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<!DOCTYPE html>
<html>
<head>
<meta content="text/html; charset=UTF-8" http-equiv="content-type">
<title>ch04-03</title>
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<body>
<div class="os1"> 4.3 自定义信号和槽 </div>
<br>
本节首先介绍一下 C++ 编程中常用的传递数据机制，包括类对象的公有成员变量、友元类/函数 、公有函数、回调函数等，这些机制在 Qt
程序中也是可以使用的。然后重点介绍如何在 Qt
类里面自定义信号和槽，通过手动触发信号来调用槽函数，完成两个对象之间的信息传递，本节最后示范一个信号接力触发的例子。本节内容较多，可以分两三次来学。<br>
<br>
<div class="os2"> 4.3.1 C++ 的沟通方式 </div>
<br>
C++ 编程中常遇到各个对象之间进行沟通的情景，需要将数据从一个对象传递给另一个对象来处理。大致的方法有如下几种：<br>
<ul>
<li>接收端定义公有成员变量以供源端修改，然后接收端处理数据；
（<span style="font-weight: bold;">不建议用！</span>）</li>
<li>接收端将私有成员变量通过友元方式共享给源端，源端可以修改接收端变量；
（<span style="font-weight: bold;">除了特殊情况，一般不建议用！</span>）</li>
<li>接收端定义公开的 get 和 set 函数，提供给源端调用
（<span style="font-weight: bold;">推荐使用，可以与信号和槽机制协同工作</span>）；</li>
<li>源端给出回调函数约定，接收端定义相同参数和返回值类型的静态成员函数，将静态成员函数作为回调函数交给源端，源端再调用该函数
（<span style="font-weight: bold;">可以使用</span>）。</li>
</ul>
<span style="font-weight: bold;">这些方式都是基于标准 C++ 的，在 Qt 中都可以用，但由于 Qt
有更好的信号和槽机制，因此一般更推荐使用信号和槽机制实现通信。</span>下面对 C++
常见的传递数据方式依次举例示范，主要是让读者大致了解一下传递数据的过程。<br>
<br>
本小节的示例是多个独立的 cpp 文件，可以通过 2.1 节命令行方式编译运行，这几个 cpp 文件都放到
D:\QtProjects\ch04\cppcom 文件夹里的。<br>
首先是公有成员变量的例子，publicmember.cpp 代码：<br>
<div class="code"><span style=" color:#008000;"></span><span style=" color:#008000;">//publicmember.cpp</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000080;">#include</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;"><iostream></span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">using</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#808000;">namespace</span><span style=" color:#c0c0c0;"> </span>std<span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#008000;">//接收端类</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">class</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">Dst</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">public</span><span style=" color:#000000;">:</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//处理数据函数</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">DoSomething</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800000;">m_dblValue</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">/</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblResult</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">};</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#008000;">//源端类</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">class</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">Src</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">public</span><span style=" color:#000000;">:</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">void</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">SendDataTo</span><span
style=" color:#000000;">(</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Dst</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">&</span><span
style=" color:#000000;">theDst</span><span style=" color:#000000;">)</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//设置接收端公有变量</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">.</span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">2.0</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">.</span><span style=" color:#800000;">m_nCount</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">3</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">};</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">int</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">main</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//定义两个对象</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Dst</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Src</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theSrc</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//传递数据</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theSrc</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">SendDataTo</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//接收端处理</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span>cout<span style=" color:#000000;"><<</span><span
style=" color:#000000;">theDst</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">DoSomething</span><span style=" color:#000000;">();</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">}</span></pre>
</div>
接收端的类对象 theDst 有两个公有成员 m_nCount 和 m_dblValue，源端对象 theSrc 在 SendDataTo
函数里面修改了接收端的成员变量，然后接收端再对数据进行处理。<br>
这种传递数据方式最大的问题是谁都可以修改接收端的公有成员变量，如果有其他代码也修改了
theDst，那么处理结果是很难预知的，尤其是在多线程程序里，公有变量在另一个线程被修改了，本线程很可能都不知道。如果通过全局变量传递数据的情景，这种负面效果是
类 似的，而且更严重。<br>
公有成员变量和全局变量方式都会破坏类数据的封装特性，谁都能修改，结果不可控。另外，这种方式不方便做数值有效性鉴定，比如把 m_nCount 设置为 0，
除 0 会直接导致程序出错。<br>
所以不建议使用这种方式。<br>
<br>
接下来是友元类的例子，通过友元声明，源端可以直接设置接收端的私有成员变量，如 friend.cpp 代码：<br>
<div class="code"><span style=" color:#008000;"></span><span style=" color:#008000;">//friend.cpp</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000080;">#include</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;"><iostream></span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">using</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#808000;">namespace</span><span style=" color:#c0c0c0;"> </span>std<span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#008000;">//接收端类</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">class</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">Dst</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">private</span><span style=" color:#000000;">:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//私有变量</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">public</span><span style=" color:#000000;">:</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//处理数据函数</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">DoSomething</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800000;">m_dblValue</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">/</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblResult</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//友元类</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">friend</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">class</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Src</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">};</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#008000;">//源端类</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">class</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">Src</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">public</span><span style=" color:#000000;">:</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">void</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">SendDataTo</span><span
style=" color:#000000;">(</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Dst</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">&</span><span
style=" color:#000000;">theDst</span><span style=" color:#000000;">)</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//因为是友元类，所以能设置接收端私有变量</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">.</span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">2.0</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">.</span><span style=" color:#800000;">m_nCount</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">3</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">};</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">int</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">main</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//定义两个对象</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Dst</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Src</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theSrc</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//传递数据</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theSrc</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">SendDataTo</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//接收端处理</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span>cout<span style=" color:#000000;"><<</span><span
style=" color:#000000;">theDst</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">DoSomething</span><span style=" color:#000000;">();</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">}</span></pre>
</div>
接收端的成员变量是私有的，因此不用担心成员变量被其他代码段胡乱修改，只有友元授权的 Src 类对象才能修改接收端对象私有变量。这种方式的缺陷是 Src
类和 Dst
类是紧耦合的，它们息息相关，修改了一个类的代码很可能影响另一个。除非程序员确定需要这种紧耦合设计，否则一般也不建议使用友元。另外，对于传递的数值有效性鉴定，需要
友元类对象确保不把 m_nCount 设置成 0。<br>
<br>
再来看看第三种，使用 get 和 set 函数对的方式，在 Qt 类库里面，对于设置数值的函数以 set 字样打头，而获取数值的函数默认省略 get
字样，比如 QLabel 对象，设置文本函数为 setText()，获取文本函数为 text() 。下面的示例也按照 Qt 命名风格，
getset.cpp 代码：<br>
<div class="code"><span style=" color:#008000;">//getset.cpp</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000080;">#include</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;"><iostream></span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">using</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#808000;">namespace</span><span style=" color:#c0c0c0;"> </span>std<span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#008000;">//接收端类</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">class</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">Dst</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">private</span><span style=" color:#000000;">:</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//私有变量</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">public</span><span style=" color:#000000;">:</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//get函数</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">value</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">count</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//set函数</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">void</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">setValue</span><span
style=" color:#000000;">(</span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">v</span><span style=" color:#000000;">)</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">v</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">void</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">setCount</span><span
style=" color:#000000;">(</span><span style=" color:#808000;">int</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">n</span><span style=" color:#000000;">)</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">if</span><span
style=" color:#000000;">(</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">n</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;"><</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">1</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">)</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//防止除</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">0</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">，并且计数限定为正整数</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">1</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">else</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">n</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//处理数据函数</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">DoSomething</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800000;">m_dblValue</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">/</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblResult</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">};</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#008000;">//源端类</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">class</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">Src</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">public</span><span style=" color:#000000;">:</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">void</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">SendDataTo</span><span
style=" color:#000000;">(</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Dst</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">&</span><span
style=" color:#000000;">theDst</span><span style=" color:#000000;">)</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//通过set函数传递数据</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">setValue</span><span
style=" color:#000000;">(</span><span style=" color:#000080;">2.0</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">setCount</span><span
style=" color:#000000;">(</span><span style=" color:#000080;">3</span><span style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">};</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">int</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">main</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//定义两个对象</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Dst</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Src</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theSrc</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//传递数据</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">theSrc</span><span
style=" color:#000000;">.</span><span style=" color:#000000;">SendDataTo</span><span
style=" color:#000000;">(</span><span style=" color:#000000;">theDst</span><span
style=" color:#000000;">);</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//接收端处理</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span>cout<span style=" color:#000000;"><<</span><span
style=" color:#000000;">theDst</span><span style=" color:#000000;">.</span><span
style=" color:#000000;">DoSomething</span><span style=" color:#000000;">();</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000080;">0</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">}</span></pre>
</div>
代码里通过 value()和 setValue() 函数封装私有变量 m_dblValue，通过 count()和 setCount() 函数封装
m_nCount 私有变量，在 set 函数里面可以对输入的数据做判断，确认是否合法，数据的可控性大大增强。get/set
方式是推荐的做法，下一节有规范的 Qt 属性封装介绍，与这种方式是类似的。<br>
<br>
接下来示范一个回调函数的例子，多线程编程和 Windows
编程会经常遇到类似的回调函数，通常源端会给出通用的回调函数类型声明，接收端按照该格式定义自己的回调函数。因为回调函数一般是通用的，所以参数里常用的是
void * 指针，而不会针对某一个固定的类对象传参。因为类的普通成员函数需要隐藏的 this
指针，会导致不符合回调函数类型声明，所以回调函数只能用静态成员函数或全局函数定义。回调函数示范 callback.cpp：<br>
<div class="code"><span style=" color:#008000;">//callback.cpp</span>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000080;">#include</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;"><iostream></span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">using</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#808000;">namespace</span><span style=" color:#c0c0c0;"> </span>std<span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#008000;">//源端约定回调函数类型</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">typedef</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#808000;">void</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">(*</span><span
style=" color:#800080;">PFUNC</span><span style=" color:#000000;">)(</span><span
style=" color:#808000;">double</span><span style=" color:#c0c0c0;"> </span>v<span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span>n<span style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#808000;">void</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">*</span>pObject<span
style=" color:#000000;">);</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#008000;">//接收端类</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">class</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800080;">Dst</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">private</span><span style=" color:#000000;">:</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">int</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><br></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#808000;">public</span><span style=" color:#000000;">:</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//处理数据函数</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">DoSomething</span><span
style=" color:#000000;">()</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">double</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblResult</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#800000;">m_dblValue</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">/</span><span style=" color:#c0c0c0;"> </span><span style=" color:#800000;">m_nCount</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">return</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">dblResult</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//回调函数</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">static</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">void</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">FuncCallBack</span><span style=" color:#000000;">(</span><span
style=" color:#808000;">double</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">v</span><span style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#808000;">int</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">n</span><span
style=" color:#000000;">,</span><span style=" color:#c0c0c0;"> </span><span style=" color:#808000;">void</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">*</span><span style=" color:#000000;">pObject</span><span
style=" color:#000000;">)</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//转换成</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">Dst</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#008000;">指针</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#800080;">Dst</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">*</span><span style=" color:#000000;">pDst</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">(</span><span style=" color:#800080;">Dst</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">*)</span><span style=" color:#000000;">pObject</span><span
style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//静态成员函数也是可以设置私有变量的，但需要手动传对象指针</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//设置</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">value</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">pDst</span><span
style=" color:#000000;">-></span><span style=" color:#800000;">m_dblValue</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000000;">v</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#008000;">//设置count</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">if</span><span
style=" color:#000000;">(</span><span style=" color:#c0c0c0;"> </span><span style=" color:#000000;">n</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;"><</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">1</span><span style=" color:#000000;">)</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">{</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">pDst</span><span
style=" color:#000000;">-></span><span style=" color:#800000;">m_nCount</span><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">=</span><span style=" color:#c0c0c0;"> </span><span
style=" color:#000080;">1</span><span style=" color:#000000;">;</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#000000;">}</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span
style=" color:#c0c0c0;"> </span><span style=" color:#808000;">else</span></pre>
<pre style=" margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;"><span