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Visit website Recent posts Updated on $DATE date interview Zola2026-10-07T00:00:00+00:00https://shaozk.xyz/tags/interview/atom.xml 2026-10-07T00:00:00+00:00 2026-10-07T00:00:00+00:00 shaozk https://shaozk.xyz/blog/025-a-failed-cpp-interview/ <blockquote> <p>一次 C++ 面试中遇到的题目，令人难忘。但其实《Effective C++》中提到过，只是我忘记罢了：条款37——绝不重新定义继承而来的缺省参数值</p> </blockquote> <h2 id="mian-shi-ti">面试题</h2> <pre class="giallo z-code"><code data-lang="cpp"><span class="giallo-l"><span class="z-punctuation z-definition z-directive z-keyword z-control z-directive">#include</span><span class="z-punctuation z-definition z-string z-string"> &lt;iostream&gt;</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span class="z-keyword z-storage z-type">using namespace</span><span class="z-entity z-name z-namespace"> std</span><span class="z-punctuation">;</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span class="z-storage z-type">class</span><span class="z-entity z-name z-type"> Base</span><span class="z-punctuation"> {</span></span> <span class="giallo-l"><span class="z-storage z-type">public</span><span class="z-punctuation">:</span></span> <span class="giallo-l"><span class="z-storage z-modifier z-storage z-type"> virtual void</span><span class="z-entity z-name z-function"> print</span><span class="z-punctuation">(</span><span class="z-storage z-type">int</span><span class="z-variable z-parameter"> a</span><span class="z-keyword z-operator"> =</span><span class="z-constant z-numeric"> 5</span><span class="z-punctuation">) {</span></span> <span class="giallo-l"><span class="z-source"> cout </span><span class="z-keyword z-operator">&lt;&lt;</span><span class="z-source"> a </span><span class="z-keyword z-operator">&lt;&lt;</span><span class="z-source"> endl</span><span class="z-punctuation">;</span><span class="z-source"> </span></span> <span class="giallo-l"><span class="z-punctuation"> }</span></span> <span class="giallo-l"><span class="z-punctuation">};</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span class="z-storage z-type">class</span><span class="z-entity z-name z-type"> Derived</span><span class="z-punctuation"> :</span><span class="z-storage z-type"> public</span><span class="z-entity z-name z-type"> Base</span><span class="z-punctuation"> {</span></span> <span class="giallo-l"><span class="z-storage z-type">public</span><span class="z-punctuation">:</span></span> <span class="giallo-l"><span class="z-storage z-type"> void</span><span class="z-entity z-name z-function"> print</span><span class="z-punctuation">(</span><span class="z-storage z-type">int</span><span class="z-variable z-parameter"> b</span><span class="z-keyword z-operator"> =</span><span class="z-constant z-numeric"> 10</span><span class="z-punctuation">)</span><span class="z-storage z-modifier"> override</span><span class="z-punctuation"> {</span></span> <span class="giallo-l"><span class="z-source"> cout </span><span class="z-keyword z-operator">&lt;&lt;</span><span class="z-source"> b </span><span class="z-keyword z-operator">&lt;&lt;</span><span class="z-source"> endl</span><span class="z-punctuation">;</span><span class="z-source"> </span></span> <span class="giallo-l"><span class="z-punctuation"> }</span></span> <span class="giallo-l"><span class="z-punctuation">};</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span class="z-storage z-type">int</span><span class="z-entity z-name z-function"> main</span><span class="z-punctuation">() {</span></span> <span class="giallo-l"><span class="z-source"> Base </span><span class="z-keyword z-operator">*</span><span class="z-source">base </span><span class="z-keyword z-operator">=</span><span class="z-keyword z-operator z-new"> new</span><span class="z-source"> Derived</span><span class="z-punctuation">[</span><span class="z-constant z-numeric">10</span><span class="z-punctuation">]();</span></span> <span class="giallo-l"><span class="z-source"> base</span><span class="z-punctuation">[</span><span class="z-constant z-numeric">3</span><span class="z-punctuation">].</span><span class="z-entity z-name z-function">print</span><span class="z-punctuation">();</span></span> <span class="giallo-l"><span class="z-keyword z-operator"> delete []</span><span class="z-source"> base</span><span class="z-punctuation">;</span></span> <span class="giallo-l"><span class="z-keyword"> return</span><span class="z-constant z-numeric"> 0</span><span class="z-punctuation">;</span></span> <span class="giallo-l"><span class="z-punctuation">}</span></span></code></pre><h3 id="1-new-yi-ge-lei-dui-xiang-hou-de-gua-hao-neng-bu-neng-sheng-lue">1. new 一个类对象后的括号能不能省略？</h3> <p>答案是可以的，对于类类型会调用默认构造函数。</p> <h3 id="2-dai-ma-zhong-da-yin-de-jie-guo-shi-sha">2. 代码中打印的结果是啥？</h3> <p>很明显这里存在继承关系，虚函数 base[3] 实际上是类 Derived 的对象，所以调用的是 Derived::print()，结果会打印 10。</p> <p>如果你也是这样想的话，那你就犯错了，面试官心内心 OS：这个人不能给他通过。</p> <p>正确答案是打印 5。因为函数缺省参数值是静态绑定的，而 base 的静态类型是指向 Base 的指针，所以默认参数用 Base::print() 中的 5；base 的动态类型是指向 Derived 的指针，所以调用的是 Derived::print()。</p> <h3 id="3-base-3-zai-32-wei-ji-qi-de-pian-yi-shi-duo-shao-zi-jie">3. base[3] 在 32 位机器的偏移是多少字节？</h3> <p>是 3 * sizeof(Base) 而不是 3 * sizeof(Derived)。</p> <p>其中：sizeof(Base) = vptr + 数据成员 = 4 + 0 = 4，所以结果是 3 * 4 = 12 字节。</p> <h3 id="4-dai-ma-zhong-you-sha-qian-zai-wen-ti">4. 代码中有啥潜在问题？</h3> <p>当前子类也没有任何数据成员，所以恰好没问题。如果存在数据成员的话，其 sizeof(Derived) 和 sizeof(Base) 大小会不一样，导致访问 base[3] 会落到错误的地址。同时 delete[] base 也存在问题。</p> <h2 id="jiao-xun">教训</h2> <p>这些问题我都没能答出来，这无疑不暴露我 C++ 基础知识之薄弱。</p>