play - the line below might not report "b is true"...

// Check if b is considered to be true
if (b == true) printf("b is true");

...但该测试进一步存在缺陷...

...but that test is further flawed by...

• 可能从未初始化的 bool 表示中的其他字节读取(未初始化的内存读取具有未定义的行为)

• possibly reading from other bytes in the bool representation that haven't been initialised (uninitialised memory reads have undefined behaviour)

单独使用 bool 值本身就具有未定义行为"；就对 bool 内存的二进制写入"而言，只有来自另一个正确初始化的 bool 的完整字节对字节副本才能保证保留此 bool 是可用状态.

fiddling with the bool value alone inherently has "undefined behaviour"; in terms of "binary writing" to the bool's memory, only a full byte-for-byte copy from another properly initialised bool is guaranteed to leave this bool is a usable state.

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