blocks|key|1130731|text|意思是找不到。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1130732|通常是这样定义的：|1130733|static+const+size_t+npos+=+-1;|code-block|syntax|javascript|1130734|最好与npos进行比较，而不是-1，因为代码更易读。|1130735|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|N|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|O|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

It means not found.

It is usually defined like so:

<pre><code>static const size_t npos = -1;
</code></pre>

It is better to compare to npos instead of -1 because the code is more legible.

blocks|key|1130802|text|string::npos是一个代表非位置的常量(可能是-1)。当找不到模式时，方法find会返回它。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1130803|entityMap^0|0|C|R|2|15|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

<code>string::npos</code> is a constant (probably <code>-1</code>) representing a non-position. It's returned by method <code>find</code> when the pattern was not found.

blocks|key|1130880|text|size_t是一个无符号变量，因此'unsigned+value+=-+1‘自动使其成为size_t：18446744073709551615的最大可能值|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1130881|entityMap^0|0|6|18|6|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

<code>size_t</code> is an unsigned variable, thus 'unsigned value = - 1' automatically makes it the largest possible value for <code>size_t</code>: 18446744073709551615

blocks|key|916073|text|std::string::npos是实现定义的索引，它总是超出任何std::string实例的界限。各种std::string函数都会返回它或接受它，以发出超出字符串结尾的信号。它通常是一些无符号整数类型，它的值通常是std::numeric_limits<std::string::size_type>::max+()+(感谢标准的整数提升)，通常与-1相当。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|916074|entityMap^0|0|H|X|B|1G|B|31|1F|4W|2|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|G|$]]

<code>std::string::npos</code> is implementation defined index that is always out of bounds of any <code>std::string</code> instance. Various <code>std::string</code> functions return it or accept it to signal beyond the end of the string situation. It is usually of some unsigned integer type and its value is usually <code>std::numeric_limits&lt;std::string::size_type&gt;::max ()</code> which is (thanks to the standard integer promotions) usually comparable to <code>-1</code>.

blocks|key|1130777|text|如果在搜索字符串中找不到子字符串，found将为npos。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1130778|entityMap^0|H|5|O|4|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

<code>found</code> will be <code>npos</code> in case of failure to find the substring in the search string.

blocks|key|1130929|text|我们必须为find函数的返回类型使用string::size_type，否则与string::npos的比较可能不起作用。由字符串的分配器定义的size_type必须是unsigned整数类型。默认分配器使用类型size_t作为size_type。因为-1被转换为无符号整数类型，所以npos是其类型的最大无符号值。但是，确切的值取决于size_type类型的确切定义。不幸的是，这些最大值不同。事实上，如果类型的大小不同，(unsigned+long)-1就不同于(unsigned+short)-1。因此，比较|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1130930|idx+==+std::string::npos|code-block|syntax|javascript|1130931|如果idx的值为-1，而idx和string::npos的类型不同，则可能会产生false：|1130932|std::string+s;
...
int+idx+=+s.find("not+found");+//+assume+it+returns+npos
if+(idx+==+std::string::npos)+{+//+ERROR:+comparison+might+not+work
...
}|1130933|避免此错误的一种方法是直接检查搜索是否失败：|1130934|if+(s.find("hi")+==+std::string::npos)+{
...
}|1130935|但是，您通常需要匹配字符位置的索引。因此，另一个简单的解决方案是为npos定义您自己的签名值：|1130936|const+int+NPOS+=+-1;|1130937|现在比较看起来有点不同，甚至更方便：|1130938|if+(idx+==+NPOS)+{+//+works+almost+always
...
}|1130939|entityMap^0|I|H|13|C|20|9|2C|8|2Y|6|36|9|3I|2|4O|9|5W|H|6H|H|0|0|8|2|G|C|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|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]|$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]|$9|1L|A|1M|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1N|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1O|8|@$9|1P|A|1Q|B|C]|$9|1R|A|1S|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|1T|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1U|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|1V|8|@]|D|@]|E|$I|J]]|$1|S|3|T|5|6|7|1W|8|@]|D|@]|E|$]]|$1|U|3|V|5|H|7|1X|8|@]|D|@]|E|$I|J]]|$1|W|3|X|5|6|7|1Y|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|H|7|1Z|8|@]|D|@]|E|$I|J]]|$1|10|3|-4|5|6|7|20|8|@]|D|@]|E|$]]]|11|$]]

we have to use <code>string::size_type</code> for the return type of the find function otherwise the comparison with <code>string::npos</code> might not work.
<code>size_type</code>, which is defined by the allocator of the string, must be an <code>unsigned</code>
integral type. The default allocator, allocator, uses type <code>size_t</code> as <code>size_type</code>. Because <code>-1</code> is
converted into an unsigned integral type, npos is the maximum unsigned value of its type. However,
the exact value depends on the exact definition of type <code>size_type</code>. Unfortunately, these maximum
values differ. In fact, <code>(unsigned long)-1</code> differs from <code>(unsigned short)-</code>1 if the size of the
types differs. Thus, the comparison

<pre><code>idx == std::string::npos
</code></pre>

might yield false if idx has the value <code>-1</code> and idx and <code>string::npos</code> have different types:

<pre><code>std::string s;
...
int idx = s.find("not found"); // assume it returns npos
if (idx == std::string::npos) { // ERROR: comparison might not work
...
}
</code></pre>

One way to avoid this error is to check whether the search fails directly:

<pre><code>if (s.find("hi") == std::string::npos) {
...
}
</code></pre>

However, often you need the index of the matching character position. Thus, another simple solution
is to define your own signed value for npos:

<pre><code>const int NPOS = -1;
</code></pre>

Now the comparison looks a bit different and even more convenient:

<pre><code>if (idx == NPOS) { // works almost always
...
}
</code></pre>

blocks|key|1130863|text|$21.4+-+"static+const+size_type+npos+=+-1;"|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|1130864|它由字符串函数返回，指示错误/未找到等。|unstyled|1130865|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>$21.4 - "static const size_type npos = -1;"
</code></pre>

It is returned by string functions indicating error/not found etc.

blocks|key|1131036|text|当我们有std::optional的时候，这是对这些天C%2B%2B17的答案|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1131037|如果你稍微斜视一下，假装std::string::find()返回了一个std::optional<std::string::size_type>+(它应该...)-那么条件就变成了：|1131038|auto+position+=+str.find(str2);

if+(+position.has_value()+)+{
++++std::cout+<<+"first+'needle'+found+at:+"+<<+found.value()+<<+std::endl;
}|code-block|syntax|javascript|1131039|entityMap|0|LINK|mutability|MUTABLE|url|https://en.cppreference.com/w/cpp/utility/optional^0|4|D|4|D|0|0|C|J|10|11|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|F|3|G|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|15|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

An answer for these days of C++17, when we have <a href="https://en.cppreference.com/w/cpp/utility/optional" rel="nofollow noreferrer"><code>std::optional</code></a>:

If you squint a bit and pretend <code>std::string::find()</code> returns an <code>std::optional&lt;std::string::size_type&gt;</code> (which it sort of should...) - then the condition becomes:

<pre><code>auto position = str.find(str2);

if ( position.has_value() ) {
 std::cout &lt;&lt; "first 'needle' found at: " &lt;&lt; found.value() &lt;&lt; std::endl;
}
</code></pre>

blocks|key|916298|text|字符串：：npos的值为18446744073709551615。如果找不到字符串，则返回一个值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|916299|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Value of string::npos is 18446744073709551615. Its a value returned if there is no string found.

blocks|key|916220|text|静态常量size_t+npos+=+-1；|type|unstyled|depth|inlineStyleRanges|entityRanges|data|916221|size_t的最大值|916222|npos是一个静态成员常量值，对于size_t类型的元素，它具有最大的可能值。|916223|当该值用作字符串的成员函数中的len+(或sublen)参数的值时，表示“直到字符串结束”。|916224|作为返回值，它通常用于表示没有匹配项。|916225|此常量定义为值-1，因为size_t是无符号整数类型，所以它是此类型的最大可表示值。|916226|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|P|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|Q|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|R|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|S|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|M|$]]

static const size_t npos = -1;

Maximum value for size_t

npos is a static member constant value with the greatest possible value for an element of type size_t.

This value, when used as the value for a len (or sublen) parameter in string's member functions, means "until the end of the string".

As a return value, it is usually used to indicate no matches.

This constant is defined with a value of -1, which because size_t is an unsigned integral type, it is the largest possible representable value for this type.

What does the phrase <code>std::string::npos</code> mean in the following snippet of code?

<pre><code>found = str.find(str2);

if (found != std::string::npos)
 std::cout &lt;&lt; "first 'needle' found at: " &lt;&lt; int(found) &lt;&lt; std::endl;
</code></pre>

What does string::npos mean in this code?

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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在下面的代码片段中，短语std::string::npos是什么意思？found = str.find(str2);if (found != std::string::npos) std::cout << "first 'needle' found at: " << int(found) << std::endl;

问在这段代码中，string::npos是什么意思？
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问在这段代码中，string::npos是什么意思？EN

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问在这段代码中，string::npos是什么意思？
EN