blocks|key|1336100|text|由于C%2B%2B标准已经被接受，实际上所有的标准库都在std名称空间中。因此，如果您不想用std::限定所有标准库调用，则需要添加using指令。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1336101|然而，|1336102|using+namespace+std;|code-block|syntax|javascript|1336103|被认为是一种糟糕的做法，因为您实际上是在导入整个标准名称空间，从而为名称冲突打开了许多可能性。最好只导入代码中实际使用的内容，例如|1336104|using+std::string;|1336105|entityMap^0|16|5|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|W|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|X|8|@]|D|@]|E|$]]|$1|O|3|P|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|R|$]]

Since the C++ standard has been accepted, practically all of the standard library is inside the std namespace. So if you don't want to qualify all standard library calls with <code>std::</code>, you need to add the using directive.

However,

<pre><code>using namespace std;
</code></pre>

is considered a bad practice because you are practically importing the whole standard namespace, thus opening up a lot of possibilities for name clashes. It is better to import only the stuff you are actually using in your code, like

<pre><code>using std::string;
</code></pre>

blocks|key|1335936|text|这是避免在该名称空间中的所有内容加上std:：前缀的一种快捷方式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1335937|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Nothing does, it's a shorthand to avoid prefixing everything in that namespace with std::

blocks|key|1336000|text|无需显式引用std::member即可引用std命名空间中的成员的能力。例如：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1336001|#include+<iostream>
using+namespace+std;

...
cout+<<+"Hi"+<<+endl;|code-block|syntax|javascript|1336002|与|1336003|#include+<iostream>

...
std::cout+<<+"Hi"+<<+std::endl;|1336004|entityMap^0|6|B|L|3|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|$]]

The ability to refer to members in the <code>std</code> namespace without the need to refer to <code>std::member</code> explicitly. For example:

<pre><code>#include &lt;iostream&gt;
using namespace std;

...
cout &lt;&lt; "Hi" &lt;&lt; endl;
</code></pre>

vs.

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

...
std::cout &lt;&lt; "Hi" &lt;&lt; std::endl;
</code></pre>

blocks|key|1121136|text|你绝对不应该说：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1121137|using+namespace+std;|code-block|syntax|javascript|1121138|在您的C%2B%2B头中，因为这超越了使用名称空间的全部意义(这样做将构成“名称空间污染”)。以下是有关此主题的一些有用资源：|1121139|1)+Standard+convention+for+using+“std”上的堆栈溢出线程|offset|length|1121140|2)+Herb+Sutter在Migrating+to+Namespaces上的一篇文章|1121141|3)来自Marshall+Cline的C%2B%2B常见问题精简版的FAQ+27.5。|1121142|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/1961561/standard-convention-for-using-std|1|http://www.gotw.ca/publications/migrating_to_namespaces.htm|2|http://www.parashift.com/c%2B%2B-faq-lite/coding-standards.html#faq-27.5^0|0|0|0|3|Z|0|0|F|N|1|0|U|8|2|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|14|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|15|8|@]|9|@$K|16|L|17|1|18]]|A|$]]|$1|M|3|N|5|6|7|19|8|@]|9|@$K|1A|L|1B|1|1C]]|A|$]]|$1|O|3|P|5|6|7|1D|8|@]|9|@$K|1E|L|1F|1|1G]]|A|$]]|$1|Q|3|-4|5|6|7|1H|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]|Y|$5|T|U|V|A|$W|Z]]|10|$5|T|U|V|A|$W|11]]]]

You should definitely not say:

<pre><code>using namespace std;
</code></pre>

in your C++ headers, because that beats the whole point of using namespaces (doing that would constitute "namespace pollution"). Some useful resources on this topic are the following:

1) stackoverflow thread on <a href="https://stackoverflow.com/questions/1961561/standard-convention-for-using-std">Standard convention for using “std”</a> 

2) an article by Herb Sutter on <a href="http://www.gotw.ca/publications/migrating_to_namespaces.htm" rel="nofollow noreferrer">Migrating to Namespaces</a>

3) <a href="http://www.parashift.com/c++-faq-lite/coding-standards.html#faq-27.5" rel="nofollow noreferrer">FAQ 27.5</a> from Marshall Cline's C++ Faq lite.

blocks|key|1336323|text|命名空间是一种包装代码的方式，以避免混淆和名称冲突。例如：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1336324|文件common1.h：|1336325|namespace+intutils
{
++++int+addNumbers(int+a,+int+b)
++++{
++++++++return+a+%2B+b;
++++}
}|code-block|syntax|javascript|1336326|用法文件：|1336327|#include+"common1.h"++++
int+main()
{
++++int+five+=+0;
++++five+=+addNumbers(2,+3);+//+Will+fail+to+compile+since+the+function+is+in+a+different+namespace.
++++five+=+intutils::addNumbers(2,+3);+//+Will+compile+since+you+have+made+explicit+which+namespace+the+function+is+contained+within.

++++using+namespace+intutils;
++++five+=+addNumbers(2,+3);+//+Will+compile+because+the+previous+line+tells+the+compiler+that+if+in+doubt+it+should+check+the+"intutils"+namespace.
}|1336328|所以，当您编写using+namespace+std时，您所做的就是告诉编译器，如果有疑问，它应该在std命名空间中查找函数，等等，它无法找到这些函数的定义。这通常用在示例(和生产)代码中，因为它使得输入通用函数，等等，像cout比必须将每个函数完全限定为std::cout更快。|offset|length|style|CODE|1336329|entityMap^0|0|0|0|0|0|7|J|1E|3|33|4|3K|9|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|Y|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|Z|8|@$O|10|P|11|Q|R]|$O|12|P|13|Q|R]|$O|14|P|15|Q|R]|$O|16|P|17|Q|R]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|18|8|@]|9|@]|A|$]]]|T|$]]

Namespaces are a way of wrapping code to avoid confusion and names from conflicting. For example:

File common1.h:

<pre><code>namespace intutils
{
 int addNumbers(int a, int b)
 {
 return a + b;
 }
}
</code></pre>

Usage file:

<pre><code>#include "common1.h" 
int main()
{
 int five = 0;
 five = addNumbers(2, 3); // Will fail to compile since the function is in a different namespace.
 five = intutils::addNumbers(2, 3); // Will compile since you have made explicit which namespace the function is contained within.

 using namespace intutils;
 five = addNumbers(2, 3); // Will compile because the previous line tells the compiler that if in doubt it should check the "intutils" namespace.
}
</code></pre>

So, when you write <code>using namespace std</code> all you are doing is telling the compiler that if in doubt it should look in the <code>std</code> namespace for functions, etc., which it can't find definitions for. This is commonly used in example (and production) code simply because it makes typing common functions, etc. like <code>cout</code> is quicker than having to fully qualify each one as <code>std::cout</code>.

blocks|key|1120909|text|C%2B%2B标准库中的所有文件都在std名称空间中声明了它的所有实体。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1120910|例如:使用iostream中定义的cin,cout|offset|length|style|CODE|1120911|替代方案：|1120912|using+std::cout;
using+std::endl;
cout+<<+"Hello"+<<+endl;|code-block|syntax|javascript|1120913|std::cout+<<+"Hello"+<<+std::endl;|1120914|​|1120915|entityMap^0|0|H|8|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|Y|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|Z|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|L|7|10|8|@]|9|@]|A|$M|N]]|$1|Q|3|R|5|6|7|11|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|T|$]]

All the files in the C++ standard library declare all of its entities within the std namespace. 
e.g: To use <code>cin,cout</code> defined in iostream

Alternatives:

<pre><code>using std::cout;
using std::endl;
cout &lt;&lt; "Hello" &lt;&lt; endl;</code></pre>

<pre><code>std::cout &lt;&lt; "Hello" &lt;&lt; std::endl;</code></pre>

blocks|key|1121033|text|每当您使用在名称空间中声明的内容时，就会使用它。C%2B%2B标准库是在名称空间std中声明的。因此，你必须这样做|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1121034|using+namespace+std;|code-block|syntax|javascript|1121035|除非您希望在调用另一个命名空间内的函数时指定命名空间，如下所示：|1121036|std::cout+<<+"cout+is+declared+within+the+namespace+std";|1121037|你可以在http://www.cplusplus.com/doc/tutorial/namespaces/上阅读更多关于它的内容。|offset|length|1121038|entityMap|0|LINK|mutability|MUTABLE|url|http://www.cplusplus.com/doc/tutorial/namespaces/^0|0|0|0|0|4|1D|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@]|9|@$M|11|N|12|1|13]]|A|$]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

It's used whenever you're using something that is declared within a namespace. The C++ standard library is declared within the namespace std. Therefore you have to do

<pre><code>using namespace std;
</code></pre>

unless you want to specify the namespace when calling functions within another namespace, like so:

<pre><code>std::cout &lt;&lt; "cout is declared within the namespace std";
</code></pre>

You can read more about it at <a href="http://www.cplusplus.com/doc/tutorial/namespaces/" rel="nofollow noreferrer">http://www.cplusplus.com/doc/tutorial/namespaces/</a>.

blocks|key|1121076|text|你永远不需要使用名称空间std来声明；使用它是不好的做法，如果你不想输入std:：always，你应该使用std:：，在某些情况下，你可以这样做：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1121077|using+std::cout;|code-block|syntax|javascript|1121078|通过使用std:：，你还可以知道你的程序的哪部分使用了标准库，哪部分没有使用，更重要的是，这可能会与包含的其他函数发生冲突。|1121079|Rgds+Layne|1121080|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]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|P|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

You never have to declare using namespace std; using it is is bad practice and you should use std:: if you don't want to type std:: always you could do something like this in some cases:

<pre><code>using std::cout;
</code></pre>

By using std:: you can also tell which part of your program uses the standard library and which doesn't. Which is even more important that there might be conflicts with other functions which get included.

Rgds
Layne

This question may be a duplicate, but I can't find a good answer. Short and simple, what requires me to declare

<pre><code>using namespace std;
</code></pre>

in C++ programs?

What requires me to declare "using namespace std;"?

这个问题可能是重复的，但我找不到一个好的答案。简明扼要，我需要声明的是using namespace std;在C++程序中？

问什么需要我声明"using namespace std;"？
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问什么需要我声明"using namespace std;"？
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