blocks|key|325865|text|如果你已经在使用Boost，你可以使用boost+string+algorithms+%2B+boost+lexical+cast:来完成|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|325866|#include+<boost/algorithm/string/predicate.hpp>
#include+<boost/lexical_cast.hpp>

try+{++++
++++if+(boost::starts_with(argv[1],+"--foo="))
++++++++foo_value+=+boost::lexical_cast<int>(argv[1]%2B6);
}+catch+(boost::bad_lexical_cast)+{
++++//+bad+parameter
}|code-block|syntax|javascript|325867|这种方法，就像这里提供的许多其他答案一样，对于非常简单的任务是可以的，但从长远来看，使用命令行解析库通常会更好。Boost有一个(Boost.Program_options)，如果你碰巧已经在使用Boost，这可能是有意义的。|325868|否则，搜索"c%2B%2B命令行解析器“将会出现许多选项。|325869|entityMap|0|LINK|mutability|MUTABLE|url|http://www.boost.org/doc/libs/1_41_0/doc/html/string_algo.html|1|http://www.boost.org/doc/libs/1_41_0/libs/conversion/lexical_cast.htm|2|http://www.boost.org/doc/libs/1_65_1/doc/html/program_options.html^0|J|N|0|19|J|1|0|0|1T|L|2|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@$A|Z|B|10|1|11]|$A|12|B|13|1|14]]|C|$]]|$1|D|3|E|5|F|7|15|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|16|8|@]|9|@$A|17|B|18|1|19]]|C|$]]|$1|K|3|L|5|6|7|1A|8|@]|9|@]|C|$]]|$1|M|3|-4|5|6|7|1B|8|@]|9|@]|C|$]]]|N|$O|$5|P|Q|R|C|$S|T]]|U|$5|P|Q|R|C|$S|V]]|W|$5|P|Q|R|C|$S|X]]]]

If you're already using Boost, you can do it with <a href="http://www.boost.org/doc/libs/1_41_0/doc/html/string_algo.html" rel="noreferrer">boost string algorithms</a> + <a href="http://www.boost.org/doc/libs/1_41_0/libs/conversion/lexical_cast.htm" rel="noreferrer">boost lexical cast:</a>

<pre><code>#include &lt;boost/algorithm/string/predicate.hpp&gt;
#include &lt;boost/lexical_cast.hpp&gt;

try { 
 if (boost::starts_with(argv[1], "--foo="))
 foo_value = boost::lexical_cast&lt;int&gt;(argv[1]+6);
} catch (boost::bad_lexical_cast) {
 // bad parameter
}
</code></pre>

This kind of approach, like many of the other answers provided here is ok for very simple tasks, but in the long run you are usually better off using a command line parsing library. Boost has one (<a href="http://www.boost.org/doc/libs/1_65_1/doc/html/program_options.html" rel="noreferrer">Boost.Program_options</a>), which may make sense if you happen to be using Boost already.

Otherwise a search for "c++ command line parser" will yield a number of options.

blocks|key|323148|text|你可以这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|323149|std::string+prefix("--foo=");
if+(!arg.compare(0,+prefix.size(),+prefix))
++++foo_value+=+std::stoi(arg.substr(prefix.size()));|code-block|syntax|javascript|323150|寻找像Boost.ProgramOptions这样的库来为您做这件事也是一个好主意。|323151|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

You would do it like this:
<pre><code>std::string prefix(&quot;--foo=&quot;);
if (!arg.compare(0, prefix.size(), prefix))
 foo_value = std::stoi(arg.substr(prefix.size()));
</code></pre>
Looking for a lib such as Boost.ProgramOptions that does this for you is also a good idea.

blocks|key|323157|text|if(boost::starts_with(string_to_search,+string_to_look_for))
++++intval+=+boost::lexical_cast<int>(string_to_search.substr(string_to_look_for.length()));|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|323158|这是完全未经测试的。其原理与Python相同。需要Boost.StringAlgo和Boost.LexicalCast。|unstyled|323159|检查字符串是否以另一个字符串开头，然后获取第一个字符串的子字符串('slice')，并使用词法转换对其进行转换。|323160|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|L|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|F|7|N|8|@]|9|@]|A|$]]]|J|$]]

<pre><code>if(boost::starts_with(string_to_search, string_to_look_for))
 intval = boost::lexical_cast&lt;int&gt;(string_to_search.substr(string_to_look_for.length()));
</code></pre>

This is completely untested. The principle is the same as the Python one. Requires Boost.StringAlgo and Boost.LexicalCast.

Check if the string starts with the other string, and then get the substring ('slice') of the first string and convert it using lexical cast.

blocks|key|323221|text|冒着使用C结构而被激怒的风险，我确实认为这个sscanf示例比大多数Boost解决方案更优雅。如果你在任何有Python解释器的地方运行，你也不必担心链接问题！|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|323222|#include+<stdio.h>
#include+<string.h>

int+main(int+argc,+char+**argv)
{
++++for+(int+i+=+1;+i+!=+argc;+%2B%2Bi)+{
++++++++int+number+=+0;
++++++++int+size+=+0;
++++++++sscanf(argv[i],+"--foo=%25d%25n",+&number,+&size);
++++++++if+(size+==+strlen(argv[i]))+{
++++++++++++printf("number:+%25d\n",+number);
++++++++}
++++++++else+{
++++++++++++printf("not-a-number\n");
++++++++}
++++}
++++return+0;
}|code-block|syntax|javascript|323223|下面是一些示例输出，演示了该解决方案处理前导/尾随垃圾的方式与等效的Python代码一样正确，而且比使用atoi+(它会错误地忽略非数字后缀)更正确。|323224|$+./scan+--foo=2+--foo=2d+--foo='2+'+'+--foo=2'
number:+2
not-a-number
not-a-number
not-a-number|323225|entityMap^0|M|6|0|0|1G|4|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|T|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

At the risk of being flamed for using C constructs, I do think this <code>sscanf</code> example is more elegant than most Boost solutions. And you don't have to worry about linkage if you're running anywhere that has a Python interpreter!

<pre><code>#include &lt;stdio.h&gt;
#include &lt;string.h&gt;

int main(int argc, char **argv)
{
 for (int i = 1; i != argc; ++i) {
 int number = 0;
 int size = 0;
 sscanf(argv[i], "--foo=%d%n", &amp;number, &amp;size);
 if (size == strlen(argv[i])) {
 printf("number: %d\n", number);
 }
 else {
 printf("not-a-number\n");
 }
 }
 return 0;
}
</code></pre>

Here's some example output that demonstrates the solution handles leading/trailing garbage as correctly as the equivalent Python code, and more correctly than anything using <code>atoi</code> (which will erroneously ignore a non-numeric suffix).

<pre><code>$ ./scan --foo=2 --foo=2d --foo='2 ' ' --foo=2'
number: 2
not-a-number
not-a-number
not-a-number
</code></pre>

blocks|key|323227|text|好吧，为什么要复杂地使用库之类的东西呢？C%2B%2B字符串对象重载了[]运算符，因此您可以只比较字符。就像我刚才所做的，因为我想列出一个目录中的所有文件，并忽略不可见的文件和..还有。伪片。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|323228|while+((ep+=+readdir(dp)))
{
++++string+s(ep->d_name);
++++if+(!(s[0]+==+'.'))+//+Omit+invisible+files+and+..+or+.
++++++++files.push_back(s);
}|code-block|syntax|javascript|323229|就这么简单..。|323230|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Ok why the complicated use of libraries and stuff? C++ String objects overload the [] operator, so you can just compare chars.. Like what I just did, because I want to list all files in a directory and ignore invisible files and the .. and . pseudofiles. 

<pre><code>while ((ep = readdir(dp)))
{
 string s(ep-&gt;d_name);
 if (!(s[0] == '.')) // Omit invisible files and .. or .
 files.push_back(s);
}
</code></pre>

It's that simple..

blocks|key|3074889|text|使用STL，这可能看起来像这样：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3074890|std::string+prefix+=+"--foo=";
std::string+arg+=+argv[1];
if+(prefix.size()<=arg.size()+&&+std::equal(prefix.begin(),+prefix.end(),+arg.begin()))+{
++std::istringstream+iss(arg.substr(prefix.size()));
++iss+>>+foo_value;
}|code-block|syntax|javascript|3074891|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|$]]

Using STL this could look like:

<pre><code>std::string prefix = "--foo=";
std::string arg = argv[1];
if (prefix.size()&lt;=arg.size() &amp;&amp; std::equal(prefix.begin(), prefix.end(), arg.begin())) {
 std::istringstream iss(arg.substr(prefix.size()));
 iss &gt;&gt; foo_value;
}
</code></pre>

blocks|key|3288307|text|我自己使用的代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3288308|std::string+prefix+=+"-param=";
std::string+argument+=+argv[1];
if(argument.substr(0,+prefix.size())+==+prefix)+{
++++std::string+argumentValue+=+argument.substr(prefix.size());
}|code-block|syntax|javascript|3288309|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|$]]

Code I use myself:

<pre><code>std::string prefix = "-param=";
std::string argument = argv[1];
if(argument.substr(0, prefix.size()) == prefix) {
 std::string argumentValue = argument.substr(prefix.size());
}
</code></pre>

blocks|key|3288311|text|您也可以使用strstr|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3288312|if+(strstr(str,+substr)+==+substr)+{
++++//+'str'+starts+with+'substr'
}|code-block|syntax|javascript|3288313|但我认为它只适用于短字符串，因为当字符串实际上不是以'substr‘开头时，它必须遍历整个字符串。|3288314|entityMap^0|6|6|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

You can also use <code>strstr</code>:

<pre><code>if (strstr(str, substr) == substr) {
 // 'str' starts with 'substr'
}
</code></pre>

but I think it's good only for short strings because it has to loop through the whole string when the string doesn't actually start with 'substr'.

blocks|key|3074953|text|还没有人使用STL+algorithm/mismatch函数。如果返回true，则prefix是‘toCheck’的前缀：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3074954|std::mismatch(prefix.begin(),+prefix.end(),+toCheck.begin()).first+==+prefix.end()|code-block|syntax|javascript|3074955|完整的prog示例：|3074956|#include+<algorithm>
#include+<string>
#include+<iostream>

int+main(int+argc,+char**+argv)+{
++++if+(argc+!=+3)+{
++++++++std::cerr+<<+"Usage:+"+<<+argv[0]+<<+"+prefix+string"+<<+std::endl
++++++++++++++++++<<+"Will+print+true+if+'prefix'+is+a+prefix+of+string"+<<+std::endl;
++++++++return+-1;
++++}
++++std::string+prefix(argv[1]);
++++std::string+toCheck(argv[2]);
++++if+(prefix.length()+>+toCheck.length())+{
++++++++std::cerr+<<+"Usage:+"+<<+argv[0]+<<+"+prefix+string"+<<+std::endl
++++++++++++++++++<<+"'prefix'+is+longer+than+'string'"+<<++std::endl;
++++++++return+2;
++++}
++++if+(std::mismatch(prefix.begin(),+prefix.end(),+toCheck.begin()).first+==+prefix.end())+{
++++++++std::cout+<<+'"'+<<+prefix+<<+'"'+<<+"+is+a+prefix+of+"+<<+'"'+<<+toCheck+<<+'"'+<<+std::endl;
++++++++return+0;
++++}+else+{
++++++++std::cout+<<+'"'+<<+prefix+<<+'"'+<<+"+is+NOT+a+prefix+of+"+<<+'"'+<<+toCheck+<<+'"'+<<+std::endl;
++++++++return+1;
++++}
}|3074957|3074958|编辑：|style|BOLD|3074959|正如@James+T.+Huggett所建议的，std::equal更适合这个问题:A是B的前缀吗？并且代码略短：|3074960|std::equal(prefix.begin(),+prefix.end(),+toCheck.begin())|3074961|3074962|#include+<algorithm>
#include+<string>
#include+<iostream>

int+main(int+argc,+char+**argv)+{
++if+(argc+!=+3)+{
++++std::cerr+<<+"Usage:+"+<<+argv[0]+<<+"+prefix+string"+<<+std::endl
++++++++++++++<<+"Will+print+true+if+'prefix'+is+a+prefix+of+string"
++++++++++++++<<+std::endl;
++++return+-1;
++}
++std::string+prefix(argv[1]);
++std::string+toCheck(argv[2]);
++if+(prefix.length()+>+toCheck.length())+{
++++std::cerr+<<+"Usage:+"+<<+argv[0]+<<+"+prefix+string"+<<+std::endl
++++++++++++++<<+"'prefix'+is+longer+than+'string'"+<<+std::endl;
++++return+2;
++}
++if+(std::equal(prefix.begin(),+prefix.end(),+toCheck.begin()))+{
++++std::cout+<<+'"'+<<+prefix+<<+'"'+<<+"+is+a+prefix+of+"+<<+'"'+<<+toCheck
++++++++++++++<<+'"'+<<+std::endl;
++++return+0;
++}+else+{
++++std::cout+<<+'"'+<<+prefix+<<+'"'+<<+"+is+NOT+a+prefix+of+"+<<+'"'
++++++++++++++<<+toCheck+<<+'"'+<<+std::endl;
++++return+1;
++}
}|3074963|entityMap|0|LINK|mutability|MUTABLE|url|http://www.cplusplus.com/reference/algorithm/mismatch/^0|A|I|0|0|0|0|0|0|0|3|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@$A|17|B|18|1|19]]|C|$]]|$1|D|3|E|5|F|7|1A|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|1B|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|1C|8|@]|9|@]|C|$G|H]]|$1|M|3|-4|5|6|7|1D|8|@]|9|@]|C|$]]|$1|N|3|O|5|6|7|1E|8|@$A|1F|B|1G|P|Q]]|9|@]|C|$]]|$1|R|3|S|5|6|7|1H|8|@]|9|@]|C|$]]|$1|T|3|U|5|F|7|1I|8|@]|9|@]|C|$G|H]]|$1|V|3|J|5|6|7|1J|8|@]|9|@]|C|$]]|$1|W|3|X|5|F|7|1K|8|@]|9|@]|C|$G|H]]|$1|Y|3|-4|5|6|7|1L|8|@]|9|@]|C|$]]]|Z|$10|$5|11|12|13|C|$14|15]]]]

Nobody used the STL <a href="http://www.cplusplus.com/reference/algorithm/mismatch/">algorithm/mismatch</a> function yet. If this returns true, prefix is a prefix of 'toCheck':

<pre><code>std::mismatch(prefix.begin(), prefix.end(), toCheck.begin()).first == prefix.end()
</code></pre>

Full example prog:

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

int main(int argc, char** argv) {
 if (argc != 3) {
 std::cerr &lt;&lt; "Usage: " &lt;&lt; argv[0] &lt;&lt; " prefix string" &lt;&lt; std::endl
 &lt;&lt; "Will print true if 'prefix' is a prefix of string" &lt;&lt; std::endl;
 return -1;
 }
 std::string prefix(argv[1]);
 std::string toCheck(argv[2]);
 if (prefix.length() &gt; toCheck.length()) {
 std::cerr &lt;&lt; "Usage: " &lt;&lt; argv[0] &lt;&lt; " prefix string" &lt;&lt; std::endl
 &lt;&lt; "'prefix' is longer than 'string'" &lt;&lt; std::endl;
 return 2;
 }
 if (std::mismatch(prefix.begin(), prefix.end(), toCheck.begin()).first == prefix.end()) {
 std::cout &lt;&lt; '"' &lt;&lt; prefix &lt;&lt; '"' &lt;&lt; " is a prefix of " &lt;&lt; '"' &lt;&lt; toCheck &lt;&lt; '"' &lt;&lt; std::endl;
 return 0;
 } else {
 std::cout &lt;&lt; '"' &lt;&lt; prefix &lt;&lt; '"' &lt;&lt; " is NOT a prefix of " &lt;&lt; '"' &lt;&lt; toCheck &lt;&lt; '"' &lt;&lt; std::endl;
 return 1;
 }
}
</code></pre>

<hr>

Edit:

As @James T. Huggett suggests, std::equal is a better fit for the question: Is A a prefix of B? and is slight shorter code:

<pre><code>std::equal(prefix.begin(), prefix.end(), toCheck.begin())
</code></pre>

Full example prog:

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

int main(int argc, char **argv) {
 if (argc != 3) {
 std::cerr &lt;&lt; "Usage: " &lt;&lt; argv[0] &lt;&lt; " prefix string" &lt;&lt; std::endl
 &lt;&lt; "Will print true if 'prefix' is a prefix of string"
 &lt;&lt; std::endl;
 return -1;
 }
 std::string prefix(argv[1]);
 std::string toCheck(argv[2]);
 if (prefix.length() &gt; toCheck.length()) {
 std::cerr &lt;&lt; "Usage: " &lt;&lt; argv[0] &lt;&lt; " prefix string" &lt;&lt; std::endl
 &lt;&lt; "'prefix' is longer than 'string'" &lt;&lt; std::endl;
 return 2;
 }
 if (std::equal(prefix.begin(), prefix.end(), toCheck.begin())) {
 std::cout &lt;&lt; '"' &lt;&lt; prefix &lt;&lt; '"' &lt;&lt; " is a prefix of " &lt;&lt; '"' &lt;&lt; toCheck
 &lt;&lt; '"' &lt;&lt; std::endl;
 return 0;
 } else {
 std::cout &lt;&lt; '"' &lt;&lt; prefix &lt;&lt; '"' &lt;&lt; " is NOT a prefix of " &lt;&lt; '"'
 &lt;&lt; toCheck &lt;&lt; '"' &lt;&lt; std::endl;
 return 1;
 }
}
</code></pre>

blocks|key|323881|text|为什么不使用gnu+getopts呢？这是一个基本的例子(没有安全检查)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|323882|#include+<getopt.h>
#include+<stdio.h>

int+main(int+argc,+char**+argv)
{
++option+long_options[]+=+{
++++{"foo",+required_argument,+0,+0},
++++{0,0,0,0}
++};

++getopt_long(argc,+argv,+"f:",+long_options,+0);

++printf("%25s\n",+optarg);
}|code-block|syntax|javascript|323883|对于以下命令：|323884|$+./a.out+--foo=33|323885|你会得到|323886|33|323887|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Why not use gnu getopts? Here's a basic example (without safety checks):

<pre><code>#include &lt;getopt.h&gt;
#include &lt;stdio.h&gt;

int main(int argc, char** argv)
{
 option long_options[] = {
 {"foo", required_argument, 0, 0},
 {0,0,0,0}
 };

 getopt_long(argc, argv, "f:", long_options, 0);

 printf("%s\n", optarg);
}
</code></pre>

For the following command:

<pre><code>$ ./a.out --foo=33
</code></pre>

You will get

<pre><code>33
</code></pre>

blocks|key|3074995|text|考虑到argv[1]和"--foo"这两个字符串都是C字符串，@FelixDombek's+answer无疑是最好的解决方案。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3074996|然而，看到其他答案，我认为值得注意的是，如果您的文本已经以std::string的形式提供，那么存在一个简单的、零拷贝的、效率最高的解决方案，到目前为止还没有提到：|3074997|const+char+*+foo+=+"--foo";
if+(text.rfind(foo,+0)+==+0)
++++foo_value+=+text.substr(strlen(foo));|code-block|syntax|javascript|3074998|如果foo已经是一个字符串：|3074999|std::string+foo("--foo");
if+(text.rfind(foo,+0)+==+0)
++++foo_value+=+text.substr(foo.length());|3075000|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/7142763/9990^0|3|7|B|7|V|L|0|0|T|B|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@$9|13|A|14|1|15]]|E|$]]|$1|F|3|G|5|6|7|16|8|@$9|17|A|18|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|19|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1A|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|1B|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|R|$S|$5|T|U|V|E|$W|X]]]]

Given that both strings — <code>argv[1]</code> and <code>"--foo"</code> — are C strings, <a href="https://stackoverflow.com/a/7142763/9990">@FelixDombek's answer</a> is hands-down the best solution.

Seeing the other answers, however, I thought it worth noting that, if your text is already available as a <code>std::string</code>, then a simple, zero-copy, maximally efficient solution exists that hasn't been mentioned so far:

<pre><code>const char * foo = "--foo";
if (text.rfind(foo, 0) == 0)
 foo_value = text.substr(strlen(foo));
</code></pre>

And if foo is already a string:

<pre><code>std::string foo("--foo");
if (text.rfind(foo, 0) == 0)
 foo_value = text.substr(foo.length());
</code></pre>

blocks|key|3288416|text|使用带有搜索位置pos参数的rfind重载，并为其传递零：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3288417|std::string+s+=+"tititoto";
if+(s.rfind("titi",+0)+==+0)+{+//+pos=0+limits+the+search+to+the+prefix
++//+s+starts+with+prefix
}|code-block|syntax|javascript|3288418|还需要别的什么吗？纯STL！|3288419|许多人将其误读为“在整个字符串中反向搜索前缀”。这将给出错误的结果(例如，string("tititito").rfind("titi")返回2，因此与==+0相比将返回false)，而且效率将很低(查看整个字符串，而不仅仅是开始部分)。但它没有这样做，因为它将pos参数作为0传递，这将搜索限制为仅在该位置或更早的位置匹配。例如：|3288420|std::string+test+=+"0123123";
size_t+match1+=+test.rfind("123");++++//+returns+4+(rightmost+match)
size_t+match2+=+test.rfind("123",+2);+//+returns+1+(skipped+over+later+match)
size_t+match3+=+test.rfind("123",+0);+//+returns+std::string::npos+(i.e.+not+found)|3288421|entityMap|0|LINK|mutability|MUTABLE|url|http://www.cplusplus.com/reference/string/string/rfind/^0|8|3|E|5|E|5|0|0|0|0|11|W|24|4|3N|3|3U|1|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@$9|13|A|14|1|15]]|E|$]]|$1|F|3|G|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|17|8|@]|D|@]|E|$]]|$1|M|3|N|5|6|7|18|8|@$9|19|A|1A|B|C]|$9|1B|A|1C|B|C]|$9|1D|A|1E|B|C]|$9|1F|A|1G|B|C]]|D|@]|E|$]]|$1|O|3|P|5|H|7|1H|8|@]|D|@]|E|$I|J]]|$1|Q|3|-4|5|6|7|1I|8|@]|D|@]|E|$]]]|R|$S|$5|T|U|V|E|$W|X]]]]

Use <a href="http://www.cplusplus.com/reference/string/string/rfind/" rel="noreferrer"><code>rfind</code></a> overload that takes the search position <code>pos</code> parameter, and pass zero for it:
<pre><code>std::string s = &quot;tititoto&quot;;
if (s.rfind(&quot;titi&quot;, 0) == 0) { // pos=0 limits the search to the prefix
 // s starts with prefix
}
</code></pre>
Who needs anything else? Pure STL!
Many have misread this to mean &quot;search backwards through the whole string looking for the prefix&quot;. That would give the wrong result (e.g. <code>string(&quot;tititito&quot;).rfind(&quot;titi&quot;)</code> returns 2 so when compared against <code>== 0</code> would return false) and it would be inefficient (looking through the whole string instead of just the start). But it does not do that because it passes the <code>pos</code> parameter as <code>0</code>, which limits the search to only match at that position or earlier. For example:
<pre><code>std::string test = &quot;0123123&quot;;
size_t match1 = test.rfind(&quot;123&quot;); // returns 4 (rightmost match)
size_t match2 = test.rfind(&quot;123&quot;, 2); // returns 1 (skipped over later match)
size_t match3 = test.rfind(&quot;123&quot;, 0); // returns std::string::npos (i.e. not found)
</code></pre>

blocks|key|3075029|text|std::string+text+=+"--foo=98";
std::string+start+=+"--foo=";

if+(text.find(start)+==+0)
{
++++int+n+=+stoi(text.substr(start.length()));
++++std::cout+<<+n+<<+std::endl;
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3075030|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>std::string text = "--foo=98";
std::string start = "--foo=";

if (text.find(start) == 0)
{
 int n = stoi(text.substr(start.length()));
 std::cout &lt;&lt; n &lt;&lt; std::endl;
}
</code></pre>

blocks|key|321443|text|我使用包装在实用方法中的std::string::compare，如下所示：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|321444|static+bool+startsWith(const+string&+s,+const+string&+prefix)+{
++++return+s.size()+>=+prefix.size()+&&+s.compare(0,+prefix.size(),+prefix)+==+0;
}|code-block|syntax|javascript|321445|entityMap^0|C|K|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|$]]

I use <code>std::string::compare</code> wrapped in utility method like below:

<pre><code>static bool startsWith(const string&amp; s, const string&amp; prefix) {
 return s.size() &gt;= prefix.size() &amp;&amp; s.compare(0, prefix.size(), prefix) == 0;
}
</code></pre>

blocks|key|3288436|text|text.substr(0,+start.length())+==+start|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|3288437|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code>text.substr(0, start.length()) == start
</code></pre>

blocks|key|3288439|text|对于C%2B%2B17，您可以将std::basic_string_view与C%2B%2B20、std::basic_string::starts_with或std::basic_string_view::starts_with一起使用。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3288440|与std::string相比，std::string_view在内存管理方面的优势在于，它只持有一个指向“字符串”(类字符对象的连续序列)的指针，并且知道它的大小。示例中没有移动/复制源字符串，只是为了获得整数值：|3288441|#include+<exception>
#include+<iostream>
#include+<string>
#include+<string_view>

int+main()
{
++++constexpr+auto+argument+=+"--foo=42";+//+Emulating+command+argument.
++++constexpr+auto+prefix+=+"--foo=";
++++auto+inputValue+=+0;

++++constexpr+auto+argumentView+=+std::string_view(argument);
++++if+(argumentView.starts_with(prefix))
++++{
++++++++constexpr+auto+prefixSize+=+std::string_view(prefix).size();
++++++++try
++++++++{
++++++++++++//+The+underlying+data+of+argumentView+is+nul-terminated,+therefore+we+can+use+data().
++++++++++++inputValue+=+std::stoi(argumentView.substr(prefixSize).data());
++++++++}
++++++++catch+(std::exception+&+e)
++++++++{
++++++++++++std::cerr+<<+e.what();
++++++++}
++++}
++++std::cout+<<+inputValue;+//+42
}|code-block|syntax|javascript|3288442|entityMap|0|LINK|mutability|MUTABLE|url|http://en.cppreference.com/w/cpp/string/basic_string_view|1|http://en.cppreference.com/w/cpp/string/basic_string/starts_with|2|http://en.cppreference.com/w/cpp/string/basic_string_view/starts_with^0|C|M|15|U|20|Z|C|M|0|15|U|1|20|Z|2|0|1|B|F|G|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@$9|15|A|16|1|17]|$9|18|A|19|1|1A]|$9|1B|A|1C|1|1D]]|E|$]]|$1|F|3|G|5|6|7|1E|8|@$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|1J|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|1K|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]|U|$5|P|Q|R|E|$S|V]]|W|$5|P|Q|R|E|$S|X]]]]

With C++17 you can use <a href="http://en.cppreference.com/w/cpp/string/basic_string_view" rel="noreferrer"><code>std::basic_string_view</code></a> &amp; with C++20 <a href="http://en.cppreference.com/w/cpp/string/basic_string/starts_with" rel="noreferrer"><code>std::basic_string::starts_with</code></a> or <a href="http://en.cppreference.com/w/cpp/string/basic_string_view/starts_with" rel="noreferrer"><code>std::basic_string_view::starts_with</code></a>. 

The benefit of <code>std::string_view</code> in comparison to <code>std::string</code> - regarding memory management - is that it only holds a pointer to a "string" (contiguous sequence of char-like objects) and knows its size. Example without moving/copying the source strings just to get the integer value:

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

int main()
{
 constexpr auto argument = "--foo=42"; // Emulating command argument.
 constexpr auto prefix = "--foo=";
 auto inputValue = 0;

 constexpr auto argumentView = std::string_view(argument);
 if (argumentView.starts_with(prefix))
 {
 constexpr auto prefixSize = std::string_view(prefix).size();
 try
 {
 // The underlying data of argumentView is nul-terminated, therefore we can use data().
 inputValue = std::stoi(argumentView.substr(prefixSize).data());
 }
 catch (std::exception &amp; e)
 {
 std::cerr &lt;&lt; e.what();
 }
 }
 std::cout &lt;&lt; inputValue; // 42
}
</code></pre>

blocks|key|323910|text|因为C%2B%2B11+std::regex_search还可以用于提供更复杂的表达式匹配。下面的示例还处理通过std::stof和后续转换为int的浮点数。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|323911|但是，如果前缀不匹配，下面显示的parseInt方法可能会抛出std::invalid_argument异常；这可以根据给定的应用程序轻松调整：|323912|#include+<iostream>
#include+<regex>

int+parseInt(const+std::string+&str,+const+std::string+&prefix)+{
++std::smatch+match;
++std::regex_search(str,+match,+std::regex("%5E"+%2B+prefix+%2B+"([%2B-]?(?=\\.?\\d)\\d*(?:\\.\\d*)?(?:[Ee][%2B-]?\\d%2B)?)$"));
++return+std::stof(match[1]);
}

int+main()+{
++++std::cout+<<+parseInt("foo=13.3",+"foo=")+<<+std::endl;
++++std::cout+<<+parseInt("foo=-.9",+"foo=")+<<+std::endl;
++++std::cout+<<+parseInt("foo=%2B13.3",+"foo=")+<<+std::endl;
++++std::cout+<<+parseInt("foo=-0.133",+"foo=")+<<+std::endl;
++++std::cout+<<+parseInt("foo=%2B00123456",+"foo=")+<<+std::endl;
++++std::cout+<<+parseInt("foo=-06.12e%2B3",+"foo=")+<<+std::endl;

//++++throw+std::invalid_argument
//++++std::cout+<<+parseInt("foo=1",+"bar=")+<<+std::endl;

++++return+0;
}|code-block|syntax|javascript|323913|下面的answer详细介绍了正则表达式模式的魔力。|323914|EDIT:上一个答案没有执行到整数的转换。|BOLD|323915|entityMap|0|LINK|mutability|MUTABLE|url|http://en.cppreference.com/w/cpp/regex/regex_search|1|https://en.cppreference.com/w/cpp/string/basic_string/stof|2|https://en.cppreference.com/w/cpp/error/invalid_argument|3|https://stackoverflow.com/questions/9853950/this-regular-expression-fail-to-parse-all-valid-floating-numbers^0|8|H|1G|9|1V|3|8|H|0|1G|9|1|0|G|8|V|L|V|L|2|0|0|3|6|3|0|0|5|0^^$0|@$1|2|3|4|5|6|7|15|8|@$9|16|A|17|B|C]|$9|18|A|19|B|C]|$9|1A|A|1B|B|C]]|D|@$9|1C|A|1D|1|1E]|$9|1F|A|1G|1|1H]]|E|$]]|$1|F|3|G|5|6|7|1I|8|@$9|1J|A|1K|B|C]|$9|1L|A|1M|B|C]]|D|@$9|1N|A|1O|1|1P]]|E|$]]|$1|H|3|I|5|J|7|1Q|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1R|8|@]|D|@$9|1S|A|1T|1|1U]]|E|$]]|$1|O|3|P|5|6|7|1V|8|@$9|1W|A|1X|B|Q]]|D|@]|E|$]]|$1|R|3|-4|5|6|7|1Y|8|@]|D|@]|E|$]]]|S|$T|$5|U|V|W|E|$X|Y]]|Z|$5|U|V|W|E|$X|10]]|11|$5|U|V|W|E|$X|12]]|13|$5|U|V|W|E|$X|14]]]]

Since C++11 <a href="http://en.cppreference.com/w/cpp/regex/regex_search" rel="nofollow noreferrer"><code>std::regex_search</code></a> can also be used to provide even more complex expressions matching. The following example handles also floating numbers thorugh <a href="https://en.cppreference.com/w/cpp/string/basic_string/stof" rel="nofollow noreferrer"><code>std::stof</code></a> and a subsequent cast to <code>int</code>.

However the <code>parseInt</code> method shown below could throw a <a href="https://en.cppreference.com/w/cpp/error/invalid_argument" rel="nofollow noreferrer"><code>std::invalid_argument</code></a> exception if the prefix is not matched; this can be easily adapted depending on the given application:

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

int parseInt(const std::string &amp;str, const std::string &amp;prefix) {
 std::smatch match;
 std::regex_search(str, match, std::regex("^" + prefix + "([+-]?(?=\\.?\\d)\\d*(?:\\.\\d*)?(?:[Ee][+-]?\\d+)?)$"));
 return std::stof(match[1]);
}

int main() {
 std::cout &lt;&lt; parseInt("foo=13.3", "foo=") &lt;&lt; std::endl;
 std::cout &lt;&lt; parseInt("foo=-.9", "foo=") &lt;&lt; std::endl;
 std::cout &lt;&lt; parseInt("foo=+13.3", "foo=") &lt;&lt; std::endl;
 std::cout &lt;&lt; parseInt("foo=-0.133", "foo=") &lt;&lt; std::endl;
 std::cout &lt;&lt; parseInt("foo=+00123456", "foo=") &lt;&lt; std::endl;
 std::cout &lt;&lt; parseInt("foo=-06.12e+3", "foo=") &lt;&lt; std::endl;

// throw std::invalid_argument
// std::cout &lt;&lt; parseInt("foo=1", "bar=") &lt;&lt; std::endl;

 return 0;
}
</code></pre>

The kind of magic of the regex pattern is well detailed in the following <a href="https://stackoverflow.com/questions/9853950/this-regular-expression-fail-to-parse-all-valid-floating-numbers">answer</a>.

EDIT: the previous answer did not performed the conversion to integer.

blocks|key|3075052|text|如果您需要与C%2B%2B11兼容但不能使用boost，这里有一个boost兼容插件，其中包含一个使用示例：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3075053|#include+<iostream>
#include+<string>

static+bool+starts_with(const+std::string+str,+const+std::string+prefix)
{
++++return+((prefix.size()+<=+str.size())+&&+std::equal(prefix.begin(),+prefix.end(),+str.begin()));
}

int+main(int+argc,+char*+argv[])
{
++++bool+usage+=+false;
++++unsigned+int+foos+=+0;+//+default+number+of+foos+if+no+parameter+was+supplied

++++if+(argc+>+1)
++++{
++++++++const+std::string+fParamPrefix+=+"-f=";+//+shorthand+for+foo
++++++++const+std::string+fooParamPrefix+=+"--foo=";

++++++++for+(unsigned+int+i+=+1;+i+<+argc;+%2B%2Bi)
++++++++{
++++++++++++const+std::string+arg+=+argv[i];

++++++++++++try
++++++++++++{
++++++++++++++++if+((arg+==+"-h")+%7C%7C+(arg+==+"--help"))
++++++++++++++++{
++++++++++++++++++++usage+=+true;
++++++++++++++++}+else+if+(starts_with(arg,+fParamPrefix))+{
++++++++++++++++++++foos+=+std::stoul(arg.substr(fParamPrefix.size()));
++++++++++++++++}+else+if+(starts_with(arg,+fooParamPrefix))+{
++++++++++++++++++++foos+=+std::stoul(arg.substr(fooParamPrefix.size()));
++++++++++++++++}
++++++++++++}+catch+(std::exception&+e)+{
++++++++++++++++std::cerr+<<+"Invalid+parameter:+"+<<+argv[i]+<<+std::endl+<<+std::endl;
++++++++++++++++usage+=+true;
++++++++++++}
++++++++}
++++}

++++if+(usage)
++++{
++++++++std::cerr+<<+"Usage:+"+<<+argv[0]+<<+"+[OPTION]..."+<<+std::endl;
++++++++std::cerr+<<+"Example+program+for+parameter+parsing."+<<+std::endl+<<+std::endl;
++++++++std::cerr+<<+"++-f,+--foo=N+++use+N+foos+(optional)"+<<+std::endl;
++++++++return+1;
++++}

++++std::cerr+<<+"number+of+foos+given:+"+<<+foos+<<+std::endl;
}|code-block|syntax|javascript|3075054|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|$]]

In case you need C++11 compatibility and cannot use boost, here is a boost-compatible drop-in with an example of usage:

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

static bool starts_with(const std::string str, const std::string prefix)
{
 return ((prefix.size() &lt;= str.size()) &amp;&amp; std::equal(prefix.begin(), prefix.end(), str.begin()));
}

int main(int argc, char* argv[])
{
 bool usage = false;
 unsigned int foos = 0; // default number of foos if no parameter was supplied

 if (argc &gt; 1)
 {
 const std::string fParamPrefix = "-f="; // shorthand for foo
 const std::string fooParamPrefix = "--foo=";

 for (unsigned int i = 1; i &lt; argc; ++i)
 {
 const std::string arg = argv[i];

 try
 {
 if ((arg == "-h") || (arg == "--help"))
 {
 usage = true;
 } else if (starts_with(arg, fParamPrefix)) {
 foos = std::stoul(arg.substr(fParamPrefix.size()));
 } else if (starts_with(arg, fooParamPrefix)) {
 foos = std::stoul(arg.substr(fooParamPrefix.size()));
 }
 } catch (std::exception&amp; e) {
 std::cerr &lt;&lt; "Invalid parameter: " &lt;&lt; argv[i] &lt;&lt; std::endl &lt;&lt; std::endl;
 usage = true;
 }
 }
 }

 if (usage)
 {
 std::cerr &lt;&lt; "Usage: " &lt;&lt; argv[0] &lt;&lt; " [OPTION]..." &lt;&lt; std::endl;
 std::cerr &lt;&lt; "Example program for parameter parsing." &lt;&lt; std::endl &lt;&lt; std::endl;
 std::cerr &lt;&lt; " -f, --foo=N use N foos (optional)" &lt;&lt; std::endl;
 return 1;
 }

 std::cerr &lt;&lt; "number of foos given: " &lt;&lt; foos &lt;&lt; std::endl;
}
</code></pre>

blocks|key|323922|text|对于C%2B%2B11或更高版本，您可以使用find()和find_first_of()|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|323923|使用find查找单个字符的示例：|323924|#include+<string>
std::string+name+=+"Aaah";
size_t+found_index+=+name.find('a');
if+(found_index+!=+std::string::npos)+{
++++//+Found+string+containing+'a'
}|code-block|syntax|javascript|323925|使用find查找从位置5开始的完整字符串的示例：|323926|std::string+name+=+"Aaah";
size_t+found_index+=+name.find('h',+3);
if+(found_index+!=+std::string::npos)+{
++++//+Found+string+containing+'h'
}|323927|使用find_first_of()和仅第一个字符的示例，仅在开始时搜索：|323928|std::string+name+=+".hidden._di.r";
size_t+found_index+=+name.find_first_of('.');
if+(found_index+==+0)+{
++++//+Found+'.'+at+first+position+in+string
}|323929|323930|More+about+find|unordered-list-item|323931|More+about+find_first_of|323932|323933|祝好运!|323934|entityMap|0|LINK|mutability|MUTABLE|url|http://www.cplusplus.com/reference/string/string/find/|1|http://www.cplusplus.com/reference/string/string/find_first_of/^0|I|6|P|F|0|0|0|0|0|2|F|0|0|0|0|F|0|0|0|O|1|0|0|0^^$0|@$1|2|3|4|5|6|7|1D|8|@$9|1E|A|1F|B|C]|$9|1G|A|1H|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1I|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1J|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1K|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|1L|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1M|8|@$9|1N|A|1O|B|C]]|D|@]|E|$]]|$1|S|3|T|5|J|7|1P|8|@]|D|@]|E|$K|L]]|$1|U|3|-4|5|6|7|1Q|8|@]|D|@]|E|$]]|$1|V|3|W|5|X|7|1R|8|@]|D|@$9|1S|A|1T|1|1U]]|E|$]]|$1|Y|3|Z|5|X|7|1V|8|@]|D|@$9|1W|A|1X|1|1Y]]|E|$]]|$1|10|3|-4|5|6|7|1Z|8|@]|D|@]|E|$]]|$1|11|3|12|5|6|7|20|8|@]|D|@]|E|$]]|$1|13|3|-4|5|6|7|21|8|@]|D|@]|E|$]]]|14|$15|$5|16|17|18|E|$19|1A]]|1B|$5|16|17|18|E|$19|1C]]]]

With C++11 or higher you can use <code>find()</code> and <code>find_first_of()</code> 

Example using find to find a single char:

<pre><code>#include &lt;string&gt;
std::string name = "Aaah";
size_t found_index = name.find('a');
if (found_index != std::string::npos) {
 // Found string containing 'a'
}
</code></pre>

Example using find to find a full string &amp; starting from position 5:

<pre><code>std::string name = "Aaah";
size_t found_index = name.find('h', 3);
if (found_index != std::string::npos) {
 // Found string containing 'h'
}
</code></pre>

Example using the <code>find_first_of()</code> and only the first char, to search at the start only:

<pre><code>std::string name = ".hidden._di.r";
size_t found_index = name.find_first_of('.');
if (found_index == 0) {
 // Found '.' at first position in string
}
</code></pre>

<ul>
<li><a href="http://www.cplusplus.com/reference/string/string/find/" rel="nofollow noreferrer">More about find</a></li>
<li><a href="http://www.cplusplus.com/reference/string/string/find_first_of/" rel="nofollow noreferrer">More about find_first_of</a></li>
</ul>

Good luck!

blocks|key|3075104|text|从C%2B%2B20开始，您可以使用starts_with方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3075105|std::string+s+=+"abcd";
if+(s.starts_with("abc"))+{
++++...
}|code-block|syntax|javascript|3075106|entityMap^0|E|B|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|$]]

Starting with C++20, you can use the <code>starts_with</code> method.

<pre><code>std::string s = "abcd";
if (s.starts_with("abc")) {
 ...
}
</code></pre>

blocks|key|3075128|text|在C%2B%2B20中，现在可以使用starts_with作为std::string的成员函数，其定义如下：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|3075129|constexpr+bool+starts_with(string_view+sv)+const+noexcept;

constexpr+bool+starts_with(CharT+c)+const+noexcept;

constexpr+bool+starts_with(const+CharT*+s)+const;|code-block|syntax|javascript|3075130|所以你的代码可能是这样的：|3075131|std::string+s{argv[1]};

if+(s.starts_with("--foo="))|3075132|entityMap^0|E|B|R|B|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|Y|8|@]|D|@]|E|$]]]|P|$]]

In C++20 now there is <code>starts_with</code> available as a member function of <code>std::string</code> defined as:
<pre><code>constexpr bool starts_with(string_view sv) const noexcept;

constexpr bool starts_with(CharT c) const noexcept;

constexpr bool starts_with(const CharT* s) const;
</code></pre>
So your code could be something like this:
<pre><code>std::string s{argv[1]};

if (s.starts_with(&quot;--foo=&quot;))
</code></pre>

blocks|key|3288544|text|C%2B%2B20更新：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3288545|使用std::string::starts_with的|offset|length|style|CODE|3288546|unordered-list-item|3288547|3288548|https://en.cppreference.com/w/cpp/string/basic_string/starts_with|3288549|std::string+str_value+=+/*+smthg+*/;
const+auto+starts_with_foo+=+str_value.starts_with(std::string_view{"foo"});|code-block|syntax|javascript|3288550|entityMap|0|LINK|mutability|MUTABLE|url^0|0|2|O|0|0|0|0|1T|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@$D|10|E|11|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|I|7|12|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|13|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|14|8|@]|9|@$D|15|E|16|1|17]]|A|$]]|$1|M|3|N|5|O|7|18|8|@]|9|@]|A|$P|Q]]|$1|R|3|-4|5|6|7|19|8|@]|9|@]|A|$]]]|S|$T|$5|U|V|W|A|$X|L]]]]

<h3>C++20 update :</h3>
<ul>
<li>Use <code>std::string::starts_with</code></li>
</ul>
<a href="https://en.cppreference.com/w/cpp/string/basic_string/starts_with" rel="nofollow noreferrer">https://en.cppreference.com/w/cpp/string/basic_string/starts_with</a>
<pre class="lang-cpp prettyprint-override"><code>std::string str_value = /* smthg */;
const auto starts_with_foo = str_value.starts_with(std::string_view{&quot;foo&quot;});
</code></pre>

How do I implement the following (Python pseudocode) in C++?

<pre><code>if argv[1].startswith('--foo='):
 foo_value = int(argv[1][len('--foo='):])
</code></pre>

(For example, if <code>argv[1]</code> is <code>--foo=98</code>, then <code>foo_value</code> is <code>98</code>.)

Update: I'm hesitant to look into Boost, since I'm just looking at making a very small change to a simple little command-line tool (I'd rather not have to learn how to link in and use Boost for a minor change).

How do I check if a C++ std::string starts with a certain string, and convert a substring to an int?

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如何在C++中实现以下代码(Python伪代码)？if argv[1].startswith('--foo='):    foo_value = int(argv[1][len('--foo='):])(例如，如果argv[1]为--foo=98，则foo_value为98。)我对Boost犹豫不决，因为我只是在考虑对...

问如何检查C++ std::string是否以某个字符串开头，并将子字符串转换为整型？
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