memset bool到0是否安全？ [英] Is it safe to memset bool to 0?
问题描述
假设我有一些旧版代码，除非发现错误，且该代码包含以下代码，因此无法更改：
Suppose I have some legacy code which cannot be changed unless a bug is discovered, and it contains this code:
bool data[32];
memset(data, 0, sizeof(data));
这是一个安全的方法来设置 bool
Is this a safe way to set all bool in the array to a false value?
更多一般来说， memset a bool 到 0 ，以使其值 false ？
More generally, is it safe to memset a bool to 0 in order to make its value false?
它是否可以在所有编译器上正常工作？
Is it guaranteed to work on all compilers? Or do I to request a fix?
推荐答案
我相信这个未指定，虽然它似乎可能是 false 将全部为零。 Boost.Container依靠此well （ emphasis mine ）：
I believe this unspecified although it seems likely the underlying representation of false would be all zeros. Boost.Container relies on this as well (emphasis mine):
Boost.Container使用std :: memset值来初始化一些
类型，因为在大多数平台中，初始化产生期望的
值初始化，并且性能提高。
Boost.Container uses std::memset with a zero value to initialize some types as in most platforms this initialization yields to the desired value initialization with improved performance.
遵循C11标准，Boost .Container假定对于任何
整数类型，所有位为零的对象表示
将是该类型中的零值的表示。 由于
_Bool / wchar_t / char16_t / char32_t也是C中的整数类型，它认为所有C ++整数类型都可以通过std :: memset初始化。
Following the C11 standard, Boost.Container assumes that for any integer type, the object representation where all the bits are zero shall be a representation of the value zero in that type. Since _Bool/wchar_t/char16_t/char32_t are also integer types in C, it considers all C++ integral types as initializable via std::memset.
他们指出的C11引用实际上来自C99缺陷：缺陷263：全零位表示，添加了以下内容：
This C11 quote they they point to as a rationale actually comes from a C99 defect: defect 263: all-zero bits representations which added the following:
对于任何整数类型，其中所有位都是
zero的对象表示应该是该类型中零值的表示。
For any integer type, the object representation where all the bits are zero shall be a representation of the value zero in that type.
那么这里的问题是假设正确，是C和C ++之间的整数兼容的底层对象表示吗？
提案解决C之间的区别和C ++关于整数的对象表示试图回答这在一定程度上，据我可以告诉没有解决。我在标准草案中找不到这个的确凿证据。我们有几种情况下，它明确链接到C标准类型。
So then the question here is the assumption correct, are the underlying object representation for integer compatible between C and C++?
The proposal Resolving the difference between C and C++ with regards to object representation of integers sought to answer this to some extent which as far as I can tell was not resolved. I can not find conclusive evidence of this in the draft standard. We have a couple of cases where it links to the C standard explicitly with respect to types. Section 3.9.1 [basic.fundamental] says:
[...]签名和无符号整数类型应满足C标准第5.2.4.2.1节中给出的
约束。
[...] The signed and unsigned integer types shall satisfy the constraints given in the C standard, section 5.2.4.2.1.
和 3.9 [basic.types] ：
类型T的对象的对象表示是由类型T的对象占用的N
个unsigned char对象的序列，其中N等于
sizeof（T ）。对象的值表示是保存类型T的值的一组位
。对于普通可复制类型，值
表示是对象表示中的一组位，
确定一个值，它是
实现定义的一组值的一个离散元素。 44
其中脚注44（不是规范）说：
目的是记忆模型C ++与
ISO / IEC 9899编程语言C的兼容性。
The intent is that the memory model of C++ is compatible with that of ISO/IEC 9899 Programming Language C.
草案标准最远的部分中 $
The farthest the draft standard gets to specifying the underlying representation of bool is in section 3.9.1:
类型bool，char，char16_t，char32_t，wchar_t和signed和
无符号整数类型统称为积分类型。A整数类型的
同义词是整数类型。
整数类型的表示应通过使用纯二进制数字
system.51定义值[例如：本国际标准允许2的
补码，1的补码和符号幅度表示为
积分类型。 -end example]
Types bool, char, char16_t, char32_t, wchar_t, and the signed and unsigned integer types are collectively called integral types.50 A synonym for integral type is integer type. The representations of integral types shall define values by use of a pure binary numeration system.51 [ Example: this International Standard permits 2’s complement, 1’s complement and signed magnitude representations for integral types. —end example ]
该部分还说：
bool类型的值为true或false。
Values of type bool are either true or false.
但我们知道 true 和 false 是：
布尔文字关键字false和true。这样的文字
是prvalues并且类型为bool。
The Boolean literals are the keywords false and true. Such literals are prvalues and have type bool.
，我们知道它们可以转换为 0 1 ：
and we know they are convertible to 0 an 1:
可以转换为int类型的prvalue，
false变为零并成为true。
A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one.
不接近底层表示。
据我所知，标准引用除了padding位之外的实际底层位的唯一地方是通过缺陷报告1796：空字符的全位零是有意义的要求吗？ ：
As far as I can tell the only place where the standard references the actual underlying bit value besides padding bits was removed via defect report 1796: Is all-bits-zero for null characters a meaningful requirement? :
不清楚可移植程序可以检查表示的位;相反，似乎仅限于检查与值表示相对应的数字的位（3.9.1 [basic.fundamental] paragraph 1）。可能更适合的是要求空字符值比较等于0或'\0'，而不是指定表示的位模式。
It is not clear that a portable program can examine the bits of the representation; instead, it would appear to be limited to examining the bits of the numbers corresponding to the value representation (3.9.1 [basic.fundamental] paragraph 1). It might be more appropriate to require that the null character value compare equal to 0 or '\0' rather than specifying the bit pattern of the representation.
还有更多缺陷报告处理标准中的差距，关于什么是有点和值和对象表示之间的差异。
There are more defect reports that deal with the gaps in the standard with respect to what is a bit and difference between the value and object representation.
实际上，我期望这个工作，我不会考虑它的安全，因为我们不能把这个标准。你需要改变它，不清楚，你显然有一个不平凡的权衡涉及。所以假设它现在工作的问题是，我们认为它可能会打破与各种编译器的未来版本，这是未知的。
Practically, I would expect this to work, I would not consider it safe since we can not nail this down in the standard. Do you need to change it, not clear, you clearly have a non-trivial trade-off involved. So assuming it works now the question is do we consider it likely to break with future versions of various compilers, that is unknown.
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