[軟體工程]如何寫出意圖使人犯錯的程式碼

因為一些因素，讓我學習到一段程式碼，這段程式碼就是這篇主題的主角，意圖使人出錯的程式碼。
這段code的規格是
初始化玩遊戲後，確保有一個使用者在遊戲裡面，所以要在init結束前呼叫join。
至於規格為什麼要這樣設計，不要問我XD
 原本的code是python的。我把python的code轉成用c++來表達，所以function前面都會有virtual，以符合python情形。

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

<span class="com">#include</span><span class="pln"> </span><span class="str"><cstdio></span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init start\n"</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init end\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">();</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join start\n"</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> main</span><span class="pun">(){</span><span class="pln">  </span><span class="typ">Father</span><span class="pln"> father</span><span class="pun">;</span><span class="pln">  father</span><span class="pun">.</span><span class="pln">init</span><span class="pun">();</span><span class="pln">   </span><span class="pun">}</span><span class="pln"> </span>

坦白講，這段程式碼看起來還好。除了一個function做兩件事情外。也沒什麼。執行順序也如我們的預期

father init start 
father init end
father join start 
father join end

可是如果他是基底類別。會有子類別繼承他

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

<span class="com">#include</span><span class="pln"> </span><span class="str"><cstdio></span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init start\n"</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init end\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">();</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join start\n"</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Child</span><span class="pun">:</span><span class="kwd">public</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child init start\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> ret </span><span class="pun">=</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">::</span><span class="pln">init</span><span class="pun">();</span><span class="pln">   </span><span class="com">// do something It need.</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child init end\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> ret</span><span class="pun">;</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child join start\n"</span><span class="pun">);</span><span class="pln">   </span><span class="typ">Father</span><span class="pun">::</span><span class="kwd">join</span><span class="pun">();</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> main</span><span class="pun">(){</span><span class="pln">  </span><span class="typ">Child</span><span class="pln"> child</span><span class="pun">;</span><span class="pln">  child</span><span class="pun">.</span><span class="pln">init</span><span class="pun">();</span><span class="pln">   </span><span class="pun">}</span>

程式碼看起來還好，只是執行解果有點詭異。恩....子類別join發生在子類別init完成前。

child init start
father init start
father init end
child join start
father join start
father join end
child join end
child init end

假設你不care這情形，過了一段時間，你突然想新增一些東西像這樣

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

<span class="com">#include</span><span class="pln"> </span><span class="str"><cstdio></span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init start\n"</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init end\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">();</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join start\n"</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Child</span><span class="pun">:</span><span class="kwd">public</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">int</span><span class="pln"> special</span><span class="pun">;</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child init start\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> ret </span><span class="pun">=</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">::</span><span class="pln">init</span><span class="pun">();</span><span class="pln">   special </span><span class="pun">=</span><span class="pln"> </span><span class="lit">1</span><span class="pun">;</span><span class="pln">   </span><span class="com">// do something It need.</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child init end\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> ret</span><span class="pun">;</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child join start\n"</span><span class="pun">);</span><span class="pln">                 </span><span class="typ">Father</span><span class="pun">::</span><span class="kwd">join</span><span class="pun">();</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"special = %d\n"</span><span class="pun">,</span><span class="pln"> special</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> main</span><span class="pun">(){</span><span class="pln">  </span><span class="typ">Child</span><span class="pln"> child</span><span class="pun">;</span><span class="pln">  child</span><span class="pun">.</span><span class="pln">init</span><span class="pun">();</span><span class="pln">   </span><span class="pun">}</span>

你在子類別新增一個變數，初始化時需要給他一個值，子類別的join的時候會需要他。你會發現這樣寫，special的值不合你預期(廢話，因為join發生在special = 1之前)。

怎麼辦，把special = 1移到Father::init前面就好了。 恩....
問題好像解決了，也沒什麼呀

有一天，父類別要做一些更動，要多加一個屬性name，父類別的name是從1開始算，子類別的name是從2開始算。
像這樣

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

<span class="com">#include</span><span class="pln"> </span><span class="str"><cstdio></span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">int</span><span class="pln"> name</span><span class="pun">;</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init start\n"</span><span class="pun">);</span><span class="pln">   name </span><span class="pun">=</span><span class="pln"> </span><span class="lit">1</span><span class="pun">;</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init end\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">();</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join start\n"</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"name = %d\n"</span><span class="pun">,</span><span class="pln"> name</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Child</span><span class="pun">:</span><span class="kwd">public</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">int</span><span class="pln"> special</span><span class="pun">;</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child init start\n"</span><span class="pun">);</span><span class="pln">   name </span><span class="pun">=</span><span class="pln"> </span><span class="lit">2</span><span class="pun">;</span><span class="pln">   special </span><span class="pun">=</span><span class="pln"> </span><span class="lit">1</span><span class="pun">;</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> ret </span><span class="pun">=</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">::</span><span class="pln">init</span><span class="pun">();</span><span class="pln">   </span><span class="com">// do something It need.</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child init end\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> ret</span><span class="pun">;</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child join start\n"</span><span class="pun">);</span><span class="pln">                 name </span><span class="pun">+=</span><span class="pln"> </span><span class="lit">1</span><span class="pun">;</span><span class="pln">   </span><span class="typ">Father</span><span class="pun">::</span><span class="kwd">join</span><span class="pun">();</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"special = %d\n"</span><span class="pun">,</span><span class="pln"> special</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> main</span><span class="pun">(){</span><span class="pln">  </span><span class="typ">Child</span><span class="pln"> child</span><span class="pun">;</span><span class="pln">  child</span><span class="pun">.</span><span class="pln">init</span><span class="pun">();</span><span class="pln">   </span><span class="pun">}</span>

?

1

你會發現，印出來的 name 值會是父類別的值再加一，而不是子類別的值再加一。怎麼辦?
在join那邊初始化?

不行，join在那邊會被呼叫很多次，所以在join初始化會一直讓那個變數被初始化
所以不能在那初始化。

所以怎麼辦?
不要呼叫父類別的函數好了，把父類別的函數程式碼直接複製到子類別好了
很好，問題解決了，只是這樣幹嘛用繼承呢?
如果你哪天父類別的init改了，恭喜你。你要把父類別的code重新複製到子類別去。
每改一次，複製一次 ...
不小心改錯，你的code就gg了。

有人會說，我就是要初始化完呼叫，那該怎麼辦呢?
事實上也不難。

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

<span class="com">#include</span><span class="pln"> </span><span class="str"><cstdio></span><span class="pln"> </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">int</span><span class="pln"> name</span><span class="pun">;</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init start\n"</span><span class="pun">);</span><span class="pln">   name </span><span class="pun">=</span><span class="pln"> </span><span class="lit">1</span><span class="pun">;</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father init end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join start\n"</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"name = %d\n"</span><span class="pun">,</span><span class="pln"> name</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"father join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln">    </span><span class="kwd">int</span><span class="pln"> init_and_join</span><span class="pun">(){</span><span class="pln">   </span><span class="com">//the functino can't be overrided.</span><span class="pln">   init</span><span class="pun">();</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">();</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">class</span><span class="pln"> </span><span class="typ">Child</span><span class="pun">:</span><span class="kwd">public</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">{</span><span class="pln"> </span><span class="kwd">public</span><span class="pun">:</span><span class="pln">  </span><span class="kwd">int</span><span class="pln"> special</span><span class="pun">;</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> init</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child init start\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> ret </span><span class="pun">=</span><span class="pln"> </span><span class="typ">Father</span><span class="pun">::</span><span class="pln">init</span><span class="pun">();</span><span class="pln">   name </span><span class="pun">=</span><span class="pln"> </span><span class="lit">2</span><span class="pun">;</span><span class="pln">   special </span><span class="pun">=</span><span class="pln"> </span><span class="lit">1</span><span class="pun">;</span><span class="pln">   </span><span class="com">// do something It need.</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child init end\n"</span><span class="pun">);</span><span class="pln">   </span><span class="kwd">return</span><span class="pln"> ret</span><span class="pun">;</span><span class="pln">  </span><span class="pun">}</span><span class="pln">  </span><span class="kwd">virtual</span><span class="pln"> </span><span class="kwd">int</span><span class="pln"> </span><span class="kwd">join</span><span class="pun">(){</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child join start\n"</span><span class="pun">);</span><span class="pln">   name </span><span class="pun">+=</span><span class="pln"> </span><span class="lit">1</span><span class="pun">;</span><span class="pln">   </span><span class="typ">Father</span><span class="pun">::</span><span class="kwd">join</span><span class="pun">();</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"special = %d\n"</span><span class="pun">,</span><span class="pln"> special</span><span class="pun">);</span><span class="pln">   printf</span><span class="pun">(</span><span class="str">"child join end\n"</span><span class="pun">);</span><span class="pln">  </span><span class="pun">}</span><span class="pln"> </span><span class="pun">};</span><span class="pln">   </span><span class="kwd">int</span><span class="pln"> main</span><span class="pun">(){</span><span class="pln">  </span><span class="typ">Child</span><span class="pln"> child</span><span class="pun">;</span><span class="pln">  child</span><span class="pun">.</span><span class="pln">init_and_join</span><span class="pun">();</span><span class="pln">   </span><span class="pun">}</span>

這樣問題就解決了，我們只要呼叫initandjoin()就可以了。他會幫我們完成初始化和join的動作。如果你在寫java，你還可以再initandjoin()前面加個final，防止子類別override，導致行為改變。