blocks|key|1269301|text|另一种使用标准流的方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1269302|#include+<sstream>
#include+<iostream>
#include+<string>

int+main()
{
++++std::stringstream+convertor;
++++std::string+numberString+=+"Not+a+number!";
++++int+number;

++++convertor+<<+numberString;
++++convertor+>>+number;

++++if(convertor.fail())
++++{
++++++++//+numberString+is+not+a+number!
++++++++std::cout+<<+"Not+a+Number!";
++++}
}|code-block|syntax|javascript|1269303|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Another way using standard streams :

<pre><code>#include &lt;sstream&gt;
#include &lt;iostream&gt;
#include &lt;string&gt;

int main()
{
 std::stringstream convertor;
 std::string numberString = "Not a number!";
 int number;

 convertor &lt;&lt; numberString;
 convertor &gt;&gt; number;

 if(convertor.fail())
 {
 // numberString is not a number!
 std::cout &lt;&lt; "Not a Number!";
 }
}
</code></pre>

blocks|key|1483988|text|使用streams的其他答案将成功，即使字符串在有效数字后包含无效字符，例如"123abc“。我不熟悉boost，所以不能评论它的行为。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1483989|如果您想知道字符串是否包含数字并且只包含数字，则必须使用strtol：|1483990|#include+<iostream>
#include+<string>

int+main(void)
{
++++std::string+s;
++++std::cin+>>+s;

++++char+*end;
++++long+i+=+strtol(+s.c_str(),+&end,+10+);
++++if+(+*end+==+'\0'+)
++++{
++++++++//+Success
++++}
++++else
++++{
++++++++//+Failure
++++}
}|code-block|syntax|javascript|1483991|strtol返回一个指向结束解析的字符的指针，因此可以很容易地检查是否解析了整个字符串。|1483992|请注意，strtol返回的是long类型，而不是int类型，但根据编译器的不同，它们可能是相同的。在标准库中没有strtoi函数，只有atoi函数，它不返回解析结束字符。|1483993|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|P|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|Q|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

The other answers that use streams will succeed even if the string contains invalid characters after a valid number e.g. "123abc". I'm not familiar with boost, so can't comment on its behavior. 

If you want to know if the string contains a number and only a number, you have to use strtol:

<pre><code>#include &lt;iostream&gt;
#include &lt;string&gt;

int main(void)
{
 std::string s;
 std::cin &gt;&gt; s;

 char *end;
 long i = strtol( s.c_str(), &amp;end, 10 );
 if ( *end == '\0' )
 {
 // Success
 }
 else
 {
 // Failure
 }
}
</code></pre>

strtol returns a pointer to the character that ended the parse, so you can easily check if the entire string was parsed.

Note that strtol returns a long not an int, but depending on your compiler these are probably the same. There is no strtoi function in the standard library, only atoi, which doesn't return the parse ending character.

blocks|key|1269470|text|在boost的lexical_cast可用之前，我曾经做过以下事情：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1269471|namespace+detail+{

++++template<+typename+Target,+typename+Source+>
++++struct+stream_caster+{
++++++++static+Target+stream_cast(const+Source&+s)
++++++++{
++++++++++++std::stringstream+ss;
++++++++++++if(+(ss+<<+s).fail()+)+{
++++++++++++++++throw+std::bad_cast("could+not+stream+from+source");
++++++++++++}
++++++++++++Target+t;
++++++++++++if(+(ss+>>+t).fail()+%7C%7C+!(ss+>>+ws).eof())+{
++++++++++++++++throw+std::bad_cast("could+not+stream+to+target");
++++++++++++}
++++++++++++return+t;
++++++++}
++++};

++++template<+typename+T+>
++++struct+stream_caster<T,T>+{
++++++++static+const+T&+stream_cast(const+T&+s)
++++++++{
++++++++++++return+s;
++++++++}
++++};

++++template<+typename+Source+>
++++struct+stream_caster<std::string,Source>+{
++++++++static+std::string+stream_cast(const+Source&+s)
++++++++{
++++++++++++std::ostringstream+oss;
++++++++++++if(+(oss+<<+s).fail()+)+{
++++++++++++++++throw+std::bad_cast("could+not+stream+from+source");
++++++++++++}
++++++++++++return+oss.str();
++++++++}
++++};

++++template<+typename+Target+>
++++struct+stream_caster<Target,std::string>+{
++++++++static+Target+stream_cast(const+std::string&+s)
++++++++{
++++++++++++std::stringstream+ss(s);
++++++++++++Target+t;
++++++++++++if(+(ss+>>+t).fail()+%7C%7C+!(ss+>>+ws).eof())+{
++++++++++++++++throw+std::bad_cast("could+not+stream+to+target");
++++++++++++}
++++++++++++return+t;
++++++++}
++++};

++++template<>
++++struct+stream_caster<std::string,std::string>+{
++++++++static+const+std::string&+stream_cast(const+std::string&+s)
++++++++{
++++++++++++return+s;
++++++++}
++++};

}

template<+typename+Target,+typename+Source+>
inline+Target+stream_cast(const+Source&+s)
{
++++return+detail::stream_caster<Target,Source>::stream_cast(s);
}|code-block|syntax|javascript|1269472|entityMap^0|7|C|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|P|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Q|8|@]|D|@]|E|$]]]|L|$]]

Before boost's <code>lexical_cast</code> was available, I used to do the following:
<pre><code>namespace detail {

 template&lt; typename Target, typename Source &gt;
 struct stream_caster {
 static Target stream_cast(const Source&amp; s)
 {
 std::stringstream ss;
 if( (ss &lt;&lt; s).fail() ) {
 throw std::bad_cast(&quot;could not stream from source&quot;);
 }
 Target t;
 if( (ss &gt;&gt; t).fail() || !(ss &gt;&gt; ws).eof()) {
 throw std::bad_cast(&quot;could not stream to target&quot;);
 }
 return t;
 }
 };

 template&lt; typename T &gt;
 struct stream_caster&lt;T,T&gt; {
 static const T&amp; stream_cast(const T&amp; s)
 {
 return s;
 }
 };

 template&lt; typename Source &gt;
 struct stream_caster&lt;std::string,Source&gt; {
 static std::string stream_cast(const Source&amp; s)
 {
 std::ostringstream oss;
 if( (oss &lt;&lt; s).fail() ) {
 throw std::bad_cast(&quot;could not stream from source&quot;);
 }
 return oss.str();
 }
 };

 template&lt; typename Target &gt;
 struct stream_caster&lt;Target,std::string&gt; {
 static Target stream_cast(const std::string&amp; s)
 {
 std::stringstream ss(s);
 Target t;
 if( (ss &gt;&gt; t).fail() || !(ss &gt;&gt; ws).eof()) {
 throw std::bad_cast(&quot;could not stream to target&quot;);
 }
 return t;
 }
 };

 template&lt;&gt;
 struct stream_caster&lt;std::string,std::string&gt; {
 static const std::string&amp; stream_cast(const std::string&amp; s)
 {
 return s;
 }
 };

}

template&lt; typename Target, typename Source &gt;
inline Target stream_cast(const Source&amp; s)
{
 return detail::stream_caster&lt;Target,Source&gt;::stream_cast(s);
}
</code></pre>

I have a <code>std::string</code> which could be a string or could be a value (such as <code>0</code>).

What is the best or easiest way to convert the <code>std::string</code> to <code>int</code> with the ability to fail? I want a C++ version of C#'s <code>Int32.TryParse</code>.

Convert string to int with bool/fail in C++

编译器

我有一个std::string，它可以是一个字符串，也可以是一个值(比如0)。将std::string转换为具有故障能力的int的最佳或最简单的方法是什么？我想要一个C#的Int32.TryParse的C++版本。

问在C++中使用bool/fail将字符串转换为整型
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