blocks|key|340109|text|正确的方法总是|type|unstyled|depth|inlineStyleRanges|entityRanges|data|340110|for(auto&&+elem+:+container)|code-block|syntax|javascript|340111|这将保证所有语义的保留。|340112|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|$]]

The correct means is always

<pre><code>for(auto&amp;&amp; elem : container)
</code></pre>

This will guarantee the preservation of all semantics.

blocks|key|122410|text|虽然range-for循环的最初动机可能是易于迭代容器的元素，但其语法足够通用，即使对于不是纯粹容器的对象也是有用的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|122411|for循环的语法要求是range_expression支持begin()和end()作为函数--或者作为它计算的类型的成员函数，或者作为接受该类型实例的非成员函数。|offset|length|style|CODE|122412|作为一个人为设计的示例，您可以生成一个数字范围，并使用以下类在该范围内迭代。|122413|struct+Range
{
+++struct+Iterator
+++{
++++++Iterator(int+v,+int+s)+:+val(v),+step(s)+{}

++++++int+operator*()+const
++++++{
+++++++++return+val;
++++++}

++++++Iterator&+operator%2B%2B()
++++++{
+++++++++val+%2B=+step;
+++++++++return+*this;
++++++}

++++++bool+operator!=(Iterator+const&+rhs)+const
++++++{
+++++++++return+(this->val+<+rhs.val);
++++++}

++++++int+val;
++++++int+step;
+++};

+++Range(int+l,+int+h,+int+s=1)+:+low(l),+high(h),+step(s)+{}

+++Iterator+begin()+const
+++{
++++++return+Iterator(low,+step);
+++}

+++Iterator+end()+const
+++{
++++++return+Iterator(high,+1);
+++}

+++int+low,+high,+step;
};+|code-block|syntax|javascript|122414|使用下面的main函数，|122415|#include+<iostream>

int+main()
{
+++Range+r1(1,+10);
+++for+(+auto+item+:+r1+)
+++{
++++++std::cout+<<+item+<<+"+";
+++}
+++std::cout+<<+std::endl;

+++Range+r2(1,+20,+2);
+++for+(+auto+item+:+r2+)
+++{
++++++std::cout+<<+item+<<+"+";
+++}
+++std::cout+<<+std::endl;

+++Range+r3(1,+20,+3);
+++for+(+auto+item+:+r3+)
+++{
++++++std::cout+<<+item+<<+"+";
+++}
+++std::cout+<<+std::endl;
}|122416|将会得到以下输出。|122417|1+2+3+4+5+6+7+8+9+
1+3+5+7+9+11+13+15+17+19+
1+4+7+10+13+16+19+|122418|entityMap^0|0|B|G|T|7|11|5|0|0|0|5|4|0|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]|$D|12|E|13|F|G]|$D|14|E|15|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|16|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|17|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|18|8|@$D|19|E|1A|F|G]]|9|@]|A|$]]|$1|Q|3|R|5|L|7|1B|8|@]|9|@]|A|$M|N]]|$1|S|3|T|5|6|7|1C|8|@]|9|@]|A|$]]|$1|U|3|V|5|L|7|1D|8|@]|9|@]|A|$M|N]]|$1|W|3|-4|5|6|7|1E|8|@]|9|@]|A|$]]]|X|$]]

While the initial motivation of the range-for loop might have been ease of iterating over the elements of a container, the syntax is generic enough to be useful even for objects that are not purely containers.

The syntactic requirement for the for-loop is that <code>range_expression</code> support <code>begin()</code> and <code>end()</code> as either functions -- either as member functions of the type that it evaluates to or as non-member functions what take an instance of the type.

As a contrived example, one can generate a range of numbers and iterate over the range using the following class.

<pre><code>struct Range
{
 struct Iterator
 {
 Iterator(int v, int s) : val(v), step(s) {}

 int operator*() const
 {
 return val;
 }

 Iterator&amp; operator++()
 {
 val += step;
 return *this;
 }

 bool operator!=(Iterator const&amp; rhs) const
 {
 return (this-&gt;val &lt; rhs.val);
 }

 int val;
 int step;
 };

 Range(int l, int h, int s=1) : low(l), high(h), step(s) {}

 Iterator begin() const
 {
 return Iterator(low, step);
 }

 Iterator end() const
 {
 return Iterator(high, 1);
 }

 int low, high, step;
}; 
</code></pre>

With the following <code>main</code> function,

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

int main()
{
 Range r1(1, 10);
 for ( auto item : r1 )
 {
 std::cout &lt;&lt; item &lt;&lt; " ";
 }
 std::cout &lt;&lt; std::endl;

 Range r2(1, 20, 2);
 for ( auto item : r2 )
 {
 std::cout &lt;&lt; item &lt;&lt; " ";
 }
 std::cout &lt;&lt; std::endl;

 Range r3(1, 20, 3);
 for ( auto item : r3 )
 {
 std::cout &lt;&lt; item &lt;&lt; " ";
 }
 std::cout &lt;&lt; std::endl;
}
</code></pre>

one would get the following output.

<pre class="lang-none prettyprint-override"><code>1 2 3 4 5 6 7 8 9 
1 3 5 7 9 11 13 15 17 19 
1 4 7 10 13 16 19 
</code></pre>

What is the correct way of using C++11's range-based <code>for</code>?

What syntax should be used? <code>for (auto elem : container)</code>, 
or <code>for (auto&amp; elem : container)</code> or <code>for (const auto&amp; elem : container)</code>? 
Or some other?

What is the correct way of using C++11's range-based for?

容器

使用C++11的基于范围的for的正确方法是什么应该使用什么语法？for (auto elem : container)、for (auto& elem : container)还是for (const auto& elem : container)？还是其他的？

问使用C++11的基于范围的函数的正确方法是什么？
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