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<h1 class="post-title">应该在何时判断一个指针是否为NULL</h1>
<span class="post-date">周五 12 十月 2007</span>
<p>今天看到一篇Blog：<a href="http://fsfoundry.org/codefreak/2007/09/16/a-couple-of-things-about-pointer-to-null/">《NULL指标两三事》</a>，文中谈的主题即是我这篇Blog的题目，我这里也只是对原文的做些简单的记录。</p>
<p>文中谈到的问题，也是我们平常出现的问题，即在操作一个指针前，我们常常在不需要判断指针是否为NULL时做了判断，而在需要判断时却想当然地把它漏掉了。</p>
<p>例如：delete时，完全可以对一个NULL指针进行操作，而我们却要多余地加上一个<code>if(ptr!=NULL)</code>判断；
又如：strlen时，如果参数给了一个NULL指针，就可能造成整个程序崩掉，我们却以为strlen自己会做NULL指针判读，并在获得一个NULL参数时return。</p>
<p>另外，在原文的评论栏中有一问一答，合理地解释了为什么strlen这样的接受指针参数的函数，会不做<code>if(NULL==ptr)return;</code>这样的操作。嗯，为了效率，这么做充分体验了C的精神。想像一下，对于一个错误的参数，检测函数的返回难道比在调用前就检测参数更省事么；何况如果同一个参数穿过层层的函数调用，一个个检测函数返回是多么可怕的事，不如在调用函数事先来个参数检测。实际上，像strlen这样的函数，也不是没有在函数内部做参数检测，只不过它用的是非常合理的assert断言机制，即满足了功能，又满足了效率。</p>
<p>以后再操作指针时，要记得有这么回事~</p>
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