📄 00205.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN">
<html>
<head>
<title>18.2.2 搜索顺序规则</title>
<meta http-equiv="Content-Type" content="text/html; charset=GB2312" />
<meta name="generator" content="Doc-O-Matic" />
<meta http-equiv="Content-Style-Type" content="text/css" />
<link rel="STYLESHEET" href="default.css" type="text/css" />
<script type="text/javascript" src="scripts.js"></script>
</head>
<body class="Element700" onload="onBodyLoadEx('systemverilog31a.html', 'topic', '00205.html');" onmousedown="onBodyMouseDown();">
<!-- Begin Popups -->
<div class="Element801" id="popup00375">
<div class="Element800">
<div class="Element14">
链接</div>
<div class="Element11">
<div class="Element10">
<a href="00203.html" target="topic">18.2 包</a></div>
</div>
</div>
</div>
<!-- End Popups -->
<!-- Begin Page Header -->
<div class="Element710" id="areafixed">
<div class="Element92">
<table width="100%" cellspacing="0" cellpadding="0">
<tr><td width="33%">
<div class="Element1">
<a href="#" onmousedown="showPopup(this, 'popup00375');"><img src="seealsolink.png" border="0" alt="" title=""></a> SystemVerilog 3.1a语言参考手册</div>
</td><td width="34%">
<div class="Element2">
</div>
</td><td width="33%">
<div class="Element90">
<a href="00204.html" target="topic"><img src="btn_prev_lightblue.gif" border="0" alt="Previous" title="Previous" onmouseover="switchImage(this, 'btn_prev_lightblue_hover.gif');" onmouseout="switchImage(this, 'btn_prev_lightblue.gif');"></a><a href="00203.html" target="topic"><img src="btn_up_lightblue.gif" border="0" alt="Up" title="Up" onmouseover="switchImage(this, 'btn_up_lightblue_hover.gif');" onmouseout="switchImage(this, 'btn_up_lightblue.gif');"></a><a href="00206.html" target="topic"><img src="btn_next_lightblue.gif" border="0" alt="Next" title="Next" onmouseover="switchImage(this, 'btn_next_lightblue_hover.gif');" onmouseout="switchImage(this, 'btn_next_lightblue.gif');"></a></div>
</td></tr></table><div class="Element5">
18.2.2 搜索顺序规则</div>
</div>
</div>
<!-- End Page Header -->
<!-- Begin Client Area -->
<div class="Element720" id="areascroll">
<div class="Element721">
<!-- Begin Page Content -->
<div class="Element58">
<a name="描述"></a><div class="Element11">
<div class="Element10">
<p class="Element10">
对于从一个包中导入的声明，表格18-1为其描述了搜索顺序规则。为了方便以后的讨论，考虑下面的包声明： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00798');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00798"><pre class="Element12"><span style="color: #0000FF">package</span> p<span style="color: #800080">;</span>
<span style="color: #0000FF">typedef enum</span> <span style="color: #800080">{</span>FALSE<span style="color: #800080">,</span> TRUE<span style="color: #800080">}</span> BOOL<span style="color: #800080">;</span>
<span style="color: #0000FF">const</span> BOOL c <span style="color: #800080">=</span> FALSE<span style="color: #800080">;</span>
<span style="color: #0000FF">endpackage</span>
<span style="color: #0000FF">package</span> q<span style="color: #800080">;</span>
<span style="color: #0000FF">const int</span> c <span style="color: #800080">= </span><span style="color: #FF0000">0</span><span style="color: #800080">;</span>
<span style="color: #0000FF">endpackage</span></pre></div></div>
<p class="Element10" style="text-align: center">
</p>
<p class="Element10" style="text-align: center">
<strong>表格18-1：包导入的作用域规则</strong></p><div class="Element63">
<div class="TableDiv">
<table cellspacing="0" class="Table1">
<tr>
<td class="Element65" valign="top" width="20%">
<div class="Element66">
例子 </div></td><td class="Element65" valign="top" width="16%">
<div class="Element66">
描述 </div></td><td class="Element65" valign="top" width="16%">
<div class="Element66">
包含了一个c的本地声明的作用域 </div></td><td class="Element65" valign="top" width="16%">
<div class="Element66">
没有包含一个c的本地声明的作用域 </div></td><td class="Element65" valign="top" width="16%">
<div class="Element66">
包含了一个使用q::c导入的c声明的作用域 </div></td><td class="Element65" valign="top" width="16%">
<div class="Element66">
包含了一个作为q::*导入的c声明的作用域 </div></td></tr><tr>
<td class="Element67" valign="top" width="20%">
<div class="Element68">
<span class="Element146">u <span style="color: #800080">=</span> p<span style="color: #800080">::</span>c<span style="color: #800080">;</span></span><br><span class="Element146">y <span style="color: #800080">=</span> p<span style="color: #800080">::</span>TRUE<span style="color: #800080">;</span></span> </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
一个限定的包标识符在任意作用域内可见（无需使用一条导入子句） </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK<br><br>对<i><span class="Element146">c</span></i>的直接引用指向本地声明的<i><span class="Element146">c</span></i>。<br><br><i><span class="Element146">p<span style="color: #800080">::</span>c</span></i>指向包<i><span class="Element146">p</span></i>中的<i><span class="Element146">c</span></i>。 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK<br><br>对<i><span class="Element146">c</span></i>的直接引用是非法的，因为它是未定义的。<br><br><i><span class="Element146">p<span style="color: #800080">::</span>c</span></i>指向包<i><span class="Element146">p</span></i>中的<i><span class="Element146">c</span></i>。 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK<br><br>对<i><span class="Element146">c</span></i>的直接引用指向从<i><span class="Element146">q</span></i>中导入的<i><span class="Element146">c</span></i>。<br><br><i><span class="Element146">p<span style="color: #800080">::</span>c</span></i>指向包<i><span class="Element146">p</span></i>中的<i><span class="Element146">c</span></i>。 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK<br><br>对<i><span class="Element146">c</span></i>的直接引用指向从<i><span class="Element146">q</span></i>中导入的<i><span class="Element146">c</span></i>。<br><br><i><span class="Element146">p<span style="color: #800080">::</span>c</span></i>指向包<i><span class="Element146">p</span></i>中的<i><span class="Element146">c</span></i>。 </div></td></tr><tr>
<td class="Element67" valign="top" width="20%">
<div class="Element68">
<span class="Element146"><span style="color: #0000FF">import</span> p<span style="color: #800080">::</span><span style="color: #FF0000">*</span><span style="color: #800080">;</span></span><br><span class="Element146">. . .</span><br><span class="Element146">y <span style="color: #800080">=</span> FALSE<span style="color: #800080">;</span></span> </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
包p内的所有声明在导入作用域内都具有潜在的直接可视性：
<ul class="Element634">
<li class="Element604"><span class="Element146">c</span></li>
<li class="Element604"><span class="Element146">BOOL</span></li>
<li class="Element604"><span class="Element146">FALSE</span></li>
<li class="Element604"><span class="Element146">TRUE</span></li>
</ul> </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK<br><br>对<i><span class="Element146">c</span></i>的直接引用指向本地声明的<i><span class="Element146">c</span></i>。<br><br>对其它标识符（例如<span class="Element146">FALSE</span>）的引用指向那些隐含地从包p中导入的标识符。 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK<br><br>对<i><span class="Element146">c</span></i>的直接引用指向从<i><span class="Element146">p</span></i>中导入的<i><span class="Element146">c</span></i>。 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK<br><br>对<i><span class="Element146">c</span></i>的直接引用指向从<i><span class="Element146">q</span></i>中导入的<i><span class="Element146">c</span></i>。 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK / ERROR<br><br><i><span class="Element146">c</span></i>在导入作用域内是未定义的。因此对<i><span class="Element146">c</span></i>的直接引用是非法的并且会产生一个错误。<br><br>然而允许使用导入子句。 </div></td></tr><tr>
<td class="Element67" valign="top" width="20%">
<div class="Element68">
<span class="Element146"><span style="color: #0000FF">import</span> p<span style="color: #800080">::</span>c<span style="color: #800080">;</span></span><br><span class="Element146">. . .</span><br><span class="Element146"><span style="color: #0000FF">if</span><span style="color: #800080">(!</span>c<span style="color: #800080">)</span> ...</span> </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
导入的标识符在导入作用域内变为直接可见 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
错误 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK<br><br>对<i><span class="Element146">c</span></i>的直接引用指向从<i><span class="Element146">p</span></i>中导入的<i><span class="Element146">c</span></i>。 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
错误<br><br>导入一个在导入作用域中定义的标识符是非法的。 </div></td><td class="Element67" valign="top" width="16%">
<div class="Element68">
OK / ERROR<br><br><span class="Element146">p<span style="color: #800080">::</span>c</span>的导入使得任何先前对<span class="Element146">c</span>的引用成为非法的。<br><br>然而，对c的直接引用指向从包p中导入的c。 </div></td></tr></table></div></div>
<p class="Element10">
</p>
<p class="Element10">
当使用一个通配导入的时候，对一个在包内部声明的未定义标识符的引用会导致这个标识符被导入到本地作用域中。然而，如果相同的标识符在后来又被声明或者被显式地导入的话则会产生一个错误。如下面的例子所示： </p><div class="Element170">
<a href="#" onclick="CopyElementToClipboard('code00799');">Copy Code</a></div>
<div class="Element13"><div class="Element12" id="code00799"><pre class="Element12"><span style="color: #0000FF">module</span> foo<span style="color: #800080">;</span>
<span style="color: #0000FF">import</span> q<span style="color: #800080">::</span><span style="color: #FF0000">*</span><span style="color: #800080">;</span>
<span style="color: #0000FF">wire</span> a <span style="color: #800080">= </span>c<span style="color: #800080">;</span> <span style="color: #008000">// 本条语句强制导入q::c;</span>
<span style="color: #0000FF">import</span> p<span style="color: #800080">::</span>c<span style="color: #800080">;</span> <span style="color: #008000">// q::c和p::c之间的冲突会产生一个错误。</span>
<span style="color: #0000FF">endmodule</span></pre></div></div>
</div>
</div>
<a name="Group"></a><div class="Element14">
<a onclick="toggleVisibilityStored('Group');" class="a_Element14"><img src="sectionminus.png" border="0" alt="" title="" id="imgGroup">Group</a></div>
<div id="divGroup">
<div class="Element11">
<div class="Element10">
<p class="Element10">
<a href="00203.html" target="topic">18.2 包</a></p></div>
</div>
</div>
<a name="Links"></a><div class="Element14">
<a onclick="toggleVisibilityStored('链接');" class="a_Element14"><img src="sectionminus.png" border="0" alt="" title="" id="img链接">链接</a></div>
<div id="div链接">
<div class="Element11">
<div class="Element10">
<a href="00203.html" target="topic">18.2 包</a></div>
</div>
</div>
</div>
<!-- End Page Content -->
<!-- Begin Page Footer -->
<hr width="98%" align="center" size="1" color="#CCCCCC" />
<table align="center" cellpadding="0" cellspacing="0" border="0">
<tbody>
<tr height="10">
<td></td>
</tr>
<tr align="center">
<td>
<script type="text/javascript"><!--
google_ad_client = "pub-5266859600380184";
google_ad_width = 468;
google_ad_height = 60;
google_ad_format = "468x60_as";
google_ad_type = "text_image";
google_ad_channel ="";
google_page_url = document.location;
//--></script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>
</td>
</tr>
<tr height="15">
<td></td>
</tr>
<tr align="center">
<td>
<font size=2>除非特别声明，原文版权归作者所有，如有转摘请注明原作者以及译者（<a href="http://www.fpgatech.net/" target="_blank">FPGA技术网</a>）信息。<br />
如果您对本主题有何建议或意见，请登陆<a href="http://www.fpgatech.net/forum/forumdisplay.php?fid=18" target="_blank">FPGA开发者家园</a>提交，您的参与是我们前进的动力。</font>
<script language="javascript" type="text/javascript" src="http://js.users.51.la/195685.js"></script>
<noscript><a href="http://www.51.la/?195685" target="_blank"><img alt="我要啦免费统计" src="http://img.users.51.la/195685.asp" style="border:none" /></a></noscript>
</td>
</tr>
</tbody>
</table>
<!-- End Page Footer -->
</div>
</div>
<!-- End Client Area -->
</body></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -