C++ bool 到 int 的转换
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bool to int conversion
提问by pic11
How portable is this conversion. Can I be sure that both assertions pass?
这种转换有多便携。我能确定这两个断言都通过了吗？
int x = 4<5;
assert(x==1);
x = 4>5;
assert(x==0);
Don't ask why. I know that it is ugly. Thank you.
不要问为什么。我知道它很丑。谢谢你。
回答by Nawaz
int x = 4<5;
Completely portable. Standard conformant. boolto intconversion is implicit!
完全便携。符合标准。bool以int转换是隐式的！
§4.7/4 from the C++ Standard says (Integral Conversion)
C++ 标准中的 §4.7/4 说（积分转换）
If the source type is bool, the value
falseis converted to zeroand the valuetrueis converted to one.
如果源类型为 bool ，则将值
false转换为零，并将值true转换为 1。
As for C, as far as I know there is no boolin C. (before 1999) So boolto intconversion is relevant in C++ only. In C, 4<5evaluates to intvalue, in this case the value is 1, 4>5would evaluate to 0.
至于C，据我所知是没有bool的C.（1999年以前），所以bool要int转换在C ++中才有意义。在 C 中，4<5计算为intvalue，在这种情况下，值为1， 4>5将计算为0。
EDIT: Jens in the comment said, C99 has _Booltype. boolis a macro defined in stdbool.hheader file. trueand falseare also macro defined in stdbool.h.
编辑：Jens 在评论中说，C99 有_Bool类型。bool是stdbool.h头文件中定义的宏。true并且false也是在stdbool.h.
§7.16 from C99 says,
C99 中的 §7.16 说，
The macro
boolexpands to _Bool.[..]
truewhich expands to the integer constant1,falsewhich expands to the integer constant0,[..]
宏
bool扩展为_Bool。[..]
true扩展为整数常量1,false扩展为整数常量0,[..]
回答by AnT
You tagged your question [C] and [C++] at the same time. The results will be consistent between the languages, but the structure of the the answer is different for each of these languages.
您同时标记了您的问题 [C] 和 [C++]。语言之间的结果将是一致的，但是对于这些语言中的每一种，答案的结构都不同。
In C language your examples has no relation to boolwhatsoever (that applies to C99 as well). In C language relational operators do not produce boolresults. Both 4 > 5and 4 < 5are expressions that produce results of type intwith values 0or 1. So, there's no "bool to int conversion" of any kind taking place in your examples in C.
在 C 语言中，您的示例与任何内容无关bool（也适用于 C99）。在 C 语言中，关系运算符不会产生bool结果。两者4 > 5并4 < 5是产生类型的结果的表达式int用值0或1。因此，在您的 C 示例中没有发生任何类型的“bool 到 int 转换”。
In C++ relational operators do indeed produce boolresults. boolvalues are convertible to inttype, with trueconverting to 1and falseconverting to 0. This is guaranteed by the language.
在 C++ 中，关系运算符确实会产生bool结果。bool值可以转换为int类型，true转换为1和false转换为0. 这是由语言保证的。
P.S. C language also has a dedicated boolean type _Bool(macro-aliased as bool), and its integral conversion rules are essentially the same as in C++. But nevertheless this is not relevant to your specific examples in C. Once again, relational operators in C always produce int(not bool) results regardless of the version of the language specification.
PS C 语言也有专用的布尔类型_Bool（宏别名为bool），其整数转换规则与 C++ 中基本相同。但是，这与您在 C 中的具体示例无关。再说一次，C 中的关系运算符总是产生int(not bool) 结果，而不管语言规范的版本如何。
回答by AnT
Section 6.5.8.6 of the C standard says:
C 标准的第 6.5.8.6 节说：
Each of the operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is false.) The result has type int.
如果指定的关系为真，则每个运算符 <（小于）、>（大于）、<=（小于或等于）和 >=（大于或等于）应产生 1，如果是，则产生 0 false。）结果的类型为 int。
回答by Alex James
There seems to be no problem since the int to bool cast is done implicitly. This works in Microsoft Visual C++, GCC and Intel C++ compiler. No problem in either C or C++.
似乎没有问题，因为 int 到 bool 的转换是隐式完成的。这适用于 Microsoft Visual C++、GCC 和 Intel C++ 编译器。在 C 或 C++ 中都没有问题。
