C++的重写，重载与充定义

阅读《Effective C++》条款33的时候产生了疑惑，在重写与重定义上有所混淆。现将网上查阅的资料记录如下：

重写（override）

首先，重写是指派生类的方法覆盖基类的方法，要求方法名、方法的参数都相同。重写是C++中实现多态这个特性基础。重写又称为覆盖，是指派生类函数覆盖基类函数，与重定义不同，重写要求被重写的基类函数为虚函数。 

例如下面的代码：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">class Base{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">Total</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> unit_price, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> num) = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
};
class Derived :<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Base{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">Total</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> unit_price, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> num)
{
cout << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"test"</span> << endl;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}
};
class Child :<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Derived{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">Total</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> unit_price, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> num)
{
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> unit_price*num;
}
};
</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul>

总结一下，重写是指派生类重新定义了基类的虚函数，基类函数必须有virtual关键字修饰，不能有static关键字；返回值类型必须是相同的，或者是协变的；此外重写函数的访问修饰符可以不同。 

注意：一般重写要求函数具有完全相同的入参，否则就会提示错误，而这个规则对于协变而言则有所放松。覆盖的返回值不区分基类或者派生类，如下代码：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">class Base{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">Total</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> unit_price, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> num)
{
cout << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Base"</span> << endl;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}
};
class Derived :<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Base{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> <span class="hljs-title" style="box-sizing: border-box;">Total</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> unit_price, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> num)
{
cout << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"test"</span> << endl;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
}
};
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//协变</span>
class Father{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> Base& <span class="hljs-title" style="box-sizing: border-box;">getClass</span>() = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
};
class Child{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> Derived& getClass()
{
Derived ret;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> ret;
}
};</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul>

在上面协变的例子中，要求Base和Derived两个类必须在使用前被完整的定义。 

协变返回类型的优势在于，若是我们使用Father，就能获得一个抽象的Base，如果我们正在处理Child，就会获得一个Derived。协变的返回机制将我们从这样一种处境中解脱出来：不得不使用易于出错的转换操作来“重新”提供类型信息，而这种信息是一开始就不应该丢掉的。

重载（overload）

然后是重载（overload），重载是应用于相同作用域之内的同名函数，由于参数列表不同而产生的不同的实现方法。此处提到的作用域有：全局作用域、局部作用域以及类作用域，当在同一个作用域内的时候同名的函数或者称之为方法，由于参数列表的不同，而获得的不同的函数。

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> Sum(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> Sum(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> c);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> Sum(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">float</span> b);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

重载是一种语言特性，是一种语法规则，与多态无关，与面向对象无关。

重定义

重定义则是经常出现在基类和派生类之间，归结起来有如下的特点： 

1）不在同一个作用域，主要是指类作用域，分别位于基类和派生类之中； 

2）函数名称相同，但是返回值可以不同； 

3）参数不同时，无论有没有virtual关键字，基类的函数都会被隐藏；参数相同时，但是基类函数没有关键字virtual，此时基类函数被隐藏。 

例如：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">class Base{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> <span class="hljs-title" style="box-sizing: border-box;">Total</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> b)
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> a + b;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> Print()
{
cout << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Base"</span> << endl;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> Sum(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b);

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span>:
//
};
class Derived :<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Base{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span>:
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> <span class="hljs-title" style="box-sizing: border-box;">Total</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> b, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">double</span> c)
{
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> a + b + c;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> Print()
{
cout << <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Derived"</span> << endl;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> Sum(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> a, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> b, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> c);
};</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul>

在运行Derived的实例的时候可以很容易的发现，基类中Sum函数被隐藏了。因此重定义又称为隐藏，是指派生类的函数屏蔽了与其同名的基类函数。

<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: "Source Code Pro", monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> _tmain(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> argc, _TCHAR* argv[])
{
Derived test;
test.Print();
Derived test_b;
Base <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">*p</span>;
p = new Derived;
p->Print();
Base <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">*p_2</span>;
p_2 = &test_b;
p_2->Print();
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">delete</span> p;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">system</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"pause"</span>);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><div class="save_code tracking-ad" data-mod="popu_249" style="box-sizing: border-box; position: absolute; width: 105px; height: 60px; right: 30px; top: 5px; color: rgb(255, 255, 255); cursor: pointer; z-index: 2;"><a target=_blank target="_blank" style="box-sizing: border-box; color: rgb(12, 137, 207);"><img src="http://static.blog.csdn.net/images/save_snippets.png" style="border: none; box-sizing: border-box;" alt="" /></a></div>

例如上面的类，我们在如上调用的时候得到了下面的输出：