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<H2>标准名字空间的理解<BR><SPAN> 作者: <A class=aa3
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<DIV class=a_text> 一、标准名字空间的理解： <SPAN class=copyctrl
style="FONT-SIZE: 0pt">V!U> .|[ </SPAN> <BR><SPAN class=copyctrl
style="FONT-SIZE: 0pt">K-4Z|ys </SPAN> <BR>using namespace
std;就是标准名字空间的引用，std是C++标准库中的名字空间名称。大家都知道，当我们定义了一个类，而声明对象的时候就要用到 class_name
后跟对象名称，标准名字空间也是这个用法。 <SPAN class=copyctrl style="FONT-SIZE: 0pt">JsbCq7c
</SPAN> <BR><SPAN class=copyctrl style="FONT-SIZE: 0pt">j*Q +"h/
</SPAN> <BR>其实，我们可以按照一个更为抽象包含更广的类来理解标准名字空间。#include<iostream>中的iostream正是包含在这个标准名字空间里面的一个“对象”。
<SPAN class=copyctrl style="FONT-SIZE: 0pt">,q6[Yh : </SPAN> <BR><SPAN
class=copyctrl style="FONT-SIZE: 0pt">Ls8Si!&\
</SPAN> <BR>namespace是为了解决C++中的名字冲突而引入的。什么是名字冲突呢？比如，在文件x.h中有个类MyClass,
在文件y.h中也有个类MyClass，而在文件z.cpp中要同时引用x.h和y.h文件。显然，按通常的方法是行不能的，那怎么办呢？引入 namespace即可。
<SPAN class=copyctrl style="FONT-SIZE: 0pt">z!GVg"*/1 </SPAN> <BR><SPAN
class=copyctrl style="FONT-SIZE: 0pt">}+,fFzX(5 </SPAN> <BR>标准名字空间的用法：
<SPAN class=copyctrl style="FONT-SIZE: 0pt">#zT5 3UvK </SPAN> <BR><SPAN
class=copyctrl style="FONT-SIZE: 0pt">Y ;D*Cl
</SPAN> <BR>我们完全可以定义自己的标准名字空间，如下例： <SPAN class=copyctrl
style="FONT-SIZE: 0pt">N7tezs13`v </SPAN> <BR><SPAN class=copyctrl
style="FONT-SIZE: 0pt">%UsJpIH </SPAN> <BR>在x.h中的内容为 <SPAN
class=copyctrl style="FONT-SIZE: 0pt">YZ:7<ly!! </SPAN> <BR>// x.h
<SPAN class=copyctrl style="FONT-SIZE: 0pt">+SLTis{%{9
</SPAN> <BR>namespace MyNamespace1 <SPAN class=copyctrl
style="FONT-SIZE: 0pt">U$ A 2 </SPAN> <BR>{ <SPAN class=copyctrl
style="FONT-SIZE: 0pt">CYM&o;e </SPAN> <BR>class MyClass <SPAN
class=copyctrl style="FONT-SIZE: 0pt">Gjl)vq>uw </SPAN> <BR>{ <SPAN
class=copyctrl style="FONT-SIZE: 0pt">2eMVU=_ </SPAN> <BR>public: <SPAN
class=copyctrl style="FONT-SIZE: 0pt">}F:npOF/o </SPAN> <BR>void f();
<SPAN class=copyctrl style="FONT-SIZE: 0pt">D)'J(s </SPAN> <BR>private:
<SPAN class=copyctrl style="FONT-SIZE: 0pt">x1-MG~> Q </SPAN> <BR>int
m; <SPAN class=copyctrl style="FONT-SIZE: 0pt">m m?'>/jt </SPAN> <BR>};
<SPAN class=copyctrl style="FONT-SIZE: 0pt">6P>.%wjC0 </SPAN> <BR>};
<SPAN class=copyctrl style="FONT-SIZE: 0pt">+LK+t B Z3 </SPAN> <BR><SPAN
class=copyctrl style="FONT-SIZE: 0pt">/q=oet}\4~ </SPAN> <BR>在y.h中的内容为
<SPAN class=copyctrl style="FONT-SIZE: 0pt">V.)TX UW </SPAN> <BR>// y.h
<SPAN class=copyctrl style="FONT-SIZE: 0pt">?8O [1zO"
</SPAN> <BR>namespace MyNamespace2 <SPAN class=copyctrl
style="FONT-SIZE: 0pt">HjT'yy (b </SPAN> <BR>{ <SPAN class=copyctrl
style="FONT-SIZE: 0pt">LF}(q`g. </SPAN> <BR>class MyClass <SPAN
class=copyctrl style="FONT-SIZE: 0pt">6zfAyL </SPAN> <BR>{ <SPAN
class=copyctrl style="FONT-SIZE: 0pt">,-}49Mv0 </SPAN> <BR>public: <SPAN
class=copyctrl style="FONT-SIZE: 0pt"> gEljQ </SPAN> <BR>void f();
<SPAN class=copyctrl style="FONT-SIZE: 0pt">wkE&Dxi"
</SPAN> <BR>private: <SPAN class=copyctrl style="FONT-SIZE: 0pt">NgC4`x
</SPAN> <BR>int m; <SPAN class=copyctrl style="FONT-SIZE: 0pt">-x, ?_Mui
</SPAN> <BR>} ; <SPAN class=copyctrl style="FONT-SIZE: 0pt">!Rng`"<H
</SPAN> <BR>}; <SPAN class=copyctrl style="FONT-SIZE: 0pt">1 a[v@cs45
</SPAN> <BR>然后在z.cpp中引入x.h和y.h <SPAN class=copyctrl
style="FONT-SIZE: 0pt">71<~&%!. </SPAN> <BR>// z.cpp <SPAN
class=copyctrl style="FONT-SIZE: 0pt">qA& P{<R
</SPAN> <BR>#include "x.h" <SPAN class=copyctrl
style="FONT-SIZE: 0pt">s^zTH x </SPAN> <BR>#include "y.h" <SPAN
class=copyctrl style="FONT-SIZE: 0pt">z9 XWV^_ </SPAN> <BR>using
namespace MyNamespace1; <SPAN class=copyctrl style="FONT-SIZE: 0pt">e fVc.Hbq
</SPAN> <BR>using namespace MyNamespace2; <SPAN class=copyctrl
style="FONT-SIZE: 0pt"> 28=I/,{ </SPAN> <BR><SPAN class=copyctrl
style="FONT-SIZE: 0pt">"8@* bv </SPAN> <BR>void main() <SPAN
class=copyctrl style="FONT-SIZE: 0pt">ID+!X+^b </SPAN> <BR>{ <SPAN
class=copyctrl style="FONT-SIZE: 0pt">zZ cMp{
</SPAN> <BR>//声明一个文件x.h中类MyClass的实例x <SPAN class=copyctrl
style="FONT-SIZE: 0pt">>U<H+9>
</SPAN> <BR>MyNamespace1::MyClass x; <SPAN class=copyctrl
style="FONT-SIZE: 0pt">.&>NY F
</SPAN> <BR>//声明一个文件x.h中类MyClass的实例x <SPAN class=copyctrl
style="FONT-SIZE: 0pt">"%[OR6H^ </SPAN> <BR>MyNamespace2::MyClass y;
<SPAN class=copyctrl style="FONT-SIZE: 0pt"> II)-z`Q </SPAN> <BR><SPAN
class=copyctrl style="FONT-SIZE: 0pt">,w%ndnU </SPAN> <BR>//调用文件x.h中的函数f
<SPAN class=copyctrl style="FONT-SIZE: 0pt">w~hu6wkuq7 </SPAN> <BR>x.f();
<SPAN class=copyctrl style="FONT-SIZE: 0pt">y/#-ww
</SPAN> <BR>//调用文件y.h中的函数f <SPAN class=copyctrl
style="FONT-SIZE: 0pt">,hq$yF </SPAN> <BR>y.f(); <SPAN class=copyctrl
style="FONT-SIZE: 0pt">UOy y4 </SPAN> <BR>} <SPAN class=copyctrl
style="FONT-SIZE: 0pt">N5~A&b|R </SPAN> <BR><SPAN class=copyctrl
style="FONT-SIZE: 0pt">2^@w 0J v
</SPAN> <BR>名字空间实质上是一个作用域。前面所述，数据成员要包含在类里面，那么做为一个作用域更大的类，要声明的话，根据需要，一般的类就可以放到标准名字空间里面。
<SPAN class=copyctrl style="FONT-SIZE: 0pt">&Rjsa# "i
</SPAN> <BR><SPAN class=copyctrl style="FONT-SIZE: 0pt">[\Sk _ZK
</SPAN> <BR>那么，当我们声明了一个自己的标准名字空间之后，是否还可以添加包含更多的类呢？ <SPAN class=copyctrl
style="FONT-SIZE: 0pt">z=Ni3 </SPAN> <BR><SPAN class=copyctrl
style="FONT-SIZE: 0pt">gp uaD$ [
</SPAN> <BR>答案是肯定的，当我们需要的时候，我们还可以按照原来的格式添加，这样，再引用的时候，在定义的标准名字空间里面就包含了所有添加的类。
<SPAN class=copyctrl style="FONT-SIZE: 0pt">Ck^)#:4+_A
</SPAN> <BR>二、关于名字空间定义 <SPAN class=copyctrl
style="FONT-SIZE: 0pt">`38H<!(L4 </SPAN> <BR>全局实体(global entity) <SPAN
class=copyctrl style="FONT-SIZE: 0pt">IAj\NI][
</SPAN> <BR>全局名字空间污染(global namespace pollution) <SPAN class=copyctrl
style="FONT-SIZE: 0pt">-\%DMP#, </SPAN> <BR><SPAN class=copyctrl
style="FONT-SIZE: 0pt">!W \z^I3 </SPAN> <BR>1、名字空间定义 <SPAN class=copyctrl
style="FONT-SIZE: 0pt">8 l !Bmh_ </SPAN> <BR>namespace 开头，后面是名字空间的名字。
<SPAN class=copyctrl style="FONT-SIZE: 0pt">~(aEatQIA </SPAN> <BR><SPAN
class=copyctrl style="FONT-SIZE: 0pt">X^UU,,- </SPAN> <BR>namespace
cplusplus_primer { <SPAN class=copyctrl style="FONT-SIZE: 0pt">z+I(h9D|'
</SPAN> <BR>class matrix {/*****/}; <SPAN class=copyctrl
style="FONT-SIZE: 0pt">EqLxZ_ </SPAN> <BR>void inverse(matrix &);
<SPAN class=copyctrl style="FONT-SIZE: 0pt">Xcg;$<4 </SPAN> <BR>matrix
operator+(const matrix &m1,matrix &m2) <SPAN class=copyctrl
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