August 2021
Intermediate to advanced
450 pages
9h 36m
Chinese
Content preview from C++程序设计：原理与实践（基础篇）（原书第2版）
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12.4.5 空指针
如果你没有其他指针用来初始化一个指针，那么使用空指针nullptr：
当0值被赋给一个指针时，它被称为空指针（null pointer）。我们经常通过检测指针是否为nullptr来检查一个指针是否有效（它是否指向什么东西）。例如：
这并不是一个完美的检测，因为p0可能包含一个碰巧不是0的“随机”值（例如我们忘记了初始化），或者一个已经被删除对象的地址（见12.4.6节）。但是，一般来说这已是我们所能做得最好的。我们实际上不必明确提及nullptr，因为if语句会检测它的条件是否为nullptr：
考虑到它更直接地表达了“p0有效”的思想，我们更喜欢这种简短的形式，但是也有不同意见。
如果我们有一个指针有时指向一个对象，而有时什么都不指向，我们就需要使用空指针。这种情况很少见，比很多人想象的更少。思考一下：如果你没有对象需用一个指针指向，那你又为什么定义那个指针呢？你不能等到有一个对象时再做吗？
用名字nullptr表示空指针是C++11的新特性，因此在旧代码中，人们通常使用0（零）或NULL代替nullptr。两种旧的替代方法都会导致混淆和/或错误，因此优先选择更专用的nullptr。 ...
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