C++ 访问权限

最新推荐文章于 2024-02-12 12:12:37 发布

转载最新推荐文章于 2024-02-12 12:12:37 发布 · 923 阅读

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三种访问权限

我们知道C++中的类，有三种访问权限(也称作访问控制)，它们分别是public、protected、private。要理解它们其实也很容易，看下面了一个例子。
父类：

<code class="hljs cs has-numbering">class Person
{
<span class="hljs-keyword">public</span>:
    <span class="hljs-title">Person</span>(<span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& name, <span class="hljs-keyword">int</span> age) : <span class="hljs-title">m_name</span>(name), <span class="hljs-title">m_age</span>(age)
    {
    }

    <span class="hljs-keyword">void</span> ShowInfo()
    {
        cout << <span class="hljs-string">"姓名："</span> << m_name << endl;
        cout << <span class="hljs-string">"年龄："</span> << m_age << endl;
    }

<span class="hljs-keyword">protected</span>:
    <span class="hljs-keyword">string</span>  m_name;     <span class="hljs-comment">//姓名</span>

<span class="hljs-keyword">private</span>:
    <span class="hljs-keyword">int</span>     m_age;      <span class="hljs-comment">//年龄</span>
};</code>

子类：

<code class="hljs cs has-numbering">class Teacher : <span class="hljs-keyword">public</span> Person
{
<span class="hljs-keyword">public</span>:
    <span class="hljs-title">Teacher</span>(<span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& name, <span class="hljs-keyword">int</span> age, <span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& title)
        : <span class="hljs-title">Person</span>(name, age), <span class="hljs-title">m_title</span>(title)
    {
    }

    <span class="hljs-keyword">void</span> ShowTeacherInfo()
    {
        ShowInfo();                             <span class="hljs-comment">//正确，public属性子类可见</span>
        cout << <span class="hljs-string">"姓名："</span> << m_name << endl;        <span class="hljs-comment">//正确，protected属性子类可见</span>
        cout << <span class="hljs-string">"年龄："</span> << m_age << endl;     <span class="hljs-comment">//错误，private属性子类不可见</span>

        cout << <span class="hljs-string">"职称："</span> << m_title << endl;   <span class="hljs-comment">//正确，本类中可见自己的所有成员</span>
    }

<span class="hljs-keyword">private</span>:
    <span class="hljs-keyword">string</span>  m_title;        <span class="hljs-comment">//职称</span>
};</code>

调用方：

<code class="hljs cpp has-numbering"><span class="hljs-keyword">void</span> test()
{
    Person person(<span class="hljs-string">"张三"</span>, <span class="hljs-number">22</span>);
    person.ShowInfo();                  <span class="hljs-comment">//public属性,对外部可见</span>
    <span class="hljs-built_in">cout</span> << person.m_name << endl;      <span class="hljs-comment">//protected属性,对外部不可见</span>
    <span class="hljs-built_in">cout</span> << person.m_age << endl;       <span class="hljs-comment">//private属性,对外部不可见</span>
}</code>

总结

我们对C++类三种方式控制权限总结如下，这与Java中的三种对应的访问权限是一样的。

访问权限	public	protected	private
对本类	可见	可见	可见
对子类	可见	可见	不可见
对外部(调用方)	可见	不可见	不可见

三种继承方式

C++中继承的方式还有多种，也分别都用public、protected、private表示。这与Java不一样，Java只有继承的概念，默认是public继承的。
1. 三种继承方式不影响子类对父类的访问权限，子类对父类只看父类的访问控制权。如下面三种继承方式都能访问父类中的public和protected成员。

<code class="hljs cs has-numbering">class Teacher : <span class="hljs-comment">/*public*/</span> <span class="hljs-comment">/*protected*/</span> <span class="hljs-keyword">private</span> Person
{
<span class="hljs-keyword">public</span>:
    <span class="hljs-title">Teacher</span>(<span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& name, <span class="hljs-keyword">int</span> age, <span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& title)
        : <span class="hljs-title">Person</span>(name, age), <span class="hljs-title">m_title</span>(title)
    {
    }

    <span class="hljs-keyword">void</span> ShowTeacherInfo()
    {
        ShowInfo();                             <span class="hljs-comment">//正确，public属性子类可见</span>
        cout << <span class="hljs-string">"姓名："</span> << m_name << endl;        <span class="hljs-comment">//正确，protected属性子类可见</span>
        <span class="hljs-comment">//cout << "年龄：" << m_age << endl;       //错误，private属性子类不可见</span>

        cout << <span class="hljs-string">"职称："</span> << m_title << endl;   <span class="hljs-comment">//正确，本类中可见自己的所有成员</span>
    }

<span class="hljs-keyword">private</span>:
    <span class="hljs-keyword">string</span>  m_title;        <span class="hljs-comment">//职称</span>
};</code>

2. 继承方式是为了控制子类(也称派生类)的调用方(也叫用户)对父类(也称基类)的访问权限。

public继承

<code class="hljs cs has-numbering">class Teacher : <span class="hljs-keyword">public</span> Person
{
<span class="hljs-keyword">public</span>:
    <span class="hljs-title">Teacher</span>(<span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& name, <span class="hljs-keyword">int</span> age, <span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& title)
        : <span class="hljs-title">Person</span>(name, age), <span class="hljs-title">m_title</span>(title)
    {
    }

    <span class="hljs-keyword">void</span> ShowTeacherInfo()
    {
        ShowInfo();                             <span class="hljs-comment">//正确，public属性子类可见</span>
        cout << <span class="hljs-string">"职称："</span> << m_title << endl;   <span class="hljs-comment">//正确，本类中可见自己的所有成员</span>
    }

<span class="hljs-keyword">private</span>:
    <span class="hljs-keyword">string</span>  m_title;        <span class="hljs-comment">//职称</span>
};</code>

<code class="hljs avrasm has-numbering">void TestPublic()
{
    Teacher teacher(<span class="hljs-string">"李四"</span>, <span class="hljs-number">35</span>, <span class="hljs-string">"副教授"</span>)<span class="hljs-comment">;</span>
    teacher<span class="hljs-preprocessor">.ShowInfo</span>()<span class="hljs-comment">;</span>
    cout << endl<span class="hljs-comment">;</span>
    teacher<span class="hljs-preprocessor">.ShowTeacherInfo</span>()<span class="hljs-comment">;</span>
}</code>

结果：

姓名：李四
年龄：35

姓名：李四
年龄：35
职称：副教授

private继承：

<code class="hljs cs has-numbering">class Teacher : <span class="hljs-keyword">private</span> Person
{
<span class="hljs-keyword">public</span>:
    <span class="hljs-title">Teacher</span>(<span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& name, <span class="hljs-keyword">int</span> age, <span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& title)
        : <span class="hljs-title">Person</span>(name, age), <span class="hljs-title">m_title</span>(title)
    {
    }

    <span class="hljs-keyword">void</span> ShowTeacherInfo()
    {
        ShowInfo();                             <span class="hljs-comment">//正确，public属性子类可见</span>
        cout << <span class="hljs-string">"职称："</span> << m_title << endl;   <span class="hljs-comment">//正确，本类中可见自己的所有成员</span>
    }

<span class="hljs-keyword">private</span>:
    <span class="hljs-keyword">string</span>  m_title;        <span class="hljs-comment">//职称</span>
};</code>

<code class="hljs scss has-numbering">void <span class="hljs-function">TestPrivate()</span>
{
    Teacher <span class="hljs-function">teacher(<span class="hljs-string">"李四"</span>, <span class="hljs-number">35</span>, <span class="hljs-string">"副教授"</span>)</span>;
    teacher<span class="hljs-class">.ShowInfo</span>();             <span class="hljs-comment">//错误，因为Teacher采用了private的继承方式，外部不可访问。</span>
    cout << endl;
    teacher<span class="hljs-class">.ShowTeacherInfo</span>();
}</code>

.
3. public、protected、private三种继承方式，相当于把父类的public访问权限在子类中变成了对应的权限。 如protected继承，把父类中的public成员在本类中变成了protected的访问控制权限；private继承，把父类的public成员和protected成员在本类中变成了private访问控制权。

protected继承：

<code class="hljs cs has-numbering">class Teacher : <span class="hljs-keyword">protected</span> Person
{
<span class="hljs-keyword">public</span>:
    <span class="hljs-title">Teacher</span>(<span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& name, <span class="hljs-keyword">int</span> age, <span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& title)
        : <span class="hljs-title">Person</span>(name, age), <span class="hljs-title">m_title</span>(title)
    {
    }

    <span class="hljs-keyword">void</span> ShowTeacherInfo()
    {
        ShowInfo();                             <span class="hljs-comment">//正确，public属性子类可见</span>
        cout << <span class="hljs-string">"职称："</span> << m_title << endl;   <span class="hljs-comment">//正确，本类中可见自己的所有成员</span>
    }

<span class="hljs-keyword">private</span>:
    <span class="hljs-keyword">string</span>  m_title;        <span class="hljs-comment">//职称</span>
};</code>

<code class="hljs scss has-numbering">void <span class="hljs-function">TestProtected()</span>
{
    Teacher <span class="hljs-function">teacher(<span class="hljs-string">"李四"</span>, <span class="hljs-number">35</span>, <span class="hljs-string">"副教授"</span>)</span>;
    teacher<span class="hljs-class">.ShowInfo</span>();         <span class="hljs-comment">//错误，基类Person的ShowInfo此时对Teacher相当于protected的，外部不可以被访问</span>
    cout << endl;
    teacher<span class="hljs-class">.ShowTeacherInfo</span>();
}</code>

<code class="hljs cs has-numbering">class Leader : <span class="hljs-keyword">public</span> Teacher
{
<span class="hljs-keyword">public</span>:
    <span class="hljs-title">Leader</span>(<span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& name, <span class="hljs-keyword">int</span> age, <span class="hljs-keyword">const</span> <span class="hljs-keyword">string</span>& title, <span class="hljs-keyword">string</span> position)
        : <span class="hljs-title">Teacher</span>(name, age, title), <span class="hljs-title">m_position</span>(position)
    {
    }

    <span class="hljs-keyword">void</span> ShowLeaderInfo()
    {
        ShowInfo();             <span class="hljs-comment">//基类Person的ShowInfo此时相当于protected的，但子类仍可以访问</span>
        ShowTeacherInfo();      <span class="hljs-comment">//ShowTeacherInfo仍然是public的，可以访问</span>
        cout << m_position << endl;
    }

<span class="hljs-keyword">private</span>:
    <span class="hljs-keyword">string</span> m_position;
};</code>