blocks|key|1621628|text|如果您确实需要隐藏“真实”迭代器返回的值(例如，因为您希望将键迭代器与标准算法一起使用，以便它们对键而不是键对进行操作)，那么可以看看Boost的transform_iterator。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1621629|提示:查看新类的Boost文档时，请先阅读末尾的“示例”。然后，您就有机会弄清楚它的其余部分到底在说什么:-)|1621630|entityMap|0|LINK|mutability|MUTABLE|url|http://www.boost.org/doc/libs/1_40_0/libs/iterator/doc/transform_iterator.html^0|21|I|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

If you really need to hide the value that the "real" iterator returns (for example because you want to use your key-iterator with standard algorithms, so that they operate on the keys instead of the pairs), then take a look at Boost's <a href="http://www.boost.org/doc/libs/1_40_0/libs/iterator/doc/transform_iterator.html" rel="noreferrer">transform_iterator</a>.

[Tip: when looking at Boost documentation for a new class, read the "examples" at the end first. You then have a sporting chance of figuring out what on earth the rest of it is talking about :-)]

blocks|key|1516671|text|map是关联容器。因此，迭代器是一对密钥val。如果只需要键，可以忽略键对中的值部分。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1516672|for(std::map<Key,Val>::iterator+iter+=+myMap.begin();+iter+!=+myMap.end();+%2B%2Biter)
{
Key+k+=++iter->first;
//ignore+value
//Value+v+=+iter->second;
}|code-block|syntax|javascript|1516673|EDIT:：如果您只想向外部公开关键字，那么可以将映射转换为矢量或关键字并公开。|offset|length|style|BOLD|1516674|entityMap^0|0|0|0|5|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

map is associative container. Hence, iterator is a pair of key,val. IF you need only keys, you can ignore the value part from the pair.

<pre><code>for(std::map&lt;Key,Val&gt;::iterator iter = myMap.begin(); iter != myMap.end(); ++iter)
{
Key k = iter-&gt;first;
//ignore value
//Value v = iter-&gt;second;
}
</code></pre>

EDIT::
In case you want to expose only the keys to outside then you can convert the map to vector or keys and expose.

blocks|key|321674|text|使用C%2B%2B11，迭代语法很简单。您仍然可以遍历对，但只访问键很容易。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|321675|#include+<iostream>
#include+<map>

int+main()
{
++++std::map<std::string,+int>+myMap;

++++myMap["one"]+=+1;
++++myMap["two"]+=+2;
++++myMap["three"]+=+3;

++++for+(+const+auto+&myPair+:+myMap+)+{
++++++++std::cout+<<+myPair.first+<<+"\n";
++++}
}|code-block|syntax|javascript|321676|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|$]]

With C++11 the iteration syntax is simple. You still iterate over pairs, but accessing just the key is easy.

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

int main()
{
 std::map&lt;std::string, int&gt; myMap;

 myMap["one"] = 1;
 myMap["two"] = 2;
 myMap["three"] = 3;

 for ( const auto &amp;myPair : myMap ) {
 std::cout &lt;&lt; myPair.first &lt;&lt; "\n";
 }
}
</code></pre>

blocks|key|112146|text|通过C%2B%2B17，您可以在range-based+for+loop中使用structured+binding+(相应地调整John+H.'s+answer+)：|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|112147|#include+<iostream>
#include+<map>

int+main()+{
++++std::map<std::string,+int>+myMap;

++++myMap["one"]+=+1;
++++myMap["two"]+=+2;
++++myMap["three"]+=+3;

++++for+(+const+auto+&[key,+value]:+myMap+)+{
++++++++std::cout+<<+key+<<+'\n';
++++}
}|code-block|syntax|javascript|112148|不幸的是，C%2B%2B17标准要求您声明value变量，即使您没有使用它(对于std::tie(..)使用的std::ignore不起作用，请参阅this+discussion)。|CODE|112149|因此，一些编译器可能会警告您未使用的value变量！在我看来，有关未使用变量的编译时警告对于任何产品代码都是行不通的。因此，这可能不适用于某些编译器版本。|112150|entityMap|0|LINK|mutability|MUTABLE|url|http://en.cppreference.com/w/cpp/language/range-for|1|http://en.cppreference.com/w/cpp/language/structured_binding|2|https://stackoverflow.com/questions/1443793/iterate-keys-in-a-c-map/21436440#21436440|3|https://en.cppreference.com/w/cpp/utility/tuple/tie|4|https://en.cppreference.com/w/cpp/utility/tuple/ignore|5|https://stackoverflow.com/questions/40673080/stdignore-with-structured-bindings^0|0|7|C|K|0|Z|I|1|1O|G|2|0|0|H|5|10|C|1F|B|10|C|3|1F|B|4|1Y|F|5|0|I|5|0^^$0|@$1|2|3|4|5|6|7|17|8|@$9|18|A|19|B|C]]|D|@$9|1A|A|1B|1|1C]|$9|1D|A|1E|1|1F]|$9|1G|A|1H|1|1I]]|E|$]]|$1|F|3|G|5|H|7|1J|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1K|8|@$9|1L|A|1M|B|M]|$9|1N|A|1O|B|M]|$9|1P|A|1Q|B|M]]|D|@$9|1R|A|1S|1|1T]|$9|1U|A|1V|1|1W]|$9|1X|A|1Y|1|1Z]]|E|$]]|$1|N|3|O|5|6|7|20|8|@$9|21|A|22|B|M]]|D|@]|E|$]]|$1|P|3|-4|5|6|7|23|8|@]|D|@]|E|$]]]|Q|$R|$5|S|T|U|E|$V|W]]|X|$5|S|T|U|E|$V|Y]]|Z|$5|S|T|U|E|$V|10]]|11|$5|S|T|U|E|$V|12]]|13|$5|S|T|U|E|$V|14]]|15|$5|S|T|U|E|$V|16]]]]

With C++17 you can use a <a href="http://en.cppreference.com/w/cpp/language/structured_binding" rel="noreferrer">structured binding</a> inside a <a href="http://en.cppreference.com/w/cpp/language/range-for" rel="noreferrer">range-based for loop</a> (adapting <a href="https://stackoverflow.com/questions/1443793/iterate-keys-in-a-c-map/21436440#21436440">John H.'s answer</a> accordingly):

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

int main() {
 std::map&lt;std::string, int&gt; myMap;

 myMap["one"] = 1;
 myMap["two"] = 2;
 myMap["three"] = 3;

 for ( const auto &amp;[key, value]: myMap ) {
 std::cout &lt;&lt; key &lt;&lt; '\n';
 }
}
</code></pre>

Unfortunately the C++17 standard requires you to declare the <code>value</code> variable, even though you're not using it (<a href="https://en.cppreference.com/w/cpp/utility/tuple/ignore" rel="noreferrer"><code>std::ignore</code></a> as one would use for <a href="https://en.cppreference.com/w/cpp/utility/tuple/tie" rel="noreferrer"><code>std::tie(..)</code></a> does not work, see <a href="https://stackoverflow.com/questions/40673080/stdignore-with-structured-bindings">this discussion</a>).

Some compilers may therefore warn you about the unused <code>value</code> variable! Compile-time warnings regarding unused variables are a no-go for any production code in my mind. So, this may not be applicable for certain compiler versions.

blocks|key|1516977|text|没有助推|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1516978|只需为该映射扩展STL迭代器，就可以做到这一点。例如，字符串到整数的映射：|1516979|#include+<map>
typedef+map<string,+int>+ScoreMap;
typedef+ScoreMap::iterator+ScoreMapIterator;

class+key_iterator+:+public+ScoreMapIterator
{
++public:
++++key_iterator()+:+ScoreMapIterator()+{};
++++key_iterator(ScoreMapIterator+s)+:+ScoreMapIterator(s)+{};
++++string*+operator->()+{+return+(string*+const)&(ScoreMapIterator::operator->()->first);+}
++++string+operator*()+{+return+ScoreMapIterator::operator*().first;+}
};|code-block|syntax|javascript|1516980|您还可以使用perform+this+extension+in+a+template，以获得更通用的解决方案。|offset|length|1516981|除了遍历映射的begin()和end()之外，您可以像使用列表迭代器一样使用迭代器。|style|CODE|1516982|ScoreMap+m;
m["jim"]+=+1000;
m["sally"]+=+2000;

for+(key_iterator+s+=+m.begin();+s+!=+m.end();+%2B%2Bs)
++++printf("\n+key+%25s",+s->c_str());|1516983|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/35262398/2063546^0|0|0|0|6|10|0|0|7|7|F|5|0|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|12|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|13|8|@]|9|@$K|14|L|15|1|16]]|A|$]]|$1|M|3|N|5|6|7|17|8|@$K|18|L|19|O|P]|$K|1A|L|1B|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|F|7|1C|8|@]|9|@]|A|$G|H]]|$1|S|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]]]

<h2>Without Boost</h2>

You can do this by simply extending the STL iterator for that map. For example, a mapping of strings to ints:

<pre><code>#include &lt;map&gt;
typedef map&lt;string, int&gt; ScoreMap;
typedef ScoreMap::iterator ScoreMapIterator;

class key_iterator : public ScoreMapIterator
{
 public:
 key_iterator() : ScoreMapIterator() {};
 key_iterator(ScoreMapIterator s) : ScoreMapIterator(s) {};
 string* operator-&gt;() { return (string* const)&amp;(ScoreMapIterator::operator-&gt;()-&gt;first); }
 string operator*() { return ScoreMapIterator::operator*().first; }
};
</code></pre>

You could also <a href="https://stackoverflow.com/a/35262398/2063546">perform this extension in a template</a>, for a more general solution. 

You use your iterator exactly like you would use a list iterator, except you're iterating over the map's <code>begin()</code> and <code>end()</code>.

<pre><code>ScoreMap m;
m["jim"] = 1000;
m["sally"] = 2000;

for (key_iterator s = m.begin(); s != m.end(); ++s)
 printf("\n key %s", s-&gt;c_str());
</code></pre>

blocks|key|111978|text|下面是Ian提到的更通用的模板化解决方案...|type|unstyled|depth|inlineStyleRanges|entityRanges|data|111979|#include+<map>

template<typename+Key,+typename+Value>
using+Map+=+std::map<Key,+Value>;

template<typename+Key,+typename+Value>
using+MapIterator+=+typename+Map<Key,+Value>::iterator;

template<typename+Key,+typename+Value>
class+MapKeyIterator+:+public+MapIterator<Key,+Value>+{

public:

++++MapKeyIterator+(+)+:+MapIterator<Key,+Value>+(+)+{+};
++++MapKeyIterator+(+MapIterator<Key,+Value>+it_+)+:+MapIterator<Key,+Value>+(+it_+)+{+};

++++Key+*operator+->+(+)+{+return+(+Key+*+const+)+&(+MapIterator<Key,+Value>::operator+->+(+)->first+);+}
++++Key+operator+*+(+)+{+return+MapIterator<Key,+Value>::operator+*+(+).first;+}
};

template<typename+Key,+typename+Value>
class+MapValueIterator+:+public+MapIterator<Key,+Value>+{

public:

++++MapValueIterator+(+)+:+MapIterator<Key,+Value>+(+)+{+};
++++MapValueIterator+(+MapIterator<Key,+Value>+it_+)+:+MapIterator<Key,+Value>+(+it_+)+{+};

++++Value+*operator+->+(+)+{+return+(+Value+*+const+)+&(+MapIterator<Key,+Value>::operator+->+(+)->second+);+}
++++Value+operator+*+(+)+{+return+MapIterator<Key,+Value>::operator+*+(+).second;+}
};|code-block|syntax|javascript|111980|所有的功劳都归伊恩。谢谢伊恩。|111981|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|$]]

Below the more general templated solution to which Ian referred...

<pre><code>#include &lt;map&gt;

template&lt;typename Key, typename Value&gt;
using Map = std::map&lt;Key, Value&gt;;

template&lt;typename Key, typename Value&gt;
using MapIterator = typename Map&lt;Key, Value&gt;::iterator;

template&lt;typename Key, typename Value&gt;
class MapKeyIterator : public MapIterator&lt;Key, Value&gt; {

public:

 MapKeyIterator ( ) : MapIterator&lt;Key, Value&gt; ( ) { };
 MapKeyIterator ( MapIterator&lt;Key, Value&gt; it_ ) : MapIterator&lt;Key, Value&gt; ( it_ ) { };

 Key *operator -&gt; ( ) { return ( Key * const ) &amp;( MapIterator&lt;Key, Value&gt;::operator -&gt; ( )-&gt;first ); }
 Key operator * ( ) { return MapIterator&lt;Key, Value&gt;::operator * ( ).first; }
};

template&lt;typename Key, typename Value&gt;
class MapValueIterator : public MapIterator&lt;Key, Value&gt; {

public:

 MapValueIterator ( ) : MapIterator&lt;Key, Value&gt; ( ) { };
 MapValueIterator ( MapIterator&lt;Key, Value&gt; it_ ) : MapIterator&lt;Key, Value&gt; ( it_ ) { };

 Value *operator -&gt; ( ) { return ( Value * const ) &amp;( MapIterator&lt;Key, Value&gt;::operator -&gt; ( )-&gt;second ); }
 Value operator * ( ) { return MapIterator&lt;Key, Value&gt;::operator * ( ).second; }
};
</code></pre>

All credits go to Ian... Thanks Ian.

blocks|key|111795|text|您正在寻找map_keys，使用它您可以编写如下内容|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|111796|BOOST_FOREACH(const+key_t+key,+the_map+%7C+boost::adaptors::map_keys)
{
++//+do+something+with+key
}|code-block|syntax|javascript|111797|entityMap|0|LINK|mutability|MUTABLE|url|http://www.boost.org/doc/libs/1_50_0/libs/range/doc/html/range/reference/adaptors/reference/map_keys.html^0|5|8|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

You are looking for <a href="http://www.boost.org/doc/libs/1_50_0/libs/range/doc/html/range/reference/adaptors/reference/map_keys.html" rel="noreferrer">map_keys</a>, with it you can write things like

<pre><code>BOOST_FOREACH(const key_t key, the_map | boost::adaptors::map_keys)
{
 // do something with key
}
</code></pre>

blocks|key|1517284|text|不使用BOOST，直接使用键和值|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1517285|for(auto+const&+[key,+value]:+m_map)
++++{
++++std::cout<<"+key="<<key;
++++std::cout<<"+value="<<value<<std::endl;
++++}|code-block|syntax|javascript|1517286|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|$]]

<h1>Without BOOST, directly using key and value</h1>
<pre><code>for(auto const&amp; [key, value]: m_map)
 {
 std::cout&lt;&lt;&quot; key=&quot;&lt;&lt;key;
 std::cout&lt;&lt;&quot; value=&quot;&lt;&lt;value&lt;&lt;std::endl;
 }
</code></pre>

blocks|key|111779|text|下面是如何使用Boost的transform_iterator完成此操作的示例|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|111780|#include+<iostream>
#include+<map>
#include+<iterator>
#include+"boost/iterator/transform_iterator.hpp"

using+std::map;
typedef+std::string+Key;
typedef+std::string+Val;

map<Key,Val>::key_type+get_key(map<Key,Val>::value_type+aPair)+{
++return+aPair.first;
}

typedef+map<Key,Val>::key_type+(*get_key_t)(map<Key,Val>::value_type);
typedef+map<Key,Val>::iterator+map_iterator;
typedef+boost::transform_iterator<get_key_t,+map_iterator>+mapkey_iterator;

int+main()+{
++map<Key,Val>+m;
++m["a"]="A";
++m["b"]="B";
++m["c"]="C";

++//+iterate+over+the+map's+(key,val)+pairs+as+usual
++for(map_iterator+i+=+m.begin();+i+!=+m.end();+i%2B%2B)+{
++++std::cout+<<+i->first+<<+"+"+<<+i->second+<<+std::endl;
++}

++//+iterate+over+the+keys+using+the+transformed+iterators
++mapkey_iterator+keybegin(m.begin(),+get_key);
++mapkey_iterator+keyend(m.end(),+get_key);
++for(mapkey_iterator+i+=+keybegin;+i+!=+keyend;+i%2B%2B)+{
++++std::cout+<<+*i+<<+std::endl;
++}
}|code-block|syntax|javascript|111781|entityMap|0|LINK|mutability|MUTABLE|url|http://www.boost.org/doc/libs/1_40_0/libs/iterator/doc/transform_iterator.html^0|D|I|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

Here's an example of how to do it using Boost's <a href="http://www.boost.org/doc/libs/1_40_0/libs/iterator/doc/transform_iterator.html" rel="noreferrer">transform_iterator</a>

<pre><code>#include &lt;iostream&gt;
#include &lt;map&gt;
#include &lt;iterator&gt;
#include "boost/iterator/transform_iterator.hpp"

using std::map;
typedef std::string Key;
typedef std::string Val;

map&lt;Key,Val&gt;::key_type get_key(map&lt;Key,Val&gt;::value_type aPair) {
 return aPair.first;
}

typedef map&lt;Key,Val&gt;::key_type (*get_key_t)(map&lt;Key,Val&gt;::value_type);
typedef map&lt;Key,Val&gt;::iterator map_iterator;
typedef boost::transform_iterator&lt;get_key_t, map_iterator&gt; mapkey_iterator;

int main() {
 map&lt;Key,Val&gt; m;
 m["a"]="A";
 m["b"]="B";
 m["c"]="C";

 // iterate over the map's (key,val) pairs as usual
 for(map_iterator i = m.begin(); i != m.end(); i++) {
 std::cout &lt;&lt; i-&gt;first &lt;&lt; " " &lt;&lt; i-&gt;second &lt;&lt; std::endl;
 }

 // iterate over the keys using the transformed iterators
 mapkey_iterator keybegin(m.begin(), get_key);
 mapkey_iterator keyend(m.end(), get_key);
 for(mapkey_iterator i = keybegin; i != keyend; i++) {
 std::cout &lt;&lt; *i &lt;&lt; std::endl;
 }
}
</code></pre>

blocks|key|1516685|text|你想这么做吗？|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1516686|std::map<type,type>::iterator+iter+=+myMap.begin();
std::map<type,type>::iterator+endIter+=+myMap.end();
for(;+iter+!=+endIter;+%2B%2Biter)
{
+++type+key+=+iter->first;++
+++.....
}|code-block|syntax|javascript|1516687|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|$]]

You want to do this?
<pre><code>std::map&lt;type,type&gt;::iterator iter = myMap.begin();
std::map&lt;type,type&gt;::iterator endIter = myMap.end();
for(; iter != endIter; ++iter)
{
 type key = iter-&gt;first; 
 .....
}
</code></pre>

blocks|key|1623691|text|当不需要显式的begin和end时，例如对于范围循环，可以使用以下命令获得键(第一个示例)或值(第二个示例)上的循环|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1623692|#include+<boost/range/adaptors.hpp>

map<Key,+Value>+m;

for+(auto+k+:+boost::adaptors::keys(m))
++cout+<<+k+<<+endl;

for+(auto+v+:+boost::adaptors::values(m))
++cout+<<+v+<<+endl;|code-block|syntax|javascript|1623693|entityMap^0|7|5|D|3|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

When no explicit <code>begin</code> and <code>end</code> is needed, ie for range-looping, the loop over keys (first example) or values (second example) can be obtained with

<pre><code>#include &lt;boost/range/adaptors.hpp&gt;

map&lt;Key, Value&gt; m;

for (auto k : boost::adaptors::keys(m))
 cout &lt;&lt; k &lt;&lt; endl;

for (auto v : boost::adaptors::values(m))
 cout &lt;&lt; v &lt;&lt; endl;
</code></pre>

blocks|key|321780|text|如果您需要一个只返回键的迭代器，则需要将map的迭代器包装在您自己的类中，以提供所需的接口。您可以从头开始声明一个新的迭代器类，如here，或者使用现有的助手构造。This+answer展示了如何使用Boost的transform_iterator将迭代器包装在一个只返回值/键的迭代器中。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|321781|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/470835/a-c-iterator-adapter-which-wraps-and-hides-an-inner-iterator-and-converts-the-i/471068#471068|1|https://stackoverflow.com/questions/485730/keys-values-functionality-to-iterators-in-c/485744#485744|2|http://www.boost.org/doc/libs/1_40_0/libs/iterator/doc/transform_iterator.html^0|2Y|I|1T|4|0|2A|B|1|2Y|I|2|0^^$0|@$1|2|3|4|5|6|7|R|8|@$9|S|A|T|B|C]]|D|@$9|U|A|V|1|W]|$9|X|A|Y|1|Z]|$9|10|A|11|1|12]]|E|$]]|$1|F|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]|N|$5|I|J|K|E|$L|O]]|P|$5|I|J|K|E|$L|Q]]]]

If you need an iterator that just returns the keys you need to wrap map's iterator in your own class that provides the desired interface. You can declare a new iterator class from scratch like <a href="https://stackoverflow.com/questions/470835/a-c-iterator-adapter-which-wraps-and-hides-an-inner-iterator-and-converts-the-i/471068#471068">here</a>, of use existing helper constructs. <a href="https://stackoverflow.com/questions/485730/keys-values-functionality-to-iterators-in-c/485744#485744">This answer</a> shows how to use Boost's <a href="http://www.boost.org/doc/libs/1_40_0/libs/iterator/doc/transform_iterator.html" rel="nofollow noreferrer"><code>transform_iterator</code></a> to wrap the iterator in one that only returns the values/keys.

blocks|key|1623482|text|你可以|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1623483|1623484|创建自定义迭代器类，使用boost::bind(+&pair::second,+_1+)函数器将map.begin()的std::map<K,V>::iterator|unordered-list-item|offset|length|style|CODE|1623485|use+std::transform聚合到map.end()+|1623486|使用for循环迭代时只需忽略->second成员。|1623487|1623488|entityMap^0|0|0|C|W|1C|B|1O|N|0|4|E|L|9|0|2|3|E|8|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|R|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|S|8|@$F|T|G|U|H|I]|$F|V|G|W|H|I]|$F|X|G|Y|H|I]]|9|@]|A|$]]|$1|J|3|K|5|E|7|Z|8|@$F|10|G|11|H|I]|$F|12|G|13|H|I]]|9|@]|A|$]]|$1|L|3|M|5|E|7|14|8|@$F|15|G|16|H|I]|$F|17|G|18|H|I]]|9|@]|A|$]]|$1|N|3|-4|5|6|7|19|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|1A|8|@]|9|@]|A|$]]]|P|$]]

You could

<ul>
<li>create a custom iterator class, aggregating the <code>std::map&lt;K,V&gt;::iterator</code></li>
<li>use <code>std::transform</code> of your <code>map.begin()</code> to <code>map.end()</code> 
with a <code>boost::bind( &amp;pair::second, _1 )</code> functor</li>
<li>just ignore the <code>-&gt;second</code> member while iterating with a <code>for</code> loop.</li>
</ul>

blocks|key|111949|text|这个答案类似于rodrigob的答案，只是没有BOOST_FOREACH。你可以改用c%2B%2B的range+based+for。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|111950|#include+<map>
#include+<boost/range/adaptor/map.hpp>
#include+<iostream>

template+<typename+K,+typename+V>
void+printKeys(std::map<K,V>+map){
+++++for(auto+key+:+map+%7C+boost::adaptors::map_keys){
++++++++++std::cout+<<+key+<<+std::endl;
+++++}
}|code-block|syntax|javascript|111951|entityMap^0|N|D|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|$]]

This answer is like rodrigob's except without the <code>BOOST_FOREACH</code>. You can use c++'s range based for instead.

<pre><code>#include &lt;map&gt;
#include &lt;boost/range/adaptor/map.hpp&gt;
#include &lt;iostream&gt;

template &lt;typename K, typename V&gt;
void printKeys(std::map&lt;K,V&gt; map){
 for(auto key : map | boost::adaptors::map_keys){
 std::cout &lt;&lt; key &lt;&lt; std::endl;
 }
}
</code></pre>

blocks|key|1621935|text|使用C%2B%2B20，我们可以访问ranges库，该库有一个很好的解决方案：std::views::keys|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1621936|#include+<ranges>

//...

std::map<int,+int>+myMap+=+{{1,2},{3,4},{5,6}};
auto+keys+=+std::views::keys(myMap);
for(auto+key+:+keys)+{
++++std::cout+<<+key+<<+std::endl;
}|code-block|syntax|javascript|1621937|你自己试试吧：https://godbolt.org/z/heeWv4Gh6|1621938|entityMap|0|LINK|mutability|MUTABLE|url|https://en.cppreference.com/w/cpp/ranges|1|https://en.cppreference.com/w/cpp/ranges/keys_view|2|https://godbolt.org/z/heeWv4Gh6^0|E|6|Z|G|E|6|0|Z|G|1|0|0|7|V|2|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]|$9|16|A|17|1|18]]|E|$]]|$1|F|3|G|5|H|7|19|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1A|8|@]|D|@$9|1B|A|1C|1|1D]]|E|$]]|$1|M|3|-4|5|6|7|1E|8|@]|D|@]|E|$]]]|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++20, we have access to the <a href="https://en.cppreference.com/w/cpp/ranges" rel="nofollow noreferrer"><code>ranges</code></a> library, which has a nifty solution for this: <a href="https://en.cppreference.com/w/cpp/ranges/keys_view" rel="nofollow noreferrer"><code>std::views::keys</code></a>
<pre class="lang-cpp prettyprint-override"><code>#include &lt;ranges&gt;

//...

std::map&lt;int, int&gt; myMap = {{1,2},{3,4},{5,6}};
auto keys = std::views::keys(myMap);
for(auto key : keys) {
 std::cout &lt;&lt; key &lt;&lt; std::endl;
}
</code></pre>
Try it out yourself: <a href="https://godbolt.org/z/heeWv4Gh6" rel="nofollow noreferrer">https://godbolt.org/z/heeWv4Gh6</a>

blocks|key|1621741|text|没有Boost，你可以这样做。如果你能写一个强制转换操作符而不是getKeyIterator()，那就好了，但是我不能编译它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1621742|#include+<map>
#include+<unordered_map>


template<typename+K,+typename+V>
class+key_iterator:+public+std::unordered_map<K,V>::iterator+{

public:

++++const+K+&operator*()+const+{
++++++++return+std::unordered_map<K,V>::iterator::operator*().first;
++++}

++++const+K+*operator->()+const+{
++++++++return+&(**this);
++++}
};

template<typename+K,typename+V>
key_iterator<K,V>+getKeyIterator(typename+std::unordered_map<K,V>::iterator+&it)+{
++++return+*static_cast<key_iterator<K,V>+*>(&it);
}

int+_tmain(int+argc,+_TCHAR*+argv[])
{
++++std::unordered_map<std::string,+std::string>+myMap;
++++myMap["one"]="A";
++++myMap["two"]="B";
++++myMap["three"]="C";
++++key_iterator<std::string,+std::string>+&it=getKeyIterator<std::string,std::string>(myMap.begin());
++++for+(;+it!=myMap.end();+%2B%2Bit)+{
++++++++printf("%25s\n",it->c_str());
++++}
}|code-block|syntax|javascript|1621743|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|$]]

Without Boost, you could do it like this. It would be nice if you could write a cast operator instead of getKeyIterator(), but I can't get it to compile.

<pre><code>#include &lt;map&gt;
#include &lt;unordered_map&gt;


template&lt;typename K, typename V&gt;
class key_iterator: public std::unordered_map&lt;K,V&gt;::iterator {

public:

 const K &amp;operator*() const {
 return std::unordered_map&lt;K,V&gt;::iterator::operator*().first;
 }

 const K *operator-&gt;() const {
 return &amp;(**this);
 }
};

template&lt;typename K,typename V&gt;
key_iterator&lt;K,V&gt; getKeyIterator(typename std::unordered_map&lt;K,V&gt;::iterator &amp;it) {
 return *static_cast&lt;key_iterator&lt;K,V&gt; *&gt;(&amp;it);
}

int _tmain(int argc, _TCHAR* argv[])
{
 std::unordered_map&lt;std::string, std::string&gt; myMap;
 myMap["one"]="A";
 myMap["two"]="B";
 myMap["three"]="C";
 key_iterator&lt;std::string, std::string&gt; &amp;it=getKeyIterator&lt;std::string,std::string&gt;(myMap.begin());
 for (; it!=myMap.end(); ++it) {
 printf("%s\n",it-&gt;c_str());
 }
}
</code></pre>

blocks|key|1517101|text|对于后人来说，由于我试图找到一种创建范围的方法，因此另一种选择是使用boost::adaptors::transform|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1517102|下面是一个小示例：|1517103|#include+<boost/range/adaptor/transformed.hpp>
#include+<iostream>
#include+<map>

int+main(int+argc,+const+char*+argv[])
{
++std::map<int,+int>+m;
++m[0]++=+1;
++m[2]++=+3;
++m[42]+=+0;

++auto+key_range+=
++++boost::adaptors::transform(
++++++m,
++++++[](std::map<int,+int>::value_type+const&+t)+
++++++{+return+t.first;+}
++++);+
++for+(auto&&+key+:+key_range)
++++std::cout+<<+key+<<+'+';
++std::cout+<<+'\n';
++return+0;
}|code-block|syntax|javascript|1517104|如果您想遍历这些值，可以在lambda中使用t.second。|style|CODE|1517105|entityMap|0|LINK|mutability|MUTABLE|url|http://www.boost.org/doc/libs/1_60_0/libs/range/doc/html/range/reference/adaptors/reference/transformed.html^0|Y|Q|0|0|0|0|M|8|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@$A|X|B|Y|1|Z]]|C|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|11|8|@]|9|@]|C|$I|J]]|$1|K|3|L|5|6|7|12|8|@$A|13|B|14|M|N]]|9|@]|C|$]]|$1|O|3|-4|5|6|7|15|8|@]|9|@]|C|$]]]|P|$Q|$5|R|S|T|C|$U|V]]]]

For posterity, and since I was trying to find a way to create a range, an alternative is to use <a href="http://www.boost.org/doc/libs/1_60_0/libs/range/doc/html/range/reference/adaptors/reference/transformed.html" rel="nofollow" title="boost::adaptors::transform">boost::adaptors::transform</a>

Here's a small example:

<pre><code>#include &lt;boost/range/adaptor/transformed.hpp&gt;
#include &lt;iostream&gt;
#include &lt;map&gt;

int main(int argc, const char* argv[])
{
 std::map&lt;int, int&gt; m;
 m[0] = 1;
 m[2] = 3;
 m[42] = 0;

 auto key_range =
 boost::adaptors::transform(
 m,
 [](std::map&lt;int, int&gt;::value_type const&amp; t) 
 { return t.first; }
 ); 
 for (auto&amp;&amp; key : key_range)
 std::cout &lt;&lt; key &lt;&lt; ' ';
 std::cout &lt;&lt; '\n';
 return 0;
}
</code></pre>

If you want to iterate over the values, use <code>t.second</code> in the lambda.

blocks|key|321747|text|这里有很多好的答案，下面是一种使用其中几个的方法，可以让你这样写：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|321748|void+main()
{
++++std::map<std::string,+int>+m+{+{"jim",+1000},+{"sally",+2000}+};
++++for+(auto+key+:+MapKeys(m))
++++++++std::cout+<<+key+<<+std::endl;
}|code-block|syntax|javascript|321749|如果这是您一直想要的，下面是MapKeys()的代码：|321750|template+<class+MapType>
class+MapKeyIterator+{
public:
++++class+iterator+{
++++public:
++++++++iterator(typename+MapType::iterator+it)+:+it(it)+{}
++++++++iterator+operator%2B%2B()+{+return+%2B%2Bit;+}
++++++++bool+operator!=(const+iterator+&+other)+{+return+it+!=+other.it;+}
++++++++typename+MapType::key_type+operator*()+const+{+return+it->first;+}++//+Return+key+part+of+map
++++private:
++++++++typename+MapType::iterator+it;
++++};
private:
++++MapType&+map;
public:
++++MapKeyIterator(MapType&+m)+:+map(m)+{}
++++iterator+begin()+{+return+iterator(map.begin());+}
++++iterator+end()+{+return+iterator(map.end());+}
};
template+<class+MapType>
MapKeyIterator<MapType>+MapKeys(MapType&+m)
{
++++return+MapKeyIterator<MapType>(m);
}|321751|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Lots of good answers here, below is an approach using a couple of them which lets you write this:

<pre><code>void main()
{
 std::map&lt;std::string, int&gt; m { {"jim", 1000}, {"sally", 2000} };
 for (auto key : MapKeys(m))
 std::cout &lt;&lt; key &lt;&lt; std::endl;
}
</code></pre>

If that's what you always wanted, then here is the code for MapKeys():

<pre><code>template &lt;class MapType&gt;
class MapKeyIterator {
public:
 class iterator {
 public:
 iterator(typename MapType::iterator it) : it(it) {}
 iterator operator++() { return ++it; }
 bool operator!=(const iterator &amp; other) { return it != other.it; }
 typename MapType::key_type operator*() const { return it-&gt;first; } // Return key part of map
 private:
 typename MapType::iterator it;
 };
private:
 MapType&amp; map;
public:
 MapKeyIterator(MapType&amp; m) : map(m) {}
 iterator begin() { return iterator(map.begin()); }
 iterator end() { return iterator(map.end()); }
};
template &lt;class MapType&gt;
MapKeyIterator&lt;MapType&gt; MapKeys(MapType&amp; m)
{
 return MapKeyIterator&lt;MapType&gt;(m);
}
</code></pre>

blocks|key|321807|text|我采用了Ian的答案来处理所有映射类型，并修复了为operator*返回引用的问题|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|321808|template<typename+T>
class+MapKeyIterator+:+public+T
{
public:
++++MapKeyIterator()+:+T()+{}
++++MapKeyIterator(T+iter)+:+T(iter)+{}
++++auto*+operator->()
++++{
++++++++return+&(T::operator->()->first);
++++}
++++auto&+operator*()
++++{
++++++++return+T::operator*().first;
++++}
};|code-block|syntax|javascript|321809|entityMap^0|P|9|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've adopted Ian's answer to work with all map types and fixed returning a reference for <code>operator*</code>

<pre><code>template&lt;typename T&gt;
class MapKeyIterator : public T
{
public:
 MapKeyIterator() : T() {}
 MapKeyIterator(T iter) : T(iter) {}
 auto* operator-&gt;()
 {
 return &amp;(T::operator-&gt;()-&gt;first);
 }
 auto&amp; operator*()
 {
 return T::operator*().first;
 }
};
</code></pre>

blocks|key|321575|text|我知道这不能回答您的问题，但您可能想要查看的一个选项是，只有两个具有相同索引的向量被“链接”信息。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|321576|所以在..|321577|std::vector<std::string>+vName;

std::vector<int>+vNameCount;|code-block|syntax|javascript|321578|如果您想要按名称计算名称，只需在vName.size()上执行快速for循环，当您找到它时，这就是您要查找的vNameCount的索引。|321579|当然，这可能不会给你地图的所有功能，依赖可能会更好，但如果你不知道键，并且不应该添加太多的处理，它可能会更容易。|321580|记住，当你从一个地方添加/删除时，你必须从另一个地方去做，否则事情会变得很疯狂。|321581|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

I know this doesn't answer your question, but one option you may want to look at is just having two vectors with the same index being "linked" information.. 

So in..

<pre><code>std::vector&lt;std::string&gt; vName;

std::vector&lt;int&gt; vNameCount;
</code></pre>

if you want the count of names by name you just do your quick for loop over vName.size(), and when ya find it that is the index for vNameCount that you are looking for. 

Sure this may not give ya all the functionality of the map, and depending may or may not be better, but it might be easier if ya don't know the keys, and shouldn't add too much processing.

Just remember when you add/delete from one you have to do it from the other or things will get crazy heh :P

Is there a way to iterate over the keys, not the pairs of a C++ map?

Iterate keys in a C++ map

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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有没有一种方法可以遍历键，而不是C++映射的对？

问迭代C++映射中的键
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问迭代C++映射中的键
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