blocks|key|3985823|text|!!的原因不是效率--任何优秀的编译器都会把它编译成与任何其他非疯狂的转换方式完全相同的东西，包括仅仅依赖于隐式转换--而是它抑制了关于隐式缩小到bool的compiler+warning，它存在于VisualStudio中的微软编译器的旧版本中，但在VS2017中被删除了。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3985824|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-3-c4800?view=vs-2017^0|0|2|21|4|26|G|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@$9|S|A|T|1|U]]|E|$]]|$1|F|3|-4|5|6|7|V|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

The reason for the <code>!!</code> is not efficiency - any decent compiler will compile it to the exact same thing as any other non-bonkers way of converting, including just relying on an implicit conversion - but that it suppresses a <a href="https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-3-c4800?view=vs-2017" rel="nofollow noreferrer">compiler warning</a> about an implicit narrowing to a <code>bool</code> present in older versions of Microsoft's compiler in VisualStudio but was removed in VS2017.

blocks|key|1669690|text|我假设您指的是Win32类型BOOL，它是一种用于int的typedef，具有历史上的C兼容性。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1669691|!!对布尔值进行规范化，将任何非零(即TRUE)值更改为1/true。至于效率，这是很难解释的。对布尔值进行规范化的其他方法(x+%7C%7C+0、x+&&+1、(x+!=+0)等)任何一个好的编译器都应该优化到相同的东西。|1669692|也就是说，如果显式地需要规范化，那么就不应该这样做，除非其目的是抑制编译器警告。|1669693|entityMap^0|E|4|P|3|T|7|0|0|2|J|4|U|4|1R|6|1Y|6|25|8|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|15|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|K|$]]

I'm assuming you are referring to the Win32 type <code>BOOL</code>, which is a <code>typedef</code> for <code>int</code> for historic C compatibility. 

<code>!!</code> normalizes a boolean, changing any non-zero (i.e. <code>TRUE</code>) value into 1/<code>true</code>. As for efficiency, that's difficult to reason about. The other methods for normalizing a boolean (<code>x || 0</code>, <code>x &amp;&amp; 1</code>, <code>(x != 0)</code>, etc.) should all be optimized to the same thing by any decent compiler.

That is, if the normalization is explicitly needed, which it shouldn't be unless the intent is to suppress a compiler warning.

blocks|key|3985893|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3985894|+bVariable+=+!!iVariable与bVariable+=+(iVariable+!=+0)|blockquote|3985895|3985896|3985897|你应该首先考虑可读性，让编译器生成高效的代码。|3985898|如果你有这样的任务，只需将一个分配给另一个：|3985899|bVariable+=+iVariable;|code-block|syntax|javascript|3985900|因为int到bool的转换是定义良好的，任何C%2B%2B程序员都应该能读懂。|offset|length|style|CODE|3985901|如果需要转换变量或表达式，请使用正确C%2B%2B方式-+static_cast|3985902|template<class+T>
void+foobar(+T+t+);

foobar(+static_cast<bool>(+iVariable+)+);+//+explicitly+let+reader+know+that+you+changing+type|3985903|entityMap^0|0|0|0|0|0|0|0|2|3|6|4|0|P|B|0|0^^$0|@$1|2|3|-4|4|5|6|10|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|11|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|12|7|@]|8|@]|9|$]]|$1|E|3|-4|4|5|6|13|7|@]|8|@]|9|$]]|$1|F|3|G|4|5|6|14|7|@]|8|@]|9|$]]|$1|H|3|I|4|5|6|15|7|@]|8|@]|9|$]]|$1|J|3|K|4|L|6|16|7|@]|8|@]|9|$M|N]]|$1|O|3|P|4|5|6|17|7|@$Q|18|R|19|S|T]|$Q|1A|R|1B|S|T]]|8|@]|9|$]]|$1|U|3|V|4|5|6|1C|7|@$Q|1D|R|1E|S|T]]|8|@]|9|$]]|$1|W|3|X|4|L|6|1F|7|@]|8|@]|9|$M|N]]|$1|Y|3|-4|4|5|6|1G|7|@]|8|@]|9|$]]]|Z|$]]

<blockquote>
 bVariable = !!iVariable vs. bVariable = (iVariable != 0)
</blockquote>

You should worry about readability first, let compiler produce efficient code.

If you have an assignment like that just assign one to another:

<pre><code>bVariable = iVariable;
</code></pre>

as <code>int</code> to <code>bool</code> conversion is well defined and should be readable by any C++ programmer.

if you need to convert variable or expression use proper C++ way - <code>static_cast</code>

<pre><code>template&lt;class T&gt;
void foobar( T t );

foobar( static_cast&lt;bool&gt;( iVariable ) ); // explicitly let reader know that you changing type
</code></pre>

blocks|key|5454921|text|因此，在C%2B%2B+(和C)中，您可以隐式地转换为bool+(_Bool)。因此，您可以简单地编写|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|5454922|bIsOk+=+m_ptr->isOpen(some+Parameters)|code-block|syntax|javascript|5454923|然而，运算符!!明确表示存在转换。它们等同于标准的强制转换(bool)m_ptr->isOpen(some+Parameters)或m_ptr->isOpen(some+Parameters)!=0。!!的唯一优点是它的代码比强制转换少。|5454924|所有这些都会产生完全相同的程序集：see+here|5454925|entityMap|0|LINK|mutability|MUTABLE|url|https://godbolt.org/z/Tl-8r0^0|N|4|T|5|0|0|6|2|T|10|1U|X|2S|2|0|H|8|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|1B|8|@]|D|@$9|1C|A|1D|1|1E]]|E|$]]|$1|O|3|-4|5|6|7|1F|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

So, in C++ (and C) you can just implicitly convert to <code>bool</code> (<code>_Bool</code>). Thus, you can simply write

<pre><code>bIsOk = m_ptr-&gt;isOpen(some Parameters)
</code></pre>

The operators <code>!!</code> however make it clear that there is a conversion. They are equivalent to a standard cast <code>(bool)m_ptr-&gt;isOpen(some Parameters)</code> or to <code>m_ptr-&gt;isOpen(some Parameters)!=0</code>. The only advantage of <code>!!</code> is that it is less code than a cast.

All of those produce exactly the same assembly: <a href="https://godbolt.org/z/Tl-8r0" rel="nofollow noreferrer">see here</a>

blocks|key|3985778|text|假设你要赋值给一个bool，这样的转换已经被编译器隐式地完成了，所以“双击”在这里是没有用的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3985779|如果需要从表达式中的BOOL获取bool，“规范化”BOOL+(或类似的东西)仍然很有用。在现代编译器上，我希望它能生成与!=+0相同的代码，唯一的优点是它的类型更少(特别是考虑到一元!具有很高的优先级，而对于!=，您可能需要添加括号)。|3985780|entityMap^0|9|4|0|A|4|G|4|Q|4|1P|4|2K|1|2X|2|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|I|$]]

Given that you are assigning to a <code>bool</code>, such a conversion is already done implicitly by the compiler, so the "double bang" is useless here. 

It can still be useful to "normalize" a <code>BOOL</code> (or similar stuff) if you need to get a <code>bool</code> from a <code>BOOL</code> inside an expression. On modern compilers I expect it to generate the same code as <code>!= 0</code>, the only advantage is that it's less to type (especially given that the unary <code>!</code> has high precedence, while with <code>!=</code> you may need to add parentheses).

I have to maintain a large codebase of rather old Visual C++ source. I found code like:

<pre><code>bIsOk = !!m_ptr-&gt;isOpen(some Parameters)
</code></pre>

The datatype of <code>bIsOk</code> is <code>bool</code>, <code>isOpen(..)</code> returns <code>BOOL</code> (defined as <code>int</code>)

The engineer told me that was said to be the most efficient way to get from <code>BOOL</code> to <code>bool</code>.

Was that correct? Is it still nowadays? 2019?

bVariable = !!iVariable vs. bVariable = (iVariable != 0)

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我必须维护一个相当老的Visual C++源代码的大型代码库。我找到了如下代码：bIsOk = !!m_ptr->isOpen(some Parameters)bIsOk的数据类型为bool，isOpen(..)返回BOOL (定义为int)工程师告诉我，据说这是从BOOL到bool的最有效的方法。这是正确的吗？现在还是这样吗?2019年？

问bVariable = !!iVariable与bVariable = (iVariable != 0)
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