数据隐藏是面向对象编程的重要函数之一，它可以防止程序的函数直接访问类的内部信息。类成员的访问限制由类主体中标有标签的 public ， private 和 protected 修饰符指定，成员和类的默认访问权限为私有。
class Base {public:
//public 区
protected:
//protected 区
private:
//private 区
};
Public 公开
public 可以在class以外但在程序内的任何位置访问，您可以在没有任何成员函数的情况下设置和获取公共变量的值，如以下示例所示:
import std.stdio;
class Line {
public:
double length;
</span><span class="kwd">double</span><span class="pln"> getLength</span><span class="pun">()</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="kwd">return</span><span class="pln"> length </span><span class="pun">;</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
</span><span class="kwd">void</span><span class="pln"> setLength</span><span class="pun">(</span><span class="pln"> </span><span class="kwd">double</span><span class="pln"> len </span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
length</span><span class="pun">=</span><span class="pln">len</span><span class="pun">;</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
}
void main( ) {
Line line=new Line();
//设置行长
line.setLength(6.0);
writeln("Length of line : ", line.getLength());
//在没有成员函数的情况下设置行长
line.length=10.0; //OK: 因为长度是公开的
writeln("Length of line : ", line.length);
}
编译并执行上述代码后，将产生以下输出-
Length of line : 6 Length of line : 10
Private 私有
private 变量或函数无法访问，甚至无法从类外部进行查看，默认情况下，一个类的所有成员都是私有的。例如，在以下类中， width 是私有成员。
class Box { double width; public: double length; void setWidth( double wid ); double getWidth( void ); }
实际上，您需要在私有部分中定义数据，并在公共部分中定义相关函数，以便可以从类外部调用它们，如以下程序所示。
import std.stdio;
class Box {
public:
double length;
</span><span class="com">//成员函数定义</span><span class="pln">
</span><span class="kwd">double</span><span class="pln"> getWidth</span><span class="pun">()</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
</span><span class="kwd">return</span><span class="pln"> width </span><span class="pun">;</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
</span><span class="kwd">void</span><span class="pln"> setWidth</span><span class="pun">(</span><span class="pln"> </span><span class="kwd">double</span><span class="pln"> wid </span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
width</span><span class="pun">=</span><span class="pln">wid</span><span class="pun">;</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
private:
double width;
}
//程序的主要功能
void main( ) {
Box box=new Box();
box.length=10.0;
writeln("Length of box : ", box.length);
box.setWidth(10.0);
writeln("Width of box : ", box.getWidth());
}
编译并执行上述代码后，将产生以下输出-
Length of box : 10 Width of box : 10
Protected 受保护
protected 成员变量或函数与私有成员非常相似，但是它提供了另一个好处，即它的子类对其进行访问。
下面的示例与上面的示例相似，并且 width 成员可由其派生类SmallBox的任何成员函数访问。
import std.stdio;
class Box {
protected:
double width;
}
class SmallBox:Box { //SmallBox 是派生类。
public:
double getSmallWidth() {
return width ;
}
</span><span class="kwd">void</span><span class="pln"> setSmallWidth</span><span class="pun">(</span><span class="pln"> </span><span class="kwd">double</span><span class="pln"> wid </span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
width</span><span class="pun">=</span><span class="pln">wid</span><span class="pun">;</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
}
void main( ) {
SmallBox box=new SmallBox();
//使用成员函数设置框宽度
box.setSmallWidth(5.0);
writeln("Width of box : ", box.getSmallWidth());
}
编译并执行上述代码后，将产生以下输出-
Width of box : 5
