blocks|key|4423082|text|你可以写一个类模板：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4423083|template+<class+K,+class+T>
struct+MapElem+{
++++K+const&+key;
++++T&+value;

++++MapElem(std::pair<K+const,+T>&+pair)
++++++++:+key(pair.first)
++++++++,+value(pair.second)
++++{+}
};|code-block|syntax|javascript|4423084|优点是能够编写key和value，但缺点是必须指定类型：|offset|length|style|CODE|4423085|for+(+MapElem<int,+std::string>+kv+:+my_map+){
++++std::cout+<<+kv.key+<<+"+-->+"+<<+kv.value;
}|4423086|如果my_map是const，这也不会起作用。你必须做一些类似这样的事情：|4423087|template+<class+K,+class+T>
struct+MapElem+{
++++K+const&+key;
++++T&+value;

++++MapElem(std::pair<K+const,+T>&+pair)
++++++++:+key(pair.first)
++++++++,+value(pair.second)
++++{+}

++++MapElem(const+std::pair<K+const,+std::remove_const_t<T>>&+pair)
++++++++:+key(pair.first)
++++++++,+value(pair.second)
++++{+}
};

for+(+MapElem<int,+const+std::string>+kv+:+my_map+){
++++std::cout+<<+kv.key+<<+"+-->+"+<<+kv.value;
}|4423088|真是一团糟。现在最好的事情就是习惯于编写.first和.second，并希望结构化绑定提案能够通过，这将允许您真正想要的东西：|4423089|for+(auto&&+[key,+value]+:+my_map)+{
++++std::cout+<<+key+<<+"+-->+"+<<+value;
}|4423090|entityMap^0|0|0|7|3|B|5|0|0|2|6|9|5|0|0|K|6|R|7|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@$I|11|J|12|K|L]|$I|13|J|14|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|16|8|@$I|17|J|18|K|L]|$I|19|J|1A|K|L]]|9|@]|A|$]]|$1|Q|3|R|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|1C|8|@$I|1D|J|1E|K|L]|$I|1F|J|1G|K|L]]|9|@]|A|$]]|$1|U|3|V|5|D|7|1H|8|@]|9|@]|A|$E|F]]|$1|W|3|-4|5|6|7|1I|8|@]|9|@]|A|$]]]|X|$]]

You could write a class template:

<pre><code>template &lt;class K, class T&gt;
struct MapElem {
 K const&amp; key;
 T&amp; value;

 MapElem(std::pair&lt;K const, T&gt;&amp; pair)
 : key(pair.first)
 , value(pair.second)
 { }
};
</code></pre>

with the advantage of being able to write <code>key</code> and <code>value</code> but with the disadvantage of having to specify the types:

<pre><code>for ( MapElem&lt;int, std::string&gt; kv : my_map ){
 std::cout &lt;&lt; kv.key &lt;&lt; " --&gt; " &lt;&lt; kv.value;
}
</code></pre>

And that won't work if <code>my_map</code> were <code>const</code> either. You'd have to do something like:

<pre><code>template &lt;class K, class T&gt;
struct MapElem {
 K const&amp; key;
 T&amp; value;

 MapElem(std::pair&lt;K const, T&gt;&amp; pair)
 : key(pair.first)
 , value(pair.second)
 { }

 MapElem(const std::pair&lt;K const, std::remove_const_t&lt;T&gt;&gt;&amp; pair)
 : key(pair.first)
 , value(pair.second)
 { }
};

for ( MapElem&lt;int, const std::string&gt; kv : my_map ){
 std::cout &lt;&lt; kv.key &lt;&lt; " --&gt; " &lt;&lt; kv.value;
}
</code></pre>

It's a mess. Best thing for now is to just get used to writing <code>.first</code> and <code>.second</code> and hope that the structured bindings proposal passes, which would allow for what you really want:

<pre><code>for (auto&amp;&amp; [key, value] : my_map) {
 std::cout &lt;&lt; key &lt;&lt; " --&gt; " &lt;&lt; value;
}
</code></pre>

blocks|key|4423108|text|最接近使用std::tie的东西|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4423109|std::map<int,+std::string>+my_map;
int+key;
std::string+value;
for(auto&&+p:+my_map)
{
++++std::tie(key,+value)+=+p;
++++std::cout+<<+key+<<+":+"+<<+value+<<+std::endl;
}|code-block|syntax|javascript|4423110|当然，表达式不能放在for+range循环中，所以可以使用宏来允许表达式：|4423111|#define+FOREACH(var,+cont)+\
++++for(auto+&&+_p:cont)+\
++++++++if(bool+_done+=+false)+{}+\
++++++++else+for(var+=+std::forward<decltype(_p)>(_p);+!_done;+_done+=+true)|4423112|这样就可以在循环中直接使用std::tie：|4423113|std::map<int,+std::string>+my_map;
int+key;
std::string+value;
FOREACH(std::tie(key,+value),+my_map)
{
++++std::cout+<<+key+<<+":+"+<<+value+<<+std::endl;
}|4423114|entityMap^0|5|8|0|0|0|0|D|8|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|10|8|@$9|11|A|12|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|T|$]]

The closest thing it to use <code>std::tie</code>:

<pre><code>std::map&lt;int, std::string&gt; my_map;
int key;
std::string value;
for(auto&amp;&amp; p: my_map)
{
 std::tie(key, value) = p;
 std::cout &lt;&lt; key &lt;&lt; ": " &lt;&lt; value &lt;&lt; std::endl;
}
</code></pre>

Of course expressions can't be put in the for range loop, so instead a macro could be used to allow the expression:

<pre><code>#define FOREACH(var, cont) \
 for(auto &amp;&amp; _p:cont) \
 if(bool _done = false) {} \
 else for(var = std::forward&lt;decltype(_p)&gt;(_p); !_done; _done = true)
</code></pre>

So then <code>std::tie</code> can be used directly in the loop:

<pre><code>std::map&lt;int, std::string&gt; my_map;
int key;
std::string value;
FOREACH(std::tie(key, value), my_map)
{
 std::cout &lt;&lt; key &lt;&lt; ": " &lt;&lt; value &lt;&lt; std::endl;
}
</code></pre>

blocks|key|4423147|text|为此，我通常更倾向于使用KISS方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4423148|template<typename+KeyValuePair>
typename+KeyValuePair::first_type&+key(KeyValuePair&+kvp)
{
++++return+kvp.first;
}

template<typename+KeyValuePair>
const+typename+KeyValuePair::first_type&+key(const+KeyValuePair&+kvp)
{
++++return+kvp.first;
}

template<typename+KeyValuePair>
void+key(const+KeyValuePair&&+kvp)+=+delete;

template<typename+KeyValuePair>
typename+KeyValuePair::second_type&+value(KeyValuePair&+kvp)
{
++++return+kvp.second;
}

template<typename+KeyValuePair>
const+typename+KeyValuePair::second_type&+value(const+KeyValuePair&+kvp)
{
++++return+kvp.second;
}

template<typename+KeyValuePair>
void+value(const+KeyValuePair&&+kvp)+=+delete;|code-block|syntax|javascript|4423149|使用示例如下所示：|4423150|for(auto&+kvp+:+my_map)+{
++++std::cout+<<+key(kvp)+<<+"+"+<<+value(kvp)+<<+"\n";
}|4423151|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|$]]

I typically prefer a KISS approach for this:

<pre><code>template&lt;typename KeyValuePair&gt;
typename KeyValuePair::first_type&amp; key(KeyValuePair&amp; kvp)
{
 return kvp.first;
}

template&lt;typename KeyValuePair&gt;
const typename KeyValuePair::first_type&amp; key(const KeyValuePair&amp; kvp)
{
 return kvp.first;
}

template&lt;typename KeyValuePair&gt;
void key(const KeyValuePair&amp;&amp; kvp) = delete;

template&lt;typename KeyValuePair&gt;
typename KeyValuePair::second_type&amp; value(KeyValuePair&amp; kvp)
{
 return kvp.second;
}

template&lt;typename KeyValuePair&gt;
const typename KeyValuePair::second_type&amp; value(const KeyValuePair&amp; kvp)
{
 return kvp.second;
}

template&lt;typename KeyValuePair&gt;
void value(const KeyValuePair&amp;&amp; kvp) = delete;
</code></pre>

with usage example like this:

<pre><code>for(auto&amp; kvp : my_map) {
 std::cout &lt;&lt; key(kvp) &lt;&lt; " " &lt;&lt; value(kvp) &lt;&lt; "\n";
}
</code></pre>

blocks|key|4423201|text|在现代的c%2B%2B17中，这可以通过structured+bindings来实现。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4423202|#include+<map>
#include+<string>
#include+<iostream>

using+namespace+std;

int+main()+{
++++map<int,+string>+my_map;

++++my_map[0]+=+"hello";
++++my_map[1]+=+"world";

++++for+(auto&&+[key,+value]+:+my_map)+{
++++++++cout+<<+key+<<+","+<<+value+<<+"\n";
++++}

++++return+0;
}|code-block|syntax|javascript|4423203|构建它：|4423204|$+clang%2B%2B+-std=c%2B%2B17+test.cpp+-o+program|4423205|输出：|4423206|$+./program
0,hello
1,world|4423207|entityMap|0|LINK|mutability|MUTABLE|url|http://en.cppreference.com/w/cpp/language/structured_binding^0|G|J|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@$A|Z|B|10|1|11]]|C|$]]|$1|D|3|E|5|F|7|12|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|13|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|14|8|@]|9|@]|C|$G|H]]|$1|M|3|N|5|6|7|15|8|@]|9|@]|C|$]]|$1|O|3|P|5|F|7|16|8|@]|9|@]|C|$G|H]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|C|$]]]|R|$S|$5|T|U|V|C|$W|X]]]]

With modern c++17 this is now possible with <a href="http://en.cppreference.com/w/cpp/language/structured_binding" rel="noreferrer">structured bindings</a>.

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

using namespace std;

int main() {
 map&lt;int, string&gt; my_map;

 my_map[0] = "hello";
 my_map[1] = "world";

 for (auto&amp;&amp; [key, value] : my_map) {
 cout &lt;&lt; key &lt;&lt; "," &lt;&lt; value &lt;&lt; "\n";
 }

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

Build it:

<pre><code>$ clang++ -std=c++17 test.cpp -o program
</code></pre>

Output:

<pre><code>$ ./program
0,hello
1,world
</code></pre>

I wasn't sure what to search for. 
I found <a href="https://stackoverflow.com/questions/1500064/renaming-first-and-second-of-a-map-iterator">Renaming first and second of a map iterator</a> but it's not quite what I want to do.

Here's what I'd like to do [see below for nonsense C++ code]. Is something close to this possible? Otherwise will just have to go with the option of "adapting" the iterator as the first line inside the loop I suppose.

<pre><code>// what I want to do:
std::map&lt;int, std::string&gt; my_map;
// ... populate my_map
for(auto key, auto &amp; value: my_map){
 // do something with integer key and string value
}
</code></pre>

C++11 is fine, but rather avoid boost if possible. 

The closest I've gotten is

<pre><code>// TODO, can this be templated?
struct KeyVal{
 int &amp; id;
 std::string &amp; info;

 template &lt;typename P&gt;
 KeyVal(P &amp; p)
 : id(p.first)
 , info(p.second)
 {
 }
};

//...
for ( KeyVal kv : my_map ){
 std::cout &lt;&lt; kv.info;
}
</code></pre>

But that means writing an adapter class for each map :(

<pre><code>// slightly joke answer/"what could possibly go wrong?"
#define key first
#define value second
</code></pre>

Is it possible in C++ to do std::map<> "for element : container" iteration with named variables (eg, key and value) instead of .first and .second?

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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我不确定要搜索什么。我找到了，但这并不是我想做的事情。下面是我想在下面看到的无意义的C++代码。有没有可能做到这一点？否则，我想只能选择“调整”迭代器作为循环中的第一行。// what I want to do:std::map<int, std::string> my_map;// ... populate my_m...

问在C++中，是否可以使用命名变量(例如键和值)代替.first和.second进行std::map<> "for element : container“迭代？
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问在C++中，是否可以使用命名变量(例如键和值)代替.first和.second进行std::map<> "for element : container“迭代？EN

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问在C++中，是否可以使用命名变量(例如键和值)代替.first和.second进行std::map<> "for element : container“迭代？
EN