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<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">int* pWidth, pHeight;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">double* pResult;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">byte*[] pFlags;</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">这段代码声明了</span><span lang="EN-US">4</span><span style="FONT-FAMILY: 宋体">个变量，</span><span lang="EN-US">pWidth</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">pHeight</span><span style="FONT-FAMILY: 宋体">是整数指针，</span><span lang="EN-US">pResult</span><span style="FONT-FAMILY: 宋体">是</span><span lang="EN-US">double</span><span style="FONT-FAMILY: 宋体">型指针，</span><span lang="EN-US">pFlags</span><span style="FONT-FAMILY: 宋体">是</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">型的指针数组。我们常常在指针变量名的前面使用前缀</span><span lang="EN-US">p</span><span style="FONT-FAMILY: 宋体">来表示这些变量是指针。在变量声明中，符号</span><span lang="EN-US">*</span><span style="FONT-FAMILY: 宋体">表示声明一个指针，换言之，就是存储特定类型的变量的地址。</span></p>
<p class="a3" style="MARGIN-TOP: 8.15pt; FTEL: 21.45pt"><span style="FONT-FAMILY: 黑体">提示：</span></p>
<p class="a1" style="FTEL: 21.45pt"><span lang="EN-US">C++</span><span style="FONT-FAMILY: 楷体_GB2312">开发人员应注意，这个语法与</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 楷体_GB2312">中的语法是不同的。</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 楷体_GB2312">语句中</span><span lang="EN-US"> int* pX, pY; </span><span style="FONT-FAMILY: 楷体_GB2312">对应于</span><span lang="EN-US">C++ </span><span style="FONT-FAMILY: 楷体_GB2312">语句中的</span><span lang="EN-US"> int *pX, *pY</span><span style="FONT-FAMILY: 楷体_GB2312">；在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 楷体_GB2312">中<strong>，</strong></span><strong><span lang="EN-US">*</span></strong><span style="FONT-FAMILY: 楷体_GB2312">符号与类型相关，而不是与变量名相关。</span></p>
<p class="aa"><span style="FONT-FAMILY: 宋体">声明了指针类型的变量后，就可以用与一般变量的方式使用它们，但首先需要学习另外两个运算符：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><a ftel="ampersand"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span></span></a><span lang="EN-US">&amp; </span><span style="FONT-FAMILY: 宋体">表示&ldquo;取地址&rdquo;，并把一个值数据类型转换为指针，例如</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">转换为</span><span lang="EN-US">*int</span><span style="FONT-FAMILY: 宋体">。这个运算符称为寻址运算符。</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><a ftel="star"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; </span></span></a><span lang="EN-US">* </span><span style="FONT-FAMILY: 宋体">表示&ldquo;获取地址的内容&rdquo;，把一个指针转换为值数据类型</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">例如，</span><span lang="EN-US">*float</span><span style="FONT-FAMILY: 宋体">转换为</span><span lang="EN-US">float)</span><span style="FONT-FAMILY: 宋体">。这个运算符称为&ldquo;间接寻址运算符&rdquo;</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">有时称为&ldquo;取消引用运算符&rdquo;</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">从这些定义中可以看出，</span><span lang="EN-US">&amp;</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">*</span><span style="FONT-FAMILY: 宋体">的作用是相反的。</span></p>
<p class="a3" style="MARGIN-TOP: 8.15pt; FTEL: 21.45pt"><span style="FONT-FAMILY: 黑体">注意：</span></p>
<p class="a1" style="FTEL: 21.45pt"><span style="FONT-FAMILY: 楷体_GB2312">符号</span><span lang="EN-US">&amp;</span><span style="FONT-FAMILY: 楷体_GB2312">和</span><span lang="EN-US">*</span><span style="FONT-FAMILY: 楷体_GB2312">也表示按位</span><span lang="EN-US">AND(&amp;)</span><span style="FONT-FAMILY: 楷体_GB2312">和乘法</span><span lang="EN-US">(*)</span><span style="FONT-FAMILY: 楷体_GB2312">运算符，那么如何以这种方式使用它们？答案是在实际使用时它们是不会混淆的</span><span lang="EN-US">:</span><span style="FONT-FAMILY: 楷体_GB2312">用户和编译器总是知道在什么情况下这两个符号有什么含义，因为按照新指针的定义，这些符号总是以一元运算符的形式出现<span style="LETTER-SPACING: -1pt">&mdash;&mdash;</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 楷体_GB2312">它们只作用于一个变量，并出现在代码中变量的前面。另一方面，按位</span><span lang="EN-US">AND</span><span style="FONT-FAMILY: 楷体_GB2312">和乘法运算符是二元运算符，它们需要两个变量。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">下面的代码说明了如何使用这些运算符：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="OLE_LINK7"><span lang="EN-US">int x = 10;</span></a></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">int* pX, pY;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">pX = &amp;x;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">pY = pX;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">*pY = 20;</span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体">首先声明一个整数</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">，接着声明两个整数指针</span><span lang="EN-US">pX</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">。然后把</span><span lang="EN-US">pX</span><span style="FONT-FAMILY: 宋体">设置为指向</span><span lang="EN-US">x(</span><span style="FONT-FAMILY: 宋体">换言之，把</span><span lang="EN-US">pX</span><span style="FONT-FAMILY: 宋体">的内容设置为</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">的地址</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。把</span><span lang="EN-US">pX</span><span style="FONT-FAMILY: 宋体">的值赋予</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">，所以</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">也指向</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">。最后，在语句</span><span lang="EN-US">*pY = 20</span><span style="FONT-FAMILY: 宋体">中，把值</span><span lang="EN-US">20</span><span style="FONT-FAMILY: 宋体">赋予</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">指向的地址。实际上是把</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">的内容改为</span><span lang="EN-US">20</span><span style="FONT-FAMILY: 宋体">，因为</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">指向</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">。注意在这里，变量</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">之间没有任何关系。只是此时</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">碰巧指向存储</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">的存储单元而已。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">要进一步理解这个过程，假定</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">存储在堆栈的存储单元</span><span lang="EN-US">0x12F8C4</span><span style="FONT-FAMILY: 宋体">到</span><span lang="EN-US">0x12F8C7</span><span style="FONT-FAMILY: 宋体">中</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">十进制就是</span><span lang="EN-US">1243332</span><span style="FONT-FAMILY: 宋体">到</span><span lang="EN-US">1243335</span><span style="FONT-FAMILY: 宋体">，即有</span><span lang="EN-US">4B</span><span style="FONT-FAMILY: 宋体">，因为</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">占用</span><span lang="EN-US">4B)</span><span style="FONT-FAMILY: 宋体">。因为堆栈向下分配内存，所以变量</span><span lang="EN-US">pX</span><span style="FONT-FAMILY: 宋体">存储在</span><span lang="EN-US">0x12F8C0</span><span style="FONT-FAMILY: 宋体">到</span><span lang="EN-US"> 0x12F8C3</span><span style="FONT-FAMILY: 宋体">的位置上，</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">存储在</span><span lang="EN-US">0x12F8BC </span><span style="FONT-FAMILY: 宋体">到</span><span lang="EN-US"> 0x12F8BF</span><span style="FONT-FAMILY: 宋体">的位置上。注意，</span><span lang="EN-US">pX</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">pY</span><span style="FONT-FAMILY: 宋体">也分别占用</span><span lang="EN-US">4B</span><span style="FONT-FAMILY: 宋体">。这不是因为</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">占用</span><span lang="EN-US">4B</span><span style="FONT-FAMILY: 宋体">，而是因为在</span><span lang="EN-US">32</span><span style="FONT-FAMILY: 宋体">位处理器上，需要用</span><span lang="EN-US">4B</span><span style="FONT-FAMILY: 宋体">存储一个地址。利用这些地址，在执行完上述代码后，堆栈应如图</span><span lang="EN-US">7-5</span><span style="FONT-FAMILY: 宋体">所示。</span></p>
<p style="FTEL: 8.15pt" align="center"><span lang="EN-US"><img height="155" src="07/image005.gif" width="226" alt="" /></span></p>
<p style="FTEL: 8.15pt" align="center"><span style="FONT-FAMILY: 宋体">图</span><span lang="EN-US">&nbsp; 7-5</span></p>
<p class="a3" style="MARGIN-TOP: 8.15pt; FTEL: 21.45pt"><span style="FONT-FAMILY: 黑体">注意：</span></p>
<p class="a1" style="FTEL: 21.45pt"><span style="FONT-FAMILY: 楷体_GB2312">这个示例使用的是</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 楷体_GB2312">来说明该过程，其中</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 楷体_GB2312">存储在</span><span lang="EN-US">32</span><span style="FONT-FAMILY: 楷体_GB2312">位处理器中堆栈的连续空间上，但并不是所有的数据类型都会存储在连续的空间中。原因是</span><span lang="EN-US">32</span><span style="FONT-FAMILY: 楷体_GB2312">位处理器最擅长于在</span><span lang="EN-US">4B</span><span style="FONT-FAMILY: 楷体_GB2312">的内存块中获取数据。这种机器上的内存会分解为</span><span lang="EN-US">4</span><span style="FONT-FAMILY: 楷体_GB2312">字节的块，在</span><span lang="EN-US">Windows</span><span style="FONT-FAMILY: 楷体_GB2312">上，每个块都时常称为</span><span lang="EN-US">DWORD</span><span style="FONT-FAMILY: 楷体_GB2312">，因为这是</span><span lang="EN-US">32</span><span style="FONT-FAMILY: 楷体_GB2312">位无符号</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 楷体_GB2312">在</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 楷体_GB2312">出现之前的名字。这是从内存中获取</span><span lang="EN-US">DWORD</span><span style="FONT-FAMILY: 楷体_GB2312">的最高效的方式<span style="LETTER-SPACING: -1pt">&mdash;&mdash;</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 楷体_GB2312">跨越</span><span lang="EN-US">DWORD</span><span style="FONT-FAMILY: 楷体_GB2312">边界存储数据通常会降低硬件的性能。因此，</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 楷体_GB2312">运行库通常会给某些数据类型加上一些空间，使它们占用的内存是</span><span lang="EN-US">4B</span><span style="FONT-FAMILY: 楷体_GB2312">的倍数。例如，</span><span lang="EN-US">short</span><span style="FONT-FAMILY: 楷体_GB2312">数据占用</span><span lang="EN-US">2B</span><span style="FONT-FAMILY: 楷体_GB2312">，但如果把一个</span><span lang="EN-US">short</span><span style="FONT-FAMILY: 楷体_GB2312">放在堆栈中，堆栈指针仍会减少</span><span lang="EN-US">4</span><span style="FONT-FAMILY: 楷体_GB2312">，而不是</span><span lang="EN-US">2</span><span style="FONT-FAMILY: 楷体_GB2312">，这样，下一个存储在堆栈中的变量就仍从</span><span lang="EN-US">DWORD</span><span style="FONT-FAMILY: 楷体_GB2312">的边界开始存储。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">可以把指针声明为任意一种数据类型<span style="LETTER-SPACING: -1pt">&mdash;&mdash;</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体">即任何预定义的数据类型</span><span lang="EN-US">uint</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">byte</span><span style="FONT-FAMILY: 宋体">等，也可以声明为一个结构。但是不能把指针声明为一个类或数组，这是因为这么做会使垃圾收集器出现问题。为了正常工作，垃圾收集器需要知道在堆上创建了什么类实例，它们在什么地方。但如果代码使用指针处理类，将很容易破坏堆中</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">运行库为垃圾收集器维护的、与类相关的信息。在这里，垃圾收集器可以访问的数据类型称为托管类型，而指针只能声明为非托管类型，因为垃圾收集器不能处理它们。</span></p></div>
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                        <h1 id="divCurrentNode2" style="color: #b83507; width: 100%; text-align: left; font-size: 12px; padding-left: 2px">当前章节:<a href='100165348.htm'><font color='red'>7.3.1  指针(1)</font></a></h1>
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                        <div style='float:left;width:49%'>·<a href='100165345.htm'>7.1  后台内存管理</a></div><div style='float:right;width:49%'>·<a href='100165346.htm'>7.2  释放未托管的资源</a></div><div style='float:left;width:49%'>·<a href='100165347.htm'>7.3  不安全的代码</a></div><div style='float:right;width:49%'>·<a href='100165349.htm'>7.3.1  指针(2)</a></div><div style='float:left;width:49%'>·<a href='100165350.htm'>7.3.1  指针(3)</a></div><div style='float:right;width:49%'>·<a href='100165351.htm'>7.3.2  使用指针优化性能</a></div></div>
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