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7.3.1 指针(2)
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C#高级编程（第3版）【全文连载】
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7.3.1 指针(2)
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2006-10-13 14:41:00
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<h1 id="divCurrentNode" style="color: #b83507; width: 100%; text-align: left; font-size: 12px; padding-left: 2px">当前章节:<a href='100165349.htm'><font color='red'>7.3.1 指针(2)</font></a></h1>
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<div style='float:left;width:49%'>·<a href='100165346.htm'>7.2 释放未托管的资源</a></div><div style='float:right;width:49%'>·<a href='100165347.htm'>7.3 不安全的代码</a></div><div style='float:left;width:49%'>·<a href='100165348.htm'>7.3.1 指针(1)</a></div><div style='float:right;width:49%'>·<a href='100165350.htm'>7.3.1 指针(3)</a></div><div style='float:left;width:49%'>·<a href='100165351.htm'>7.3.2 使用指针优化性能</a></div><div style='float:right;width:49%'>·<a href='100165352.htm'>7.4 小结</a></div></div>
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<div id="text"> <link href="css.css" rel="stylesheet" type="text/css" /><h4 style="FTEL: 21.45pt"><span lang="EN-US">3. </span><span style="FONT-FAMILY: 黑体">将指针转换为整数类型</span></h4>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><a ftel="pointerscasts"><span style="FONT-FAMILY: 宋体">由于指针实际上存储了一个表示地址的整数，所以任何指针中的地址都可以转换为任何整数类型。指针到整数类型的转换必须是显式指定的，隐式的转换是不允许的。例如，编写下面的代码是合法的：</span></a></p>
<p class="a6" style="MARGIN-TOP: 4.9pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="FR">int x = 10;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="FR">int* pX, pY;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="FR">pX = &x;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="FR">pY = pX;</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="FR">*pY = 20;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="FR">uint y = (uint)pX;</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="FR">int* pD = (int*)y;</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">把指针</span><span lang="FR">pX</span><span style="FONT-FAMILY: 宋体">中包含的地址转换为</span><span style="FONT-FAMILY: 宋体">一个</span><span lang="FR">uint</span><span style="FONT-FAMILY: 宋体">，</span><span style="FONT-FAMILY: 宋体">存储在变量</span><span lang="FR">y</span><span style="FONT-FAMILY: 宋体">中。接着把</span><span lang="FR">y</span><span style="FONT-FAMILY: 宋体">转换</span><span style="FONT-FAMILY: 宋体">回</span><span lang="FR">int*</span><span style="FONT-FAMILY: 宋体">，存储</span><span style="FONT-FAMILY: 宋体">在新变量</span><span lang="FR">pD</span><span style="FONT-FAMILY: 宋体">中。因此</span><span lang="EN-US">pD</span><span style="FONT-FAMILY: 宋体">也指向</span><span lang="EN-US">x</span><span style="FONT-FAMILY: 宋体">的值。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">把指针的值转换为整数类型的主要原因是为了显示它。</span><span lang="EN-US">Console.Write()</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">Console. WriteLine()</span><span style="FONT-FAMILY: 宋体">方法没有任何带指针的重载方法，所以必须把指针转换为整数类型，才能接受和显示它们：</span></p>
<p class="2" style="MARGIN-TOP: 9.8pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">Console.WriteLine("Address is" + pX); // wrong – will give a</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US"> // compilation error</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">Console.WriteLine("Address is" + (uint) pX); // OK</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">可以把一个指针转换为任何整数类型，但是，因为在</span><span lang="EN-US">32</span><span style="FONT-FAMILY: 宋体">位系统上，地址占用</span><span lang="EN-US">4B</span><span style="FONT-FAMILY: 宋体">，把指针转换为不是</span><span lang="EN-US">uint</span><span style="FONT-FAMILY: 宋体">、</span><span lang="EN-US">long </span><span style="FONT-FAMILY: 宋体">或</span><span lang="EN-US"> ulong</span><span style="FONT-FAMILY: 宋体">的数据类型，肯定会导致溢出错误</span><span lang="EN-US">(int</span><span style="FONT-FAMILY: 宋体">也可能导致这个问题，因为它的取值范围是–</span><span lang="EN-US">20</span><span style="FONT-FAMILY: 宋体">亿到</span><span lang="EN-US">20</span><span style="FONT-FAMILY: 宋体">亿，而地址的取值范围是</span><span lang="EN-US">0</span><span style="FONT-FAMILY: 宋体">到</span><span lang="EN-US">40</span><span style="FONT-FAMILY: 宋体">亿</span><span lang="EN-US">)</span><span style="FONT-FAMILY: 宋体">。</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">是用于</span><span lang="EN-US">64</span><span style="FONT-FAMILY: 宋体">位处理器的，地址占用</span><span lang="EN-US">8B</span><span style="FONT-FAMILY: 宋体">。因此在这样的系统上，把指针转换为非</span><span lang="EN-US">ulong</span><span style="FONT-FAMILY: 宋体">的类型，就可能导致溢出错误。还要注意，</span><span lang="EN-US">checked</span><span style="FONT-FAMILY: 宋体">关键字不能用于涉及指针的转换。对于这种转换，即使在</span><span lang="EN-US">checked</span><span style="FONT-FAMILY: 宋体">情况下，发生溢出时也不会抛出异常。</span><span lang="EN-US">.NET</span><span style="FONT-FAMILY: 宋体">运行库假定，如果要使用指针，就知道自己要做什么，并希望出现溢出。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">4. </span><span style="FONT-FAMILY: 黑体">指针类型之间的转换</span></h4>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">也可以在指向不同类型的指针之间进行显式的转换。例如：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">byte aByte = 8;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="EN-US">byte* pByte= &aByte;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">double* pDouble = (double*)pByte;</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt; FTEL: 0cm"><span style="FONT-FAMILY: 宋体">这是一段合法的代码，但如果要执行这段代码，就要小心了。在上面的示例中，如果要查找指针</span><span lang="EN-US">pDouble</span><span style="FONT-FAMILY: 宋体">指向的</span><span lang="EN-US">double</span><span style="FONT-FAMILY: 宋体">，就会查找包含</span><span lang="EN-US">1B</span><span style="FONT-FAMILY: 宋体">的内存，并和一些其他内存合并在一起，把它当作包含一个</span><span lang="EN-US">double</span><span style="FONT-FAMILY: 宋体">的内存区域来对待<span style="LETTER-SPACING: -1pt">——</span></span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体">这不会得到一个有意义的值。但是，可以在类型之间转换，实现类型的统一，或者把指针转换为其他类型，例如把指针转换为</span><span lang="EN-US">sbyte</span><span style="FONT-FAMILY: 宋体">，检查内存的单个字节。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">5.</span><span lang="EN-US" style="FONT-FAMILY: 宋体"> </span><span lang="EN-US">void</span><span style="FONT-FAMILY: 黑体">指针</span></h4>
<p class="MsoNormal"><a ftel="pointersvoid"><span style="FONT-FAMILY: 宋体">如果要使用一个指针，但不希望指定它指向的数据类型，就可以把指针声明为</span><span lang="EN-US">void</span></a><span style="FONT-FAMILY: 宋体">：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">int* pointerToInt;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="EN-US">void* pointerToVoid;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">pointerToVoid = (void*)pointerToInt; </span></p>
<p class="MsoNormal"><span lang="EN-US">void</span><span style="FONT-FAMILY: 宋体">型指针的主要用途是调用需要</span><span lang="EN-US">void*</span><span style="FONT-FAMILY: 宋体">型参数的</span><span lang="EN-US">API</span><span style="FONT-FAMILY: 宋体">函数。在</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">语言中，使用</span><span lang="EN-US">void</span><span style="FONT-FAMILY: 宋体">指针的情况并不是很多。特殊情况下，如果试图使用</span><span lang="EN-US">*</span><span style="FONT-FAMILY: 宋体">运算符间接引用</span><span lang="EN-US">void</span><span style="FONT-FAMILY: 宋体">指针，编译器就会标记一个错误。</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">6. </span><span style="FONT-FAMILY: 黑体">指针的算法</span></h4>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><a ftel="arithmetic"><span style="FONT-FAMILY: 宋体">可以给指针加减整数。但是，编译器很智能，知道如何执行这个操作。例如，假定有一个</span><span lang="EN-US">int</span></a><span style="FONT-FAMILY: 宋体">指针，要在其值上加</span><span lang="EN-US">1</span><span style="FONT-FAMILY: 宋体">。编译器会假定要查找</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">后面的存储单元，因此会给该值加上</span><span lang="EN-US">4B</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -1pt">，</span><span style="LETTER-SPACING: -1pt"> </span><span style="FONT-FAMILY: 宋体">即加上</span><span lang="EN-US">int</span><span style="FONT-FAMILY: 宋体">的字节数。如果这是一个</span><span lang="EN-US">double</span><span style="FONT-FAMILY: 宋体">指针，加</span><span lang="EN-US">1</span><span style="FONT-FAMILY: 宋体">就表示在指针的值上加</span><span lang="EN-US">8B</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">，即</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">double</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">的字节数。只有指针是指向</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">byte</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">或</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt"> sbyte(</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">都是</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">1B)</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">，才会给该指针的值加上</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">1</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">。</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">可以对指针使用运算符</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">+</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">、–、</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">+=</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">、–</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">=</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">、</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">++</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">和</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -2pt">––</span><span style="LETTER-SPACING: -2pt"> </span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">，这些运算符右边的变量必须是</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">long</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">或</span><span lang="EN-US" style="LETTER-SPACING: -0.1pt">ulong</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: -0.1pt">类型。</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">不允许针对</span><span lang="EN-US">void</span><span style="FONT-FAMILY: 楷体_GB2312">指针执行算术运算。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">例如，假定有如下定义：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="OLE_LINK5"><span lang="EN-US">uint u = 3;</span></a></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="EN-US">byte b = 8;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="EN-US">double d = 10.0;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="EN-US">uint* pUint= &u; // size of a uint is 4</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; LINE-HEIGHT: 15pt; FTEL: 18.45pt"><span lang="EN-US">byte* pByte = &b; // size of a byte is 1</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">double* pDouble = &d; // size of a double is 8</span></p>
<p class="MsoNormal" style="LINE-HEIGHT: 17pt"><span style="FONT-FAMILY: 宋体">下面假定这些指针的地址是：</span></p>
<p class="1" style="MARGIN-LEFT: 37.55pt; FTEL: -16.1pt"><span lang="EN-US">●<span style="FONT: 7pt 'Times New Roman'"> </span></span><span lang="EN-US">pUint</span><span style="FONT-FAMILY: 宋体">：</span><span lang="EN-US">1243332</span></p>
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