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<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体">就可以给它定义一个指针：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">MyStruct* pStruct;</span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体">对其进行初始化：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">MyStruct Struct = new MyStruct();</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">pStruct = &amp;Struct;</span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体">也可以通过指针访问结构的成员值：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">(*pStruct).X = 4;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">(*pStruct).F = 3.4f;</span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体">但是，这个语法有点复杂。因此，</span><span lang="EN-US">C#</span><span style="FONT-FAMILY: 宋体">定义了另一个运算符，用一种比较简单的语法，通过指针访问结构的成员，该语法称为指针成员访问运算符，其符号是一个短划线，后跟一个大于号：&ndash;</span><span lang="EN-US">&gt;</span><span style="FONT-FAMILY: 宋体">。</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 lang="EN-US">C++</span><span style="FONT-FAMILY: 楷体_GB2312">开发人员会认出指针成员访问操作符。因为</span><span lang="EN-US">C++</span><span style="FONT-FAMILY: 楷体_GB2312">使用这些符号完成相同的任务。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">使用这个指针成员访问运算符，上述代码可以重写为：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="arrow"><span lang="EN-US">pStruct</span></a><span style="FONT-FAMILY: 宋体">&ndash;</span><span lang="EN-US">&gt;X = 4;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">pStruct</span><span style="FONT-FAMILY: 宋体">&ndash;</span><span lang="EN-US">&gt;F = 3.4f;</span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体">也可以直接把合适类型的指针设置为指向结构中的一个字段：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">long* pL = &amp;(Struct.X);</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">float* pF = &amp;(Struct.F);</span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体">或者</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">long* pL = &amp;(pStruct</span><span style="FONT-FAMILY: 宋体">&ndash;</span><span lang="EN-US">&gt;X);</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">float* pF = &amp;(pStruct</span><span style="FONT-FAMILY: 宋体">&ndash;</span><span lang="EN-US">&gt;F);</span></p>
<h4 style="FTEL: 21.45pt"><span lang="EN-US">9. </span><span style="FONT-FAMILY: 黑体">类成员指针</span></h4>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">前面说过，不能创建指向类的指针，这是因为垃圾收集器不包含指针的任何信息，只包含引用的信息，因此创建指向类的指针会使垃圾收集器不能正常工作。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">但是，大多数类都包含值类型的成员，可以为这些值类型成员创建指针，但这需要一种特殊的语法。例如，假定把上面示例中的结构重写为类：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">class MyClass</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; public long X;</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; public float F;</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">} </span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">然后就可以为它的字段</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">X</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">和</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">F</span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">创建指针了，方法与前面一样。但这么做会抛出一个编译</span><span lang="EN-US" style="LETTER-SPACING: 0.1pt">&nbsp;&nbsp;&nbsp; </span><span style="FONT-FAMILY: 宋体; LETTER-SPACING: 0.1pt">错误：</span></p>
<p class="2" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="pointersfields"><span lang="EN-US">MyClass myObject = new MyClass();</span></a></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">long* pL = &amp;( myObject.X);&nbsp;&nbsp; // wrong&ndash; &ndash;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">float* pF = &amp;( myObject.F);&nbsp; &nbsp;// wrong&ndash; &ndash;compilation error</span></p>
<p class="MsoNormal"><span lang="EN-US">X</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">F</span><span style="FONT-FAMILY: 宋体">本身都是非托管类型，它们嵌入在一个对象中，存储在堆上。在垃圾收集的过程中，垃圾收集器会把</span><span lang="EN-US">MyClass</span><span style="FONT-FAMILY: 宋体">移动到内存的一个新单元上，这样，</span><span lang="EN-US"> pL</span><span style="FONT-FAMILY: 宋体">和</span><span lang="EN-US">pF</span><span style="FONT-FAMILY: 宋体">就会指向错误的存储单元。由于存在这个问题，所以编译器不允许以这种方式把托管类型的成员地址分配给指针。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">解决这个问题的方法是使用</span><span lang="EN-US">fixed</span><span style="FONT-FAMILY: 宋体">关键字，它会告诉垃圾收集器，类实例的某些成员有指向它们的指针，所以这些实例不能移动。如果要声明一个指针，使用</span><span lang="EN-US">fixed</span><span style="FONT-FAMILY: 宋体">的语法如下所示：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="fixed"><span lang="EN-US">MyClass myObject = new MyClass();</span></a></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">fixed (long* pObject = &amp;( myObject.X))</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; // do something</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal" style="FTEL: 0cm"><span style="FONT-FAMILY: 宋体">在关键字</span><span lang="EN-US">fixed</span><span style="FONT-FAMILY: 宋体">后面的圆括号中，定义和初始化指针变量。这个指针变量</span><span lang="EN-US">(</span><span style="FONT-FAMILY: 宋体">在本例中是</span><span lang="EN-US">pObject)</span><span style="FONT-FAMILY: 宋体">现在就在</span><span lang="EN-US">fixed</span><span style="FONT-FAMILY: 宋体">块的作用域内，这样，垃圾收集器知道，在执行</span><span lang="EN-US">fixed</span><span style="FONT-FAMILY: 宋体">块中的代码时，不能移动</span><span lang="EN-US">MyObject</span><span style="FONT-FAMILY: 宋体">对象。</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">如果要声明多个这样的指针，可以在同一个代码块前放置多个</span><span lang="EN-US">fixed</span><span style="FONT-FAMILY: 宋体">语句：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">MyClass myObject = new MyClass();</span></p>
<p class="2" style="MARGIN-TOP: 6pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">fixed (long* pX = &amp;( myObject.X))</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">fixed (float* pF = &amp;( myObject.F))</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; // do something</span></p>
<p class="a6" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">如果要在不同的阶段固定几个指针，还可以嵌套整个</span><span lang="EN-US">fixed</span><span style="FONT-FAMILY: 宋体">块：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">MyClass myObject = new MyClass();</span></p>
<p class="2" style="MARGIN-TOP: 6pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">fixed (long* pX = &amp;( myObject.X))</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; // do something with pX</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; fixed (float* pF = &amp;( myObject.F))</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; {</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; // do something else with pF</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; }</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">}</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">也可以在同一个</span><span lang="EN-US">fixed</span><span style="FONT-FAMILY: 宋体">语句中初始化多个变量，但这些变量的类型必须相同：</span></p>
<p class="a6" style="MARGIN-TOP: 8.15pt; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><a ftel="OLE_LINK9"><span lang="EN-US">MyClass myObject = new MyClass();</span></a></p>
<p class="a6" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">MyClass myObject2 = new MyClass();</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">fixed (long* pX = &amp;( myObject.X), pX2 = &amp;( myObject2.X))</span></p>
<p class="2" style="MARGIN-LEFT: 21.45pt; FTEL: 18.45pt"><span lang="EN-US">{</span></p>
<p class="2" style="MARGIN-TOP: 0cm; MARGIN-LEFT: 21.45pt; MARGIN-RIGHT: 0cm; FTEL: 18.45pt"><span lang="EN-US">&nbsp;&nbsp; // etc.</span></p>
<p class="MsoNormal"><span style="FONT-FAMILY: 宋体">在上述情况中，是否声明不同的指针，让它们指向相同或不同对象中的字段，或者指向不与类实例相关的静态字段，这一点是不重要的。</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='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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